Complex substrates and bedding for environmental enhancement and research: or, the nature of primates

Abstract

Complex substrates, including bedding, have generally been known to encourage nonhuman primate species-typical behaviors and have myriad uses in research, yet are still frequently resisted, especially in laboratory environments. Data shows or suggests that complex substrates have the potential to address many primate biological proclivities, ecological and social, concurrently, and to a greater degree than is commonly appreciated. A literature review, along with an informal survey and focus group involving individuals at the National Primate Research Centers and affiliated institutions, provides logistical information relevant to the selection of substrates, their respective advantages and disadvantages, and strategies for successfully arguing for, implementing, or expanding their use. In addition, complex substrates have been utilized successfully in many lines of basic research, have the potential to contribute to others, and will likely improve the quality of primates as a research model. It is hoped that this review will encourage the increased utilization of bedding and other complex substrates in primate behavioral management and science.

1. Introduction

Certain features related to “being a primate” are essential considerations when our closest living relations are maintained in laboratories or zoos for research, education, and conservation purposes. Ostensibly, this review will focus on the practice of providing or manipulating bedding and other complex substrates as a form of environmental enhancement for nonhuman primates (NHPs) and/or as a research tool for their study. As such, it will consider pertinent definitions, examples, and practical strategies. More than that, however, it will consider these topics within the scope of primate natural history. The rationale is that basic biology, much of it derived from field observations, provides the initial impetus for most, if not all, sound management decisions. While the usefulness of complex substrates has long been acknowledged in a general sense, and its value for NHP welfare supported with data, its utilization is still commonly resisted on logistical or clinical grounds, especially with laboratory NHPs. But if complex substrates bring out primate-ness in additional and unexpected guises, or even have the potential to, this becomes critical to management decisions. On the basic research side, the very process of maintaining NHPs in a lab or zoo—such as controlled manipulation of habitat characteristics and particularly complex substrates—may teach us additional things about these animals that cannot easily, or sometimes ever, be gleaned in the wild state. After all, nonhuman primates need to be studied in a range of environments, including varyingly modified ones, to know all the potentialities of their anatomy, physiology, and behavior (Kummer, 1995). In short, the use of complex substrates in experimental contexts might reveal abilities or propensities heretofore unknown, and this is the essence of science.

There is oft-interchangeable use of terms such as “substrate” and “bedding” in the literature. For purposes of this paper, a substrate is an underlying surface; in biological terms, a surface upon which an organism grows, sits, moves, or otherwise behaves. Substrates increase in complexity with increasing numbers of subunits and/or overall variation in shape, texture, or other factors. Complex substrates generally can be modified, moved, or otherwise allow for variable manipulation by the animal. Bedding, a type of complex substrate, is material introduced by a living agent—such as a human caretaker—that rests upon a harder and/or denser substrate base layer, such as a concrete floor. In naturalistic terms, the upper horizon of soil might represent something functionally like a base layer substrate, with ground litter (e.g., fallen leaves, sticks, loose stones, etc.) somewhat analogous to bedding. Ground cover, such as grasses and other small plants, also overlaps functionally with bedding in some respects, e.g., it might provide visual and other sensory barriers to other items of behavioral interest, such as food.

Bedding is generally made of natural, manipulable, soft to semi-soft materials. However, given the broad definition above, some non-intuitive materials, such as introduced stones or gravel atop a soil base layer, would also qualify for our purposes. Non-contact bedding is inaccessible to the animals’ tactile sensation and serves largely a husbandry purpose, by collecting waste and controlling odor. Wood shavings in an unreachable pan under standard caging, or on the floor under a wholly arboreal animal, would be examples. Although husbandry implications for various types of bedding are within our purview, non-contact bedding will not be considered further. Contact bedding, on the other hand, is available for direct physical contact by the animals.

There are multiple aims of this paper:

1. To discuss philosophical issues pertaining to the categories of evidence utilized in primate behavioral management decisions generally, and for assessing substrates and bedding.

2. To review relevant aspects of primate natural history—perhaps the most critical category of evidence available to us—as a backdrop to the possible uses for complex substrate.

3. To discuss prior research on the effectiveness of complex substrates in primate behavioral management.

4. To examine different types of complex substrates, and the advantages, disadvantages, and perceived or real challenges associated with their use in terms of logistical considerations and observed primate behavior. This is based on a review of the literature, as well as from an informal survey and focus group discussion involving professionals at National Primate Research Centers (NPRCs) and affiliated institutions.

5. To consider the use of complex substrates in basic research, or as a tool to improve research models.

The overall goal is to provide primate workers with a useful corpus of information pertinent to the argumentation for, implementation, expansion, and/or refinement of complex substrate and bedding programs for behavioral management and/or scientific investigation.

2. Categories of evidence

Speaking very broadly, there are two types of evidence that can be used in the innovation and/or evaluation of nonhuman primate management practices. The first relates to naturalistic evidence and educated notions derived from it. For example, maintaining primates in species-typical or acceptable groupings, providing enclosure space with adequate perching and room for locomotion, and offering environmental complexity might be justified based merely on behavioral field observations, anatomy, and physiology. Obviously, practical reality might constrain advancements; potential social partners, space, and human and material resources may be limited in a primate facility. This, however, does not make the natural history data any less vital, and in our experience there are few if any situations where refinements cannot occur. For example, adding bedding to a floor might be expected to quickly and enduringly engage primate curiosity, focused visual and tactile search, and manipulative propensity, at least in most species. More on this in a moment.

This leads naturally to a second kind of evidence, which are direct tests of effectiveness—in situ evaluation of behavioral management practices in the facilities in which they are attempted. These tie closely to the first category of evidence, in that what animals do in nature provides, to varying degrees, a yardstick for success. An effective environmental enrichment strategy encourages the animal’s expression of species-typical behaviors to facilitate successful coping with stressors (Broom & Johnson, 2019; Howell & Cheyne, 2019; Lutz et al., 2022). Animals, including those who were born in laboratory and zoo environments, are highly motivated to express behaviors that in nature are likely to influence their fitness (Dawkins, 1990). Accordingly, an environment that thwarts the use of highly motivated species-typical behaviors can lead to a sense of frustration and consequently to the development of behavioral abnormalities (Pomerantz et al., 2013).

To reiterate, the two broad varieties of evidence cannot be divorced from each other and are complementary. Evidence from nature can assist in providing directional predictions in other settings. While not all naturalistic features are desirable—predators and parasites being obvious examples (Rosenblum, 1991)—characteristics that wild primates exhibit by choice or biological constitution provide clear guideposts for improvements in laboratory or zoo environments. Consider this a “Golden Rule” of primate behavioral management. By extension, data suggesting that laboratory or zoo primates seem to prefer things differently than their wild counterparts might indeed reflect true differentiation based on ontogeny/environment but might alternatively reflect constrained experimental design or an inadequate range of outcome measures. The threshold of evidence is greater—indeed, must be greater—for those claiming disconnects with evolutionary history.

3. Primate biology

To understand why primates behave as they do, and to predict how this information might optimally be utilized in behavioral management or research, it is advantageous to consider their evolved characteristics. There have been numerous attempts to define primates by anatomical criteria (e.g., Mivart, 1873) and a subset of this work includes indispensable—in other words, non-optional—considerations of behavior and life history (Martin, 1990; McGraw, 2010; Shea, 2007). To distill “what it means to be a primate” down to a single word, however, might be considered a constructive thought experiment or, alternatively, a fool’s errand. We will readily concede that it is a little bit of both, and merely state that primates are manipulators (cf. Perry & Manson, 2008). This is not pejorative. It simply means that primates, compared to most if not all other organisms, exhibit exceptionally flexible control of both their own faculties (Beran et al., 2016) and, when given the chance, stimuli in their environments. They influence the behavior of conspecifics via oftentimes intense social learning (e.g., Rapaport & Brown, 2008), flexible behavior, and perhaps even trickery (R. W. Byrne & Whiten, 1990). Primates pay exquisite attention to detail; curiosity is comparatively monumental (Glickman & Sroges, 1966; Joubert & Vauclair, 1986; Menzel & Menzel, 1979) and it is unlikely that new information lies dormant if it is potentially functional. It is repeatedly manipulated and utilized in action plans. Brains are large, lives and development are long, litters are small, play is lengthy or exaggerated, and social networks are vast and/or nuanced in even the most dispersed or “solitary” [sic] species investigated. Anatomy and behavior are, for the most part, remarkably unspecialized (Schultz, 1969). As to why complex substrates are relevant to these and many other aspects of primate natural history, please keep reading.

Primate sensory systems are key to understanding the members of this order. While some have again noted relative non-specialization—e.g., compared to other mammals—with regards to where pertinent information comes from (Smith, 1913), nearly all commentators have remarked on the evolutionary import of the visual and tactile senses. Primates tend to have anteriorly positioned orbits, aiding in binocular, stereoscopic vision that favors close examination at the expense of sweeping, peripheral vistas (Martin, 2012). Tarsiers and diurnal anthropoids have a fovea, a retinal depression of the macula associated with a zenith in visual acuity (Ankel-Simons, 2007; Hendrickson et al., 2000; Wolin & Massopust Jr., 1967), and ideal for hashing out intricate environmental details. Trichromatic vision, found piecemeal in other primates, is characteristic of all howler monkeys and catarrhines and has been hypothesized to aid in the detection of social cues, predators, and/or foods via color contrasts (Carvalho et al., 2017).

Concerning touch, primates are especially notable for their grasping abilities with the hands and feet. Indeed, opposition of the thumb—the ability to place this digit stiffly in contact with terminal pads of other digits—has been provocatively suggested as “the single most crucial adaptation” in primate and human evolutionary history (Napier, 1980, p. 68). Pointedly for purposes of this review, other primates broadly exhibit grasping abilities, from the pseudo-opposability of prosimians and most platyrrhines, where thumbs flex in tandem with other fingers and clenched-fist power grips are common, to the more honed opposability and frequent thumb-forefinger precision grips of capuchin monkeys and catarrhines (Fragaszy et al., 2004; Napier, 1980; Welles, 1976). In primates, flat nails replace sharp claws on some or all digits, and skin ridges and Meissner’s corpuscles on lateral finger and toe surfaces improve grip and the sensitivity of touch (Martin, 2012).

Taken together, primates have a litany of characteristics related to manipulation in all its guises. How these come together in relation to behavioral phenotype and function will of course vary by species and individual. Some features, such as curiosity and attention, help identify environmental regularities and irregularities that suggest or demand further investigation by the animal. Others relate to manipulation in perhaps its most literal sense, interaction with physical aspects of an organism’s environment. One very non-subtle variety of interaction with the material world is nest building, which occurs in some prosimians and all extant great apes (Kappeler, 1998). Far more generally, many primates are largely arboreal, and this has featured in numerous models of the order’s origins (Cartmill, 1974; Jones, 1916; Smith, 1913; Sussman et al., 2013; Szalay & Delson, 1979). Primates are thus varyingly comfortable on raised substrates upon which they perch, walk, swing, and leap. In a striking experiment (Menzel, 1986), titi monkeys (Callicebus moloch) preferred walking on a small stick placed on the ground, as opposed to directly on the ground, to retrieve a food item, even in some cases where travel distance significantly increased by doing so. In some cases, a stick is not just a stick and could be considered “an abstract representation of a tree branch” (p. 167). Yet the titis showed flexibility and would walk on the ground as needed, as they occasionally do both in field enclosures and in the wild. Additionally, most primates engage in at least occasional terrestrial behavior where they will encounter varying ground surfaces and covers. As with arboreal supports, this includes assorted textures, moisture levels, and degrees of firmness. Some primates frequently maintained in laboratories and/or zoos are, in nature, moderately to largely terrestrial, including ring-tailed lemurs, gray langurs, baboons, macaques, and African apes. Thus, myriad opportunities exist for providing abstract, if not real, representations of ground cover and ground litter.

Primates, in trees and on the ground, frequently pursue food targets hidden within or amongst other structures: seeds removed from fruits, roots and tubers from the earth, invertebrates from within vegetation and under the soil or rocks, and so on. Certain aspects of primate cognitive and brain evolution have been hypothesized to relate to such extractive foraging behaviors, especially in more omnivorous species who utilize complex manipulation techniques (Gibson, 1986; Parker & Gibson, 1977). Foraging on foods visually or otherwise hidden occurs widely throughout the order, using the morphological abilities mentioned above and others besides (e.g., oral movements). In some cases extractive behaviors include complex, multistep manipulations of foods and/or unattached material tools (Byrne, 1997; Parker, 2015).

Primates, as much or more so than many, most, or all other vertebrates, do not inhabit an empty Euclidean space, and physical structures influence much of their behavior. For example, crab-eating macaques (Macaca fascicularis) find hidden food piles far more often and quickly when arranged along an environmental border, such as between stone and grass, than when arranged in an invisible straight line (Menzel, 1996b). Given the manipulative propensity, sensory acuity, and curiosity of primates, not to mention a modal ecological and dietary flexibility (Harding, 1981; Sayers & Lovejoy, 2014), there will be a general drive to investigate nooks and crannies, to explore around and within physical structures, and to engage in nuanced search routines when interesting items are found (cf. Menzel, 1991) such as, say, high-energy foods hidden under grass, sand, leaves, wood shavings, etc. Since food intake affects survival and reproduction, animals including NHPs are motivated to express behaviors optimizing such activities. Indeed, macaques, as an example genus, dedicate a major portion of their activity budgets to feeding and foraging. For example, Assamese macaques (Macaca assamensis) in seasonal, tropical limestone forest spent more than 30% of their time feeding (Li et al., 2020), plantation-living pig-tailed macaques (Macaca nemestrina) were observed foraging for more than 40% of their time (Ruppert et al., 2018), and rhesus macaques (Macaca mulatta) spent 22.4% (urban environment) to 36.2% (rural environment) of their activity budget feeding (Jaman & Huffman, 2013). It is also worth noting that food search—such as travel between food patches—adds additional time to the more general foraging component of an ethogram.

What is remarkable is that complex substrates provide means to address many of these primate evolutionary imperatives concurrently, even where not intuitively obvious. It almost certainly allows for expression of social behavior not possible on a bare concrete floor, with a wider range of potential actions (i.e., possibilities for behavioral flexibility). Consider, for example, a foraging mix containing items of moderate and high preference, given to a group of baboons. If spread in the open it would be expected that, below a certain density threshold, the high preference items would be eaten almost exclusively by a small number of higher-status individuals. Others may practice avoidance. If this same mix were distributed under wood shavings, with the more valuable food items no longer within visual sensation, lower-status individuals gain opportunity to compete for the resources. An increasing number of strategies exist for hiding food finds from their simian overlords—such as waiting for turned backs before exploiting or directly placing muzzle in substrate for covert pick up (discussed further in 7.3, below). Searching through bedding presents healthy challenge (e.g., Washburn, 2015) and might engage visual attention, tactile stimulation, manual and pedal manipulation, and curiosity regarding the substrate and what potentially lies beneath, all in species-typical fashion. New forms of play are possible. Complete foraging routines including search and handling (e.g., Charnov, 1976) become the order of the day.

These are not merely potential categories of environmental enrichment or enhancement. They are deeply rooted in primate natural history, and, thus, essential. For this reason, complex substrates also can be considered possible tools to increase our knowledge of primate behavior, cognition, and functional anatomy.

4. Direct tests of effectiveness: food presentation method and complex substrates

NHPs in spartan enclosures lack the range of foraging opportunities available to their wild counterparts (Reinhardt & Roberts, 1997). Animals in such circumstances do not need to invest many resources in looking for food. Often it is presented in aggregate and provided as concentrated chow biscuits. Thus, these conditions will likely decrease the animals’ welfare as they cannot express highly motivated species-typical foraging behaviors. This idea is further strengthened in studies that examined the effects of an increase in foraging opportunities on welfare indices. For example, providing singly housed rhesus macaques with a device that encourages foraging behavior resulted in a significant decrease in rates of abnormal behavior (Bayne et al., 1991).

How food is provided to the animals affects their interaction with it. Zoo-housed Goeldi’s marmosets (Callimico goeldii) that were provided with large food items (e.g., sizable pepper chunks, whole carrots) spent significantly more time engaging with it compared to those that were given chopped food (Bartlett et al., 2023). This stimulated the monkeys to manipulate the food similarly to the way their wild counterparts would, thus enabling and promoting the expression of natural behavior. In rhesus macaques, providing large, intact produce to animals living in complex outdoor groups resulted in a staggering reduction of 37% in rates of social trauma, presumably due to the long-lasting manipulation of the food items (Wooddell et al., 2019).

The presence of bedding or natural analogues provides NHPs increased opportunities to engage in environmental exploration and extractive foraging, and to extend foraging time. For rhesus macaques, simply spreading sunflower seeds atop woodchip substrate achieved these goals more effectively than burying regular diet within the substrate (Byrne and Suomi, 1991). Long-tail macaques (Macaca fascicularis) foraged more and self-groomed less—with no change in social grooming—when bedding was available to the monkeys than when it was not (Janavaris et al., 2022).

Sanctuary-housed chimpanzees (Pan troglodytes) experienced a higher degree of welfare when provided with nesting material, as was evidenced by a reduction in abnormal behavior and an increase in affiliative behaviors (Anderson et al., 2021). Positive effects have been additionally reported for NHPs housed in enclosures with natural ground cover. Thus, male Japanese macaques (Macaca fuscata) living in a vegetated environment played more, had fewer agonistic encounters, and expressed less stereotypic behavior in comparison to those housed in non-vegetated enclosures. In addition, animals housed in the vegetated enclosures spent more time feeding, less time resting, and had better coat quality than their counterparts in the non-vegetated environment (Alejandro et al., 2022). Similarly, rhesus macaques kept in enclosures featuring gravel substrate were 3.7 times more prone to experiencing significant hair loss compared to those housed in enclosures with grass substrate (Beisner & Isbell, 2009a).

In addition to normalizing the behavioral repertoire of captive-housed animals, many forms of complex substrate would be expected to provide comfort for animals of various life stages. Softer surfaces are likely more comfortable to locomote, play, or rest upon which may especially benefit geriatric animals, younger animals, pregnant females, and NHPs that nest and create beds every night. They might also soften the landing impact of animals falling or jumping from climbing structures depending on the type and depth provided and possibly lessen impact injuries. It is also suggested that because bedding absorbs moisture, there are fewer wet surfaces or slipping hazards. All bedding may dampen sounds in the environment (Janavaris et al., 2022), although further work is needed to draw conclusions as to the magnitude of this effect or which materials are superior in this function.

While thermoregulatory considerations have received little attention with specific reference to primates, agricultural work has shown that certain varieties of bedding can provide improved heat conduction in cold ambient temperatures and cooler warm-weather surfaces, or otherwise advantageous outcomes, as compared to bare concrete flooring (e.g., Chopra et al., 2021; Singh et al., 2020).

Taken together, these results highlight the multitudinous benefits of complex substrates to NHPs. Substrates, however, yield idiosyncratic welfare or logistical benefits based on type, for which data will be reviewed below.

5. Practical considerations relating to complex substrate and bedding

There are potential applications for complex substrates in most housing types, whether indoor, sheltered, or outdoor. At most primate facilities, the construction of entirely new enclosures is sufficiently rare to make working with what you have the most common day-to-day consideration. A corollary is that when a new habitat is to be constructed, it is an opportunity to select advantageous substrates that will have behavioral management and practical repercussions across years or decades. Grassy earth or otherwise naturalistic base layers in appropriate areas have decided advantages over concrete or similar surfaces in terms of providing ecological complexity, encouraging species-typical behaviors, and reducing by orders of magnitude the resources required for cleaning and maintenance. Contrary arguments favoring hard, artificial surfaces or standard caging for purposes of straightforward sanitation, sometimes expressed in facility reviews, are non-biological, at odds with existing data (Prongay et al., 2013; Sestak et al., 2003; Wamba et al., 2022), and otherwise unconvincing. Increasingly artificial spaces may be associated with decimated gut microflora (Houtz et al., 2021; McKenzie et al., 2017), reduced activity, and myriad other factors which can negatively influence overall health. We have already mentioned how providing access to vegetated ground, or even simple bedding on a hard top surface, can have a very positive influence on activity levels and specific behaviors in nonhuman primates. We have every reason to expect, based on natural history, that such factors far outbalance any advantages afforded by a temporarily sterile floor or cage. .

Even in cases where ground cover (e.g., grass), litter (e.g., dead leaves), or one type of bedding are present, additional introduced bedding can provide novel textures and lacunae related to idiosyncratic sensations and search requirements. In outdoor spaces, thanks to the action of sun and rain, such bedding might require infrequent, or even optional, spot cleaning or replacement. The behavioral benefits associated with provisioned bedding are likely to be especially marked with lower complexity of the base layer—smooth concrete or similar substances, sans bedding, being the zenith of low complexity.

5.1. Substrates and bedding – general logistics

When selecting substrates or contact bedding material there are many factors to consider, including enclosure design, species and number of animals using the same space, their age and activity levels, and goals. The type of base layer—e.g., concrete, tile, wire, epoxy, dirt—that bedding will cover, as well as additional structures in the enclosure, influence effectiveness. For example, caging with wire mesh bottoms might allow many/most types of bedding to fall through. In such instances, alternatives are available that still provide opportunities for manual manipulation and novel sensory experience, such as placing bedding and concealed rewards on trays or in buckets affixed to the enclosure (e.g., Vernes & Louwerse, 2010).

Nooks, crannies, and moving parts related to sliding gates or doors, such as hinges, latches, and tracks, may catch or trap some types of material. Covers, plugs, baskets or other barriers may be required to protect drains. Different types of bedding vary by price depending on suppliers and regional availability. Ease of receiving, storing, longevity in storage, handling, cleaning practices and cleaning frequency are other aspects to consider. Dustiness, which varies by bedding type, affects the amount and quality of occupational exposure to airborne contaminants (Kaliste, et al., 2004). For example, a review of allergens causing occupational asthma found moderate evidence of association with hay and straw (in the context of farming) and western red cedar dust (Baur & Bakehe, 2014). Additional Personal Protective Equipment (PPE) are advisable for staff that may be in greater contact with substrates, bedding, and cleaning agents to prevent exposure and allergic reactions. Additional PPE may include but is not limited to Powered Air Purifying Respirators (PAPR), N95 face masks, gloves with extended sleeves/cuffs, additional arm coverings that might protect against itchiness, Tyvek, and waterproof and non-slip shoes.

Some NHPs are housed in outdoor enclosures and/or may have indoor/outdoor enclosures where animals have the choice of being inside or outside. Bedding can be provided regardless of environment, but in exposed locations may become dampened in wet weather, causing animals to be chilled and/or uncomfortable and increasing labor in removal. Mud accumulation can be mitigated using geotextiles, tiles, gutters, and a combination of various substrates (e.g., sand, gravel) to manage water movement away from where animals congregate.

Substrates and bedding may have dust, dyes, or aromatics that may interfere with select study protocols. However, as noted below, dust-free, dye-free, and scent-free varieties are available and likely compatible with a very wide range of research goals.

5.2. Substrate and Contact Bedding Types

Individual substrate types have varying advantages and disadvantages. They vary widely and range from various plant-based beddings that are removed and replaced regularly, to outdoor ground and vegetated surfaces, to long-term (≥ 6 months) indoor or sheltered biofloors made of natural materials. As many relevant observations have not been disseminated, the literature review conducted for this paper was coupled with an informal survey of staff from representatives at five primate institutions associated with the Behavioral Management Consortium (BMC, Baker et al., 2017) of the NPRCs and affiliated institutions with added comments recorded between these and two additional institutions (n=7 total) in a focus group on the topic. Table 1 provides a list of common substrates and cost/benefit assessments as gathered from these sources. Some statements are necessarily subjective but highlight areas in need of further targeted research.

TABLE 1 |.

Characteristics of select complex substrates.

Type	Description	Positive aspects	Negative aspects, challenges, or other considerations
Wood products
Chips	Wood pieces, generally small to moderate in individual size	Provides opportunities for extractive foraging, manipulation, environmental exploration; available in various sizes; inexpensive and usually easy to obtain1 Small softwood chips have good absorbency2 Cebus capucinus: wood chips, wood wool, and peat precipitated higher foraging, locomotion, and overall presence than ground corn cob3 Macaca arctoides: reduced aggressive interactions when small food objects were scattered on wood chip litter versus bare floor4 Macaca mulatta: woodchips and scattered sunflower seeds increased foraging and exploratory behavior, and decreased inactivity, when compared to bare floor or the burying of monkey chow in woodchips5 Pan troglodytes: wood chips placed in an outdoor play area resulted in reduced abnormal behaviors and engagement in wood chip-related behaviors for an average of 21% of measured time budget. Area was drier and cleaner with wood chips than with concrete flooring6 Comparative: Lemur catta, Saguinus labiatus, Callithrix jaccus, Cebus apella, Cercopithecus cephus, Chlorocebus aethiops, and Macaca arctoides were less inactive with wood chip litter than bare floor. Scattering food in litter bolstered the effect; other positive behaviors involved subsets of species. No negative behaviors were associated with wood chip litter and bacterial counts showed a general decrease over time7	Heavy1 Comparatively poor absorbency1 Removal comparatively difficult especially when muddied1 Suspected increase in pests1 Large chips may be uncomfortable to rest and locomote upon1 Health concerns include dust and skin irritations1
Sani-chips®	Fine-grained (small) wood chips	Provides opportunities for extractive foraging, manipulation, environmental exploration1 Small size precipitates less worry if limited amounts go down drain1 Easy sifting during spot cleaning1 Tracks less in hallway than wood shavings1 Absorbent1 Takes up comparatively little space for disposal1 Advertised as scent and dust-free8 Largely free of dust, no chemical additives; not a food source for animals (mice)9	Many food and enrichment items are reported as easily spotted within the Sani-chips® potentially leading to less time spent foraging when compared to other substrate and bedding types1 Dense, and heavy when packed into fewer disposal bags and/or bins1 Comparatively expensive1 Large quantity needed compared to larger wood shavings1 Feels prickly with sharp edges9
Sawdust	Fine-grained (small) wood byproduct	Provides opportunities for extractive foraging, manipulation, environmental exploration1,10 Less material to dispose of when compared to straw11 Macaca: Scattering forage mix in sawdust in indoor area keeps animals foraging for entire days; groups perceived as calmer and more stable compared to bare floor; excellent absorbance of feces and urine; cleaning reduced from daily to every several weeks; also used in tubs attached to enclosure10	May be inhalation irritant, as observed in rats12 Tracks easily1
Shavings	Small, thin sections of wood (e.g., pine, aspen, cedar)	Provides opportunities for extractive foraging, manipulation, environmental exploration1 Improvements in NHP hair coverage when compared to bare floor1 Has been used as intervention for aggression in some large groups1 Potential for olfactory (e.g., aromatic) enrichment1 Generally delivered in packaged cubes so easy to store1 Soft1 Easy to spread and spot clean1 Good absorbency and moderate price2 Macaca mulatta: decreased passive feeding and agonism when mixed with food in standard caging13; increased foraging and reduced aggression and self-grooming in bedded versus non-bedded shelter housing, with reduced water use and no increase in human labor time14 Macaca fascicularis: increased foraging and reduced self-grooming with wood shavings in pen compared to no bedding condition; reduced peak and mean sound levels with shavings15 Macaca radiata: decreased husbandry costs and water and chemical use in pen housing when compared to bare floors; simple drain cover eliminated putative clogging issues16	Aromatic nature and dust may serve as irritant; birch shavings found to be far more dusty than other wood or straw-based beddings in one study17 Special drain covers needed to prevent clogging issues1 Soaks up liquids therefore can be heavy to clean1 Easily trackable1 Difficult to sift clean materials from soiled material1 Removal from pens is time intensive1 One facility reported that wood shavings “required an additional 5 minutes extra per run compared to days when water cleaning was done without wood shavings present.” However, time savings were reported for spot cleaning days1
Wood wool or excelsior	Compact wood slivers often used as cushioning in packaging, e.g., to protect major awards	Provides opportunities for extractive foraging, manipulation, environmental exploration1 Delivered in compact bales for efficient storage1 Lightweight when fluffed1 One small flake fluffs up at least 10x its original size, a task that many NHPs are happy to tackle1 Can be given out in sheets1 Easy to rinse1 Reusable1 Not considered allergenic or toxic8 Cebus capucinus: stimulated more play activity and foraging than ground corn cobs, wood chips, or peat. Scored second in social contact, locomotion, and total presence after peat3 Macaca arctoides: more gymnastics-like play among juveniles on excelsior than wood chips, which in turn was higher than a bare floor7 Excellent for great ape (e.g., Pan troglodytes) nest-building activities18	Not absorbent1 Does not spread easily1 May break into smaller paper-clip size pieces which can cause drainage issues or require use of specialized tools to manage (e.g. using a squeegee on smooth surfaces appears to pick up small pieces of excelsior well)1 Cercopithecus wolfi: less time was spent examining excelsior than straw. Excelsior, unlike straw, was not found to increase affinitive behaviors19
Other plant-based products
Corncob	Agricultural byproduct derived from woody ear portions of maize, a grass/cereal grain—can be ground to make bedding	Good absorbency2	Expensive compared to other common bedding types2 Cebus capucinus: ground corn cob was the least desirable substrate when compared to peat, wood wool, and wood chips, in measures of total presence, foraging, locomotion, social contact, and play. Hard granules may stick on fur or cause hand and foot discomfort3 Physiological, including estrogen-related, effects reported in rodents20
Peat	Decomposed sphagnum moss and other organic materials harvested from bogs and other wetlands	Chemicals not required to process; cushion-like and not prone to compaction; some varieties are low-dust8 Highly absorbent of fluids and ammonia21 Cebus capucinus: peat and wood wool/excelsior precipitated higher social contact scores than ground corn cob and woodchips. Along with wood wool and wood chips, peat precipitated higher foraging, locomotion, and overall presence than ground corn cob3	Negative environmental impacts of using peat derived from bog draining render other substances more eco-friendly22 Expensive compared to most other agriculturally or horticulturally derived substrates8
Straw and hay	Agricultural product or byproduct composed of non-woody cereal grasses	Provides opportunities for extractive foraging, manipulation, environmental exploration1 Inexpensive1 Locally available in many areas1 Large variety of straw and hay types1 Some more absorbent than others which can aid in decreasing cleaning time, water usage, and feces smearing1 Provides warmth in colder weather1 Packaging varies from compressed bales and bagged products1 Cleaning and removal is fairly easy when dry1 No skin irritation reported in the literature among baboons or pigtail macaques1 Decreased puddle formation during weather events, and enclosures are cleaner with less mud1 Cercopithecus wolfi: more time was spent examining straw than excelsior. Straw also was associated with an increase in affinitive behaviors19 Cercocebus atys: Timothy hay containing food items—with additional added structures in enclosure—increased feeding, foraging time and manipulation and decreased aggressive behavior, self-grooming, and social affiliation23 Macaca nemestrina: more foraging occurred in straw than paper1 Pan troglodytes: considerable manipulation of straw without habituation over 9 weeks. Reductions in abnormal behavior, regurgitation, reingestion, and displays; increases in play and movement24	Staff reports of allergies to certain straw and hay varieties1, including dust/skin irritation greater in Bermuda than Timothy hay If ordering from a company that only has hay and straw in bales, pieces can fall off bales and cause extra storage maintenance1 Reports of random items in bales (rocks, clumps of dirt, metal, etc.)1 May increase pests, heat, and moisture1 Rated average to poor in absorbency, dustiness, endogen effect, and ease of disposal in a laboratory-focused review2 Employees at one facility that employs multiple types of bedding prefer straw and hay the least because of difficulties in cleaning and the tendency to track1 Heavy at cleanup1 More material to dispose of than shavings or sawdust11
Paper products
Aquasol® water soluble paper	Commercial product provided in tubes, strips, and mulch	Provides novel texture and can be extensively manipulated and altered by the animal1 Easy to clean when used in small or moderate quantities, as it dissolves when in contact with cold water, hot water, steam, and most aqueous solutions. Rate of breakdown can be controlled via the temperature of dissolving agent1,8 Video recorded observations of a Southwest NPRC male rhesus monkey evince considerable smelling, stripping, mouthing, and eating of Aquasol® paper1	Comparatively expensive and suited for targeted enrichment use as opposed to wide scale bedding1 When used in large quantities, may dissolve unevenly and create a mush-like substance1
Compressed paper bedding, such as Carefresh® and similar products	Processed cellulose fibers	Free of inks, dyes, and dust to reduce allergy issues; lightweight; controls odors; can absorb three times its weight; available certified and irradiated in autoclavable packaging8 Material Safety Data Sheet and Certificate of Analysis may be available1 In mouse study, no differences in health concerns reports with compressed paper versus corncob bedding25 In mouse study, compressed paper more absorbent than ground corn cob26	Expensive1 Reports it can become “cement-like” when wet and get stuck in corners or crevices1
Standard paper	Office paper shredded and repurposed for animal use	Encourages foraging behavior1 Free source materials if facility shreds its own paper1	Takes a long time to shred paper, especially for large groups—most useful as short-term or targeted enrichment1 Stray remnants missed during cleaning could clog drains1
Outdoor ground substrates
German chamomile (Matricaria chamomilla)	Near-cosmopolitan annual plant	Provides visual novelty or complexity1 Easy to grow1 Prolific1 Low maintenance due to low-growing nature1	Dies in the winter1 Contributes to mud1 Superficial root system does not prevent erosion1 May be considered invasive1
Grass	Large, near-cosmopolitan family of flowering plants	Provides opportunities for extractive foraging, manipulation, environmental exploration1 Wide variety of grass types and presentation types (clippings, clumps, sod, patches, entire enclosures, etc.)1 Naturally present, some reports of little to no maintenance1 Promotes foraging1 Macaca mulatta: myriad behavioral benefits over gravel, including more complex movement patterns, increased foraging, and less grooming and hair loss28	Muddy in rainy months1 May need to be mowed/maintained1 May require watering1 May be difficult to establish in some areas1
Gravel	Rock fragments	Provides greater complexity than smooth floors; primates can search underneath or engage in stone handling behaviors, which may have multiple developmental and welfare benefits27	Irregular in shape; sharp edges possible; may be dusty; non-absorbent8 Certain forms of enrichment, such as cardboard, may interfere with gravel cleaning procedures1 Macaca mulatta: inferior to grass in myriad behavioral measures28
River rock	Stone rounded by the actions of running water	Behavioral benefits as gravel, above Fewer sharp edges and less dusty than gravel8	Hard in texture8 Non-absorbent8
Sand	Inorganic earth texture	Provides substrate for search and play; may occur as natural ground, introduced substrate, or in “sandbox” features29	Not too absorbent, especially in finer varieties such as desert sand30 River sand absorbs liquids and makes better litter material than desert sand which is too fine and smooth30
Other complex substrates
Biofloors	Thick layer of substrate with biological activity; breaks down animal urine and feces, thereby acting as a (partially) self-cleaning floor32	Facilitates species-typical behavior and reduces abnormal behavior in multiple NHP species31 Presence of an absorbent substrate may reduce the likelihood of disease transmission from fecal contamination7 May minimize impact on joints31 Pest presence similar to bare/hard floors31 Retains heat31 May reduce noise31 Drains well31 Callithrix jaccus: decreases in maintenance time and increased enrichment opportunities compared to bare floors10 Macaca arctoides: a microbiological analysis over the course of 8 weeks found total bacteria count decreased7 No gut impactions reported in apes31	Facility constraints due to space, enclosure construction, and drain system may limit implementation; substrate materials for biofloor may be costly for initial investment, although highly economical in the long run31

Sources:

^1.Present survey and focus group.

^2. Weichbrod et al. 1986

^3.Ludes-Fraulob and Anderson 1999.

^4.Chamove and Anderson 1979.

^5.Byrne and Suomi 1991.

^6.Brent 1992.

^7.Chamove et al. 1982.

^8.Manufacturer and pricing information.

^9.Moehring 2016.

^10.Vernes and Louwerse 2010.

^11.Bradley 2019.

^12.Johard et al. 1994.

^13.Lutz and Novak 1995.

^14.Doane et al. 2013.

^15.Janavaris et al. 2022.

^16.Bennett et al. 2010.

^17.Kaliste et al. 2004.

^18.Reamer et al. 2017.

^19.Fuller et al. 2010.

^20.Trainor et al. 2013.

^21.Shephard et al, 2017.

^22.Page & Baird 2016.

^23.Crast et al. 2016.

^24.Baker 1997.

^25.Allen et al. 2021.

^26.Pallas et al. 2020.

^27.Nahallage, Leca, & Huffman, 2016.

^28.Beisner and Isbell 2008, 2009.

^29.Menzel 1997.

^30.Diarra 2021.

^31.Leinwand et al. 2021.

^32.Fiby 2020.

For example, most complex substrate types evince few or no quantitative considerations of their effects on primate behavior when compared to others, or to bare floors. The same can be said for detailed data on comparative labor or material costs, sanitation correlates (e.g., relative microorganism presence), or other outcomes (e.g., noise reductions). In addition, the nonhuman primate species that have been quantitatively studied, while broad, is uneven—with an unsurprisingly strong focus on macaques. As seen on Table 1, present evaluation of substrates is limited to some extent on qualitative observations, naturalistic arguments, and analogy with other substrate types.

Little-investigated are amalgams of surface types, which provide choice. Combinations of substrates can be used concurrently and may in some cases maximize benefits. For example, a study in laboratory swine found that by supplementing wood shavings with pine pellets, the two materials would combine to form a 5–8 cm (2–3 inches) layer of soft loam which effectively reduced ammonia and offensive odors, absorbed moisture, and prevented feces from sticking to the floor. It was reported that pigs housed on just shavings alone required substantially more bedding, storage space, and caretaker attention than those housed on mixed bedding (Grain & Mood, 2009). With NHPs, various pluses and minuses for individual bedding types have been noted with relation to the amount of foraging precipitated, weight, and ease of maintenance. A combination of two or more—such as straw, hay, and/or wood shavings—may alleviate certain concerns by decreasing the gross amount of each substrate while still providing animals with opportunities for species-specific behaviors.

5.3. Distribution, maintenance, and removal of bedding

Regardless of base layer substrate, bedding provision and maintenance involves some sequential steps. These include selection of bedding type and quantity, distribution of bedding and complementary materials, optional spot cleaning and replacement of soiled or otherwise dirty bedding, removal and disposal of bedding, and optional repeating of the process, as detailed below. The graphical abstract to this article, created with artificial intelligence tool ChatGPT based on instructions from the authors, provides a general illustration.

Providing bedding materials on top of hardscape concrete or tile is a frequent practice in many laboratories and zoos. The depth of bedding can vary from a light dusting across the surface to 30 cm (12 inches) or more deep. While even small amounts of most bedding materials encourage exploration and manipulation, it is not surprising that the placement of food targets underneath or within can be expected to precipitate more enthusiastic or prolonged search routines. Small goal objects such as sunflower seeds (Byrne & Suomi, 1991), nuts, raisins, and cereal are especially suitable, and cryptic items that blend in with the particular bedding provides additional good challenge for the primates. One option is to scatter these in a random, “fine-grained” (Post, 1984, p. 287) fashion where probabilities of encountering food are approximately equivalent across the enclosure. Another, especially relevant for cognitive study, is to bias food placement in relation to certain environmental features or arbitrary rule(s) and quantify whether the animals pick up on the pattern (see below).

Standard bedding can normally be kept down for days or weeks, depending on maintenance routine and situation (long-term biofloors are discussed below). A maintenance routine consists of periodic spot cleaning, where soiled sections of bedding are removed and additional bedding is placed down, and additional food items are added. This occurs ad hoc but daily or every-other-day spot cleans are generally the rule. The maintenance schedule drives how clean, dirty, or pathogenic an environment is. An evaluation of five sterilized bedding materials and three bacterial species common to barn conditions concluded that bacterial levels are related to more complicated factors than simply the type of substrate utilized (Zehner et al. 1986).

It is important to utilize the proper tools for substrate and bedding distribution and maintenance. Especially important considerations are the space and human maneuverability inside the enclosures. A flex rake would work well in large enclosures to pick up larger substrates and bedding like straw, whereas a sift rake would be more ideal to extract feces and debris in small-grained substrates such as sand and sawdust. For removing large quantities of bedding, wide snow shovels are preferred for lightweight bedding, and more narrow shovels for materials heavier and denser. It is critical to have dialogue with staff to determine the set of tools most appropriate for a particular substrate and bedding program.

Disposal of bedding occurs after spot cleaning or full removal of bedding. Disposal protocol will vary dependent on species, the clinical and experimental history of the individual animals, and local standard operating procedures. In some cases, bedding can be discarded as general waste. In others, the bedding must be packaged and discarded as biohazard—this is frequently required for rhesus monkeys, which can carry B virus (Rohrman, 2016), and for various primates with past infectious disease history. The Environmental Health and Safety (EHS) office at your home institution can assist in determining what path is suitable for given instances.

Bedding on hardscape surfaces will periodically be fully removed for full sanitation of the enclosure. Generally, such sanitations will occur at minimum every two weeks (National Research Council, 2010, p. 70). At that point new bedding may be distributed, and the process repeated.

Biofloors are an appealing long-term variation and consist of a deep (≥15 cm) layer of naturalistic materials (e.g., wood chips, mulch, soil) that stay down indefinitely (Leinwand et al. 2021). Maintenance includes periodic raking, turning, moistening, and supplementation of the bedding. Cleaning is accomplished with diluted bleach, vinegar, and/or other detergent and may be applied daily, bi-weekly, or ad hoc. Full replacement of the biofloor may be done periodically but is optional and dependent on local preferences and circumstances.

6. Perceived or real challenges of bedding (and solutions as needed)

As discussed, bedding allows for hardscape surfaces to be transformed into beneficial, complex substrates. While the benefits are widely known, at least in a general sense, implementation is often challenging for various reasons. We propose the following perceived or real challenges with potential solutions. While the discussion is mainly framed in terms of bedding that is changed frequently, some overlapping concerns and solutions (and/or refutations) have been provided elsewhere for long-term biofloors (e.g., Leinwand et al. 2021).

6.1. Perceived increase in human management time

One of the leading concerns of bedding use is the perceived increase in human management time. Bedding removal requires substantial cleaning time. However, published data have shown that caring for areas with bedding does not increase the overall time spent cleaning since time is saved not having to wash when bedding is present. In a 4-week study of outdoor-housed groups of rhesus macaques at the Oregon NPRC, husbandry staff tracked the amount of time spent caring for bedded versus non-bedded units. There was no difference in the time required to care between bedded and non-bedded units. Additionally, bedded units required significantly less water usage than non-bedded units (Doane et al. 2013). Bennett et al. (2010) conducted a cost-benefit analysis for bonnet macaques (Macaca radiata) living in indoor-outdoor pens and found that providing bedding reduced water and chemical use and saved labor time. The Washington NPRC examined cleaning time for full sanitization of indoor pen-housed groups of pigtailed macaques at the room level for three conditions: bare floor, paper bedding, and straw bedding. Substrate type did not significantly affect the time required to clean a pen on a full sanitization day although the mean cleaning times did vary across substrate types (in preparation).

6.2. Drain issues

At many facilities, bedding has the potential to clog drains and impact sewer systems. Drain covers can be used when bedding is present to prevent the bedding from entering the drains. Drain plugs can be purchased or specifically fabricated for unique drains. These can be assembled quickly and at low cost (e.g., Hill & Ambrose, 2003).

6.3. Cleanliness and vermin

The presence of bedding may attract vermin. To what degree likely depends on the bedding management plan, i.e., how often the bedding is changed or spot cleaned. Spot cleaning daily ensures that urine and feces do not accumulate and allows for close monitoring for the presence of vermin. Vermin infestation may be more likely in outdoor housing. Areas where vermin are more common could indicate more frequent bedding changes or perhaps reducing the amount of bedding so that vermin have less substrate to nest in.

Regular sanitation of bedded areas is an important preventative measure against vermin. When bedding is removed for sanitation, animals can be shifted into adjacent housing or temporary tunnels to allow for complete sanitation of their housing. The sanitation should follow a rotation schedule. As noted above, bedding could be provided for up to two weeks followed by complete removal of the bedding and a period of washing/sanitation for several days depending on the size of the housing area. Lastly, bedding could be sanitized before it enters the animal housing area to ensure that pests do not infiltrate the facility.

In addition, it is important to not allow anthropocentric ideas of cleanliness to negatively impact nonhuman primate welfare. The odd invertebrate might be anathema to certain humans but represent enrichment to a marmoset or a rhesus monkey.

6.4. Other enrichment and bedding

Bedding may limit other types of enrichment. For example, providing pools in the immediate vicinity of bedding might not be feasible since water will saturate the bedding. However, destructible enrichment such as cardboard, paper, pinecones, etc. are good alternative enrichment options. In addition, bedding may be removed in areas contiguous to pools, or pool deployment can be targeted for days immediately preceding bedding changeout or sanitization.

6.5. Bedding disposal

Garbage providers may not accept bedding waste. In some areas, bedding must be disposed of as biohazard. Facilities should inquire about proper waste disposal with Environmental Health and Safety (EH&S) Office. In many cases, there are still options to facilitate the process, such as convenient placement of biohazard bags, bins, and pickup locations.

6.6. Animal care staff health and safety

People may experience or develop allergies to bedding substrates. Wearing properly fitting PPE such as N95 respirators may limit the exposure to airborne allergens. Ensuring that ventilation systems are working properly may also mitigate allergens. Additionally, and as noted, some substrates are non-allergenic.

Risk of ergonomic injuries to care staff who remove bedding is also a potential concern. To reiterate, certain tools may make cleaning easier. Large shop vacuums may be helpful to remove large amounts of bedding or to remove bedding from large areas. Doane and colleagues (2013) noted that in a modification from their study using bedding for outdoor-housed social groups, personnel started using a large vacuum and a single technician could remove the bedding from an outdoor bedded unit in about 1 hour whereas it previously took 2 or 3 people about 90 minutes without the vacuum. If vacuums are not feasible, utilizing larger or smaller scoops or rakes depending on the size of the area could facilitate cleaning. In addition, using a shorter and/or wider trash receptacle would reduce the amount of lifting required for technicians and prevent muscle strains or injuries. Animal care staff should work with their EH&S Office to implement practices that limit exposure to allergens and reduce risk of ergonomic strain or injury. These are, in any event, general safety concerns that are not limited to bedding maintenance. Exposure to allergens, frequent bending and lifting, etc. are inherent in the daily routines of many primate professionals.

6.7. Potential animal health concerns

Ingestion and impaction.

Ingestion of substrates that affect appetite or lead to a blockage in the digestive tract could be of concern. Related, some animals are food regulated, and investigators may request that their animals receive no bedding or complex substrates to avoid ingestion of these materials. Although bezoars, tight masses of undigestible material, have been associated with various substrates, they are rare and usually affect individuals with a unique proclivity to engage in maladaptive ingestion behavior. While this potential issue is documented, it is a minority occurrence with only a few individuals affected out of many, over a long period of time. Indeed, one report of a vervet monkey with a phytobezoar composed of straw (Seier, Dhansay, & Davids, 2005) noted it was the only incident out of 120 monkeys maintained on this bedding over 5 years. A 1978 paper (Golarz de Bourne, 1978) on establishing a rhesus breeding colony for research mentioned two fatal occurrences of large bezoars formed from the tough indigestible blades of weeping love grass (Eragrostis curvula), which was chosen because attempts to grow rye grass were unsuccessful. Trichobezoars comprised of hair have also been reported (e.g., Mejido et al., 2009; Mook, 2002) although, to our knowledge, no one is shaving monkeys to liberate them from this danger. Another rhesus macaque breeding facility reports trichobezoars and phytobezoars and retrospectively found trichobezoars to be three times as common as phytobezoars, with respective incident rates of 1.48 and 0.43 cases per 1000 animals per year (Haertel & Garzel, 2016). The provision of bedding or natural ground cover, at any rate, would be expected to associate negatively with self-grooming and potentially hair eating. Taken together, the small risk of ingestion complication is far outweighed by the numerous benefits that bedding provides.

Respiratory concerns.

Bedding materials vary not only in their ability to absorb moisture and ammonia, but also their ability to harbor microbes or form airborne particulates. In a dairy barn environment, for example, personal exposure to bioaerosols such as endotoxin, beta-glucans, bacteria and fungi differed depending on bedding material (Samadi et al., 2012). As horses housed in captivity seem prone to respiratory conditions, there is a multitude of air quality by bedding type studies which report different findings (Fleming et al., 2008a,b; Siegers et al., 2018; Vandenput et al., 1997). An equine examination of lower airway inflammation found elevated neutrophil percentage with wood shavings as opposed to peat in an ABA design, but there was no difference in respiratory rate or tracheal mucus score and all horses remained clinically asymptomatic for the duration of the study period (Mönki et al., 2021). Such work with primates is sparse. A European biomedical primate center study of indoor air quality for rhesus and cynomolgus macaque breeding groups on wood fiber bedding recorded sneezing by the macaques; frequency of sneezes were not correlated with any of the air quality parameters (Maaskant et al., 2022). The Washington NPRC conducted a collaborative study with husbandry, veterinary and behavioral management staff on the use of bedding versus bare floors in indoor group-housed pigtail (Macaca nemestrina) macaques. Three conditions were evaluated: bare (no bedding), recycled paper bedding, and straw bedding. There was a significant difference in respiratory cases during the straw condition. However, most respiratory cases were resolved quickly (in preparation).

Diarrhea.

In the Washington NPRC pigtail macaque study, there was no statistical difference between substrate conditions and diarrhea cases opened (in preparation). Also relevant, albeit not dealing with bedding specifically, is a retrospective Oregon NPRC study that found incidences of diarrhea in rhesus macaques far lower (less than half) in corral enclosures including earth and vegetation than in smaller shelter-breeding and shelter temporary group enclosures with sealed cement flooring (Prongay et al., 2013).

6.8. Environmental sustainability

Some substrates may be more sustainable than others. Wood shavings are likely the most common type of bedding substrate used for NHPs. Facilities can choose to purchase wood shavings that are a byproduct of other wood-based products to reduce waste and save trees. Some substrates better absorb feces and urine, which may reduce the amount of water needed to clean an enclosure when the bedding is removed for sanitation.

Water conservation is an important environmental concern as drought conditions are increasingly common due to climate change. Studies have shown that water is saved by using bedding. Doane et al. (2013) compared water usage for units bedded with pine shavings and units bedded with pine-aspen shavings. They found that units with bedding used 80% less water than non-bedded units during their 4-week study; there was no difference in water usage between pine and pine-aspen shavings. Bedding has also been shown to reduce chemical use associated with frequent sanitation.

Using bedding on a rotation schedule may help reduce the amount of bedding used over time. For example, bedding can be left in an enclosure for up to two weeks in indoor areas (and much longer for biofloors). During this period, no wash downs occur and daily spot cleaning ensures that the bedding does not become overly saturated with feces and urine. To date, there have been no studies that have examined varied bedding rotation schedules to see if changing bedding more frequently saves cleaning time or resources.

6.9. General issues with promoting buy-in

Technicians often believe that using bedding, even on a rotating schedule, is more time consuming than daily washing. As discussed, studies have shown no difference or a time-savings benefit with the use of bedding (Doane et al. 2013; Bennett et al. 2010). Enclosures that are not structurally designed to use bedding might be cumbersome to remove bedding from, which temporarily disrupts workflow and can frustrate technicians. However, the labor investment should be evaluated over a period to assess whether there is time-savings in the long run. Perhaps people are resistant to changing longstanding practices despite the published evidence that conflicts with their perceptions.

Bedding is more common in zoo settings. Zoos may be more likely to use bedding since, in addition to welfare concerns, there is a greater emphasis on aesthetics. Visitors can view exhibits and bedding provides a more naturalistic design than hardscape flooring. There likely remains a lack of knowledge and experience with NHPs and bedding in research settings. Crucially, there may be misconceptions on the logistics of providing bedding or other complex substrates, or ignorance concerning their value to a behavioral management program. Education is critical. At the end of the day, increased animal welfare is more valuable than imagined savings of time and materials. Whatever the many excellent reasons people choose to work with nonhuman primates as a career, convenience (whether actual or perceived) is rarely one of them.

7. Bedding and substrates in basic research

In some cases, investigators can force the issue by proposing compelling research that explicitly requires the utilization of complex substrates—either introduced or preexisting—to address questions in primatology. In some cases, the substrate serves as a primary stimulus item where reaction to the substance(s) is measured, and elsewhere it provides a medium containing another goal object or objects. Bedding and other substrate manipulations provide an obvious, albeit for some topics little or unused, tool to investigate larger theoretical questions. These are clear opportunities for meshing environmental enhancement with basic research. Some examples, by no means exhaustive, are as follows:

7.1. Curiosity, reactions to novelty, and exploratory behavior

There is a longstanding conception that primates are markedly curious creatures (Darwin, 1871/1981; Romanes, 1897) and experimental data has clearly borne out this supposition (Butler, 1953; Glickman & Sroges, 1966; Joubert & Vauclair, 1986; Menzel & Menzel, 1979; Visalberghi et al., 2003). The importance of this fact for the evolution of the order (including human evolution) cannot be overstated. New bedding placed on the ground or raised supports, or like materials placed in boxes or trays attached to an enclosure, represent “novel objects” that can be investigated within or across taxa using measurements such as body and/or visual orientation and degrees or duration of physical manipulation. Recording response to various types of novelty is one basic methodology to investigate curiosity in a quantitative manner (Berlyne, 1950). Degrees of interaction with introduced bedding might serve as one useful measure associated with the related concept of behavioral inhibition (cf., Capitanio, 2018).

The introduction of bedding, ground cover (e.g., grass or weeds on a previously dirt substrate), or related substances might be expected to elicit exploratory behavior. Most prominently this includes foraging-related search; when measured, this increases with bedding introductions (e.g., rhesus and crab-eating macaques, Doane et al., 2013; Janavaris et al., 2022). Bedding may also introduce new or increased opportunities for exploratory play and/or manipulative behavior, including the carrying, dropping, and throwing of bedding, bodily tumbling (baboons; Sayers, personal observation), and tunneling. The rapidity and frequency of such behavior likely relate to many factors, some theoretically interesting, including individual temperament, phylogeny, and/or socioecology (e.g., modal degrees of arboreality and terrestriality). In addition, behavior after prolonged exposure to complex substrates—that is, after the novelty wears off—may have related correlates across individuals or species and has obvious repercussions for behavioral management.

7.2. Memory, attention, and related processes

In the classic delayed response procedure (direct method, Fletcher, 1965; Hunter, 1913) one of n possible locations is baited and all locations are concealed. Following a prescribed delay, the subject chooses a location and receives a reward if it is baited. While generally considered a test of working memory, there are additional factors that might influence performance (Fletcher, 1965) including, but not limited to, attentional factors, the hedonic value of reward, and prior experience in cognitive testing. Whatever the case, delayed response is a basic tool for investigating psychological processes. Trays and computer monitors are the usual platforms for testing.

Preexisting or introduced ground cover, litter, or bedding provides options for naturalistic variations on delayed response testing. In a wonderful example (Menzel, 1973), a young test chimpanzee (Pan troglodytes) was carried through a 1-acre field while an experimenter hid up to 18 pieces of food under or behind grass, leaves, and other structures. After returning the subject to a holding enclosure and a delay of ≥ 2 minutes, the experimenters released this test animal and other control chimpanzees to the field. Among the findings: the apes recovered most food items, they were by great preponderance found by the test ape, hidden fruits were on average recovered before vegetables, and the apes used efficient travel paths between test locations. Other experiments have utilized foods hidden under leaves and pine straw while a chimpanzee observed through fencing; later, this ape would “recruit” a naïve human and direct them to the locations via a combination of lexigram (arbitrary visuographic forms) use, emotional expression including vocalizations, and/or bodily orientation and gesture. Under this paradigm, apes have shown to be able to recall types of objects—as determined by lexigrams touched—and their locations after extended delays (Menzel, 1999). In addition, they will rank order the recovery of multiple hidden caches in an efficient manner showing sensitivity to such variables as food quantity and subtype (e.g., almonds with or without shells, Sayers & Menzel, 2012; food variance or risk, Sayers & Menzel, 2017). More generally, the results indicate the use of multi-step but flexible action plans (Putney, 1985, 2007) and sophisticated communication (Menzel & Menzel, 2012 and see below).

A great advantage of utilizing bedding or surface cover in naturalistic delayed response variants is the ability to test on a larger spatial scale than standard approaches, and with what is unquestionably a very intuitive—from a nonhuman primate perspective—manner of testing. This has very practical repercussions for estimating the upper limits of primate ability. From our experience, nonhuman primates readily and voluntarily participate in such tests.

7.3. Emotional expression, communication, and information pickup

The paradigm of giving one or more individual(s) information on hidden objects and then releasing this animal(s) and others without knowledge into the test area provides a simple yet effective paradigm to examine bodily expression, communicatory behavior, and social affordances. For example, uninformed chimpanzees can predict the general value of foods and quantity thereof via the behavior of their informed “leader(s)” (Menzel, 1971; Menzel, 1974). Over a series of such experiments, one leader increasingly altered behavior when one particularly aggressive, non-sharing ape was present. This included slower approach to baited locations, sitting on hiding locations as opposed to uncovering them, and eventually movement in the wrong direction until the kleptoparasite’s back was turned. A hidden fear object (rubber snake) elicited cautious approach, piloerection, hooting, ground slapping, and/or stick poking or throwing while approaching or at the location, even among uninformed apes. The modern interest in “theory of mind” began right here (de Waal, 2011; Menzel & Johnson, 1976).

7.4. Structure-guided foraging and hypothesis formation

As noted previously, nonhuman primates utilize physical structures in the environment to guide their movements and manipulative behaviors. In a striking demonstration (Menzel, 1996b), an investigator hid some food caches in a crab-eating macaque enclosure according to one of several rules, along with several visible food piles that provided evidence on what rule was active for a given trial. Upon release into the test area, the monkeys detected natural rules—such as following the edge of an environmental border (e.g., the edge of a sand field) or with all foods placed near structures of a similar category (e.g., vertical poles)—almost immediately. Conversely, it took ~ five trials to learn an arbitrary rule that involved an invisible straight line. As noted, the mere introduction of bedding or other cover may precipitate search routines. The premeditated baiting of certain locations versus others, in this case, can additionally provide information on the proximate factors that underpin the foraging strategies of primates, as well as the ways in which these animals develop and test hypotheses about their environments (see also Kummer, 2002; Menzel, 1996a).

7.5. Touch, laterality, and cross-modal perception

Various substrates including bedding provide excellent tools to investigate haptic sensation and appendage use. In one study (Parr et al., 1997) it was found that tufted capuchins (Cebus apella) more often evinced left-handedness when collecting food from a medium of pine shavings than in bipedal and especially quadrupedal reaching tasks with visible foods. The reduction in visual cues with embedded foods increased demands on the sense of touch; the results suggest laterality with relation to haptic function. An additional medium tested, water—considered to reduce visual affordances even further as this medium was more obscured in the apparatus utilized—further accentuated handedness. Similarly, rhesus macaques showed stronger left-handed biases when searching for peanuts hidden in a sand and stone mixture than when collecting within-sight rewards in sitting, tripedal, or hanging stances (Fagot et al., 1991).

Cross-modal perception involves “the interrelatedness of the senses” (Davenport & Pralinsky, 1977: 252). One variety of visual-tactile cross-modal test involves a subject seeing a novel test object and subsequently rewarded if it pulls on a visually concealed example of that object. These and vice versa (tactile to visual) abilities have been found in monkeys and apes (e.g., Davenport & Rogers, 1970; DiMattia et al., 1990). Various non-primate and primate taxa evince some combination of cross-modal abilities, which represent a potential substrate for language origins (Davenport, 1977; Fitch, 2010; Hewes et al., 1973), presumably expanded in the hominid lineage. Bedding again provides an intriguing tool for future haptic comparisons. While to some degree functionally similar, the various bedding types discussed above have strongly varying and idiosyncratic textures.

7.6. Detailed quantification of manual and other physical manipulative movements

Studies of primate hand movements, and more broadly bodily orientation, have traditionally utilized apparatuses where food is placed upon surfaces within or outside immediate visual access (Welles, 1976) and/or with techniques tailored towards the specific context of locomotion (reviewed in Vereecke & Wunderlich, 2016). The controlled manipulation of bedding/substrate—known to increase foraging search-related activities—and its embedded contents, offers complementary methods to elucidate such topics. For example, manual and other movements may vary according to substrate (e.g., straw versus sand) and reward targets they might contain (e.g., grapes versus sunflower kernels). The thoughtful manipulation of such variables, coupled with advances in video quantification (Marshall et al., 2022), will likely permit more detailed descriptions and explanatory hypotheses than have heretofore been possible.

8. Bedding and other complex substrates as tools to enhance the primate research model

As described, the provision of complex substrates provides myriad behavioral benefits to primates. These aspects of phenotype in themselves are sufficient evidence to indicate it as a useful tool to enhance the general quality of these animals as research models (see also Bayne & Würbel, 2014). There is no dearth of information on proximate and/or reductionist correlates that would also support this conclusion. For example, increasingly enriched environments have been associated with brain enlargement, hippocampal and cortical circuitry changes at the gross morphological, molecular, and electrophysiological levels (multiple species, Hirase & Shinohara, 2014; Kozorovitskiy et al., 2005), increased ontogenetic myelination (rodents, Goldstein et al., 2021), upregulated neural plasticity (multiple species, Baroncelli et al., 2010; Salvanes et al., 2013), and elevated dendritic complexity and synaptic protein presence in prefrontal cortex (marmosets, Kozorovitskiy et al., 2005). Bedding, of course, is one type of environmental enhancement and its specific neurophysiological and developmental correlates have yet to be evaluated in primates (unlike rodents, where it has the predicted effect, Walker et al., 2017). Nonetheless, behavioral findings and the more general literature on environmental enrichment are sufficient at this point to recommend bedding or analogous opportunities to nonhuman primates for purely scientific reasons. Indeed, researchers taking the initiative to insist on such measures is likely the fastest way to encourage its implementation. Research areas such as cognition and neuroscience would evince obvious benefits, but when using a “whole organism” perspective (the best perspective) its value extends to nearly any topic of investigation. This includes studies of chronic or infectious disease prevention or treatment, which is a major focus in many laboratories. Any model organism should be well-described, and abundant evidence has been presented to show that complex substrates are associated with normative (as opposed to aberrant) primate behavioral patterns. In other words, primates with naturalistic surfaces to manipulate and/or traverse are easier to define as the organisms they evolved to be. Complex substrates contribute to increased welfare, and increased welfare enhances science.

9. Summary and recommendations

The summary is where proselytizing is to be expected, and we will not break from this grand tradition. The utilization of complex substrates, including bedding, is an excellent method to address myriad biological needs of nonhuman primates concurrently. This is borne out in nearly every study that has looked at the behavioral effects of bedding or natural ground cover, especially when coupled with hidden foods, when compared to barren hardscape surfaces. In some cases, such as bedding for great apes, it is an especially obvious component of their natural history. Other benefits are less intuitive but nonetheless real, including favorable effects on social behavior and opportunities for behavioral plasticity and agency. These and other advantages—increased foraging, general activity levels, focused attention, and so on—that recommend complex substrates for behavioral management also render them as appealing tools or stimuli for basic research. In experimental settings, they are poised to improve the general quality of nonhuman primates as research models.

There are pathways to refinement in almost any imaginable circumstance. In situations where natural cover or bedding are not feasible, such as enclosures with wide mesh flooring, these materials can easily be provided on external trays or receptacles. Any housing type could benefit from the addition of uniquely textured surfaces for perching, climbing, or other forms of locomotion, such as manzanita or other wood branches. Certain types of natural vegetation (browse) provide sensory novelty but are expected to be consumed. When possible, naturalistic surfaces are recommended over artificial surfaces in outdoor areas, and, in addition to standard ephemeral bedding, long-term biofloors—more or less permanent bedding—have opened a new world of possibilities in relation to indoor areas.

For those embarking on a bedding program, it is essential to secure as much possible buy-in from relevant staff as is possible. Discuss with them the reasons for deployment in relation to primate biology, and the published literature that demonstrates that bedding does not incur significant increases in human labor or other costs and indeed may decrease it in many circumstances. Water savings may greatly interest facilities staff. Those in husbandry should be provided all the resources needed for the program to succeed. This includes proper tools, PPE, and their convenient placement for utilization in bedding deployment, cleaning, and disposal. Researchers, as applicable, can explain why complex substrates are required for their modes of investigation or highly preferable given their needs for healthy, well-adjusted subjects.

As reviewed, there are many bedding options, all with their own advantages and disadvantages. Wood shavings, straw and hay, sawdust, and wood chips are easily available and affordable in most areas. It must be remembered that while one preferred bedding option might be a staple, novelty is often enriching, so the use of combinations (permitting choice) and occasional switch-ups should be considered. Even providing additional ground litter varieties on naturalistic surfaces can be expected to result in favorable outcomes.

Nonhuman primates—like human ones—are curious and manipulative creatures, and much more than that, thanks to some 70 million years of concerted evolution. We hope that this review will encourage the further utilization of complex substrates to engage, and learn about, these deep-rooted propensities.

Research highlights.

• The evolved characteristics of primates relate strongly to “manipulation” capabilities, which broadly include all behaviors that may influence the disposition of the biotic and abiotic environment

• This review argues that providing nonhuman primates in laboratory or zoo environments with complex substrates—such as natural ground cover or introduced contact bedding materials—is an excellent method to address these biological manipulation propensities and encourage species-typical behavior, as well as experimentally study myriad components of primate natural history

• There are benefits and challenges to individual types of complex substrates, but suitable options exist for essentially any primate facility or instance