Method of Feed Presentation Affects the Growth of Mongolian Gerbils (Meriones unguiculatus)

Abstract

The Animal Welfare Regulations, which define legal requirements for the care and use of gerbils in research and testing, and the Guide for the Care and Use of Laboratory Animals describe feeding practices for several species of rodents but not Mongolian gerbils (Meriones unguiculatus). To investigate whether the method of feed presentation affected reproduction and growth of gerbils, we compared the reproductive performance and rate of growth of gerbils fed on the cage floor, by hopper (J-feeder), or by wire-bar cage lid. Reproductive parameters of 10 breeding pairs for each method of feeding were followed for 63 d and did not differ between methods. To investigate the effect of feeding method on weight gain in juvenile gerbils, groups of 80 male and 80 female weanling gerbils per feeding method were fed for 5 consecutive weeks after weaning and weighed weekly. Gerbils fed on the cage floor and by means of J-feeders were significantly heavier than were those fed by using a wire-bar top. Our findings indicate that feeding gerbils by using J-feeders or on the cage floor are both acceptable practices.

The Mongolian gerbil (Meriones unguiculatus) is a domesticated rodent commonly used in research. In the wild, M. unguiculatus is found in desert regions of Mongolia and northeastern China.⁸ As a desert-adapted animal, gerbils produce scant amounts of urine, and their fecal pellets are typically hard and dry. Their cages generally require less frequent cleaning than those of other rodents commonly used in research.⁸ Gerbils are bred and raised in a single Charles River barrier room in North America, where they are housed in solid-bottomed caging, bedded with wood-product bedding, provided with ad libitum access to water, and fed by placing commercially produced rodent chow on the floor of the cage.

The Animal Welfare Regulations (AWR), which define the legal requirements of the Animal Welfare Act,¹ under which the federal government regulates the care of gerbils used in research, describe feeding practices for several species of rodents, including hamsters and guinea pigs, but do not provide specific standards for feeding gerbils. When an individual species is not specifically identified in the AWR, general guidance is provided that states: “The food shall be wholesome, palatable, and free from contamination and of sufficient quantity and nutritive value to maintain all animals in good health.”² The Guide for the Care and Use of Laboratory Animals (the Guide) provides similar guidance regarding the need to provide palatable, nutritionally adequate, and uncontaminated food.¹³ The Guide also does not require the use of a specific feeder for gerbils. When discussing feeders, the Guide recommends that feeders should be designed to allow ease of access to food, for group-housed animals that enough feeding points should be provided to minimize competition, and that feeders should minimize the opportunity for contamination of food with urine and feces.¹³

During a routine inspection of a Charles River commercial production facility, the practice of feeding gerbils on the cage floor was questioned and the suggestion was made that food should be provided for gerbils by using hoppers (J-feeder) or wire-bar cage lids. An initial observational study was performed in which gerbils were fed by using J-feeders. Animals in all cages provided with J-feeders consistently removed food from the hopper and distributed the food on the cage floor within 24 h. After reviewing results of this initial study, a larger study was performed evaluating breeding performance and growth of gerbils based on 3 methods of food presentation: (1) on the cage floor, (2) in a J-feeder, and (3) through a wire-bar cage lid. Here we present the results of the larger study.

Materials and Methods

Animals.

Mongolian gerbils (Meriones unguiculatus; Crl:MON(Tum), Charles River, Kingston, NY) were housed in a commercial barrier production facility in solid-bottom polycarbonate cages, on hardwood bedding, and covered with stainless steel wire tops. Breeder cages were 13.125 in. × 11.125 in. × 6 5/8 in. (33.34 cm × 28.23 cm × 16.83 cm), whereas stock cages were 25.625 in. × 13.875 in. × 6.75 in. (65.1 cm × 35.24 cm × 17.15 cm). Stainless steel wire-bar cage lids used in this study included a depression in the lid top to accommodate feed and a water bottle; the interbar distance was 0.295 in. (0.75 cm). J-feeders were constructed of stainless steel and the feed opening was 2.87 in. × 2.95 in. (7.3 cm × 7.5 cm). Animals experienced a 12:12-h light:dark cycle and received HEPA-filtered air at a constant temperature of 21 ± 1 °C, with relative humidity controlled at 30% to 70%. Animals had ad libitum access to a proprietary rodent production diet (LabDiet, Richmond, IN) and UV-treated, filtered, hyperchlorinated water by means of water bottles. Animals tested negative for lymphocytic choriomeningitis virus, Clostridium piliforme, Bordetella bronchiseptica, Mycoplasma pulmonis, Salmonella spp., Helicobacter hepaticus, Klebsiella pneumoniae, Pasteurella multocida, Pseudomonas aeruginosa, Streptococcus pneumoniae, endoparasites, and ectoparasites. Cages were changed and sanitized once weekly. The facility was fully AAALAC-accredited, and animal husbandry and study procedures were approved by the Charles River Institutional Animal and Use Committee.

Experimental groups.

Stock animals.

At 28 d of age, 3 groups of 160 gerbils (per group: 80 male, 80 female; total used: 480) were weaned, individually marked, weighed, and randomly assigned to 1 of 3 feeding regimens by which feed was provided: (1) on the cage floor, (2) in a J-feeder attached to the side of the cage, or (3) in the cage-top wire-bar food hopper. Animal cage densities followed standard barrier production practices, with each cage adjusted to a specific density based on average body weight of all animals in a cage when weighed weekly: animals weighing less than 60 g were housed 16 per cage, animals weighing 61 to 80 g were housed 12 per cage, and animals 81 to 100 g were housed 10 per cage. Animals then were weighed on the same day of each week for 4 additional weeks.

Breeding pairs.

We randomly assigned 10 continuously paired (monogamously mated) proven breeder pairs that had weaned 2 previous litters to each of the 3 described feed delivery methods for 63 d. The number of pups born in each litter was recorded. The number of pups weaned was recorded at 28 d of age, at which time weanlings were removed and the breeding pair remained together. Because pairs were assigned to experimental groups after weaning of the second litter, the female breeders on study probably were pregnant at the time of assignment.

Statistical analysis.

Descriptive statistics were used to summarize production parameters and body weight data for each feeding group. Single-factor ANOVA was used to assess differences in reproductive parameters and body weight between the different feeding groups; statistically significant differences in body weight between feeding groups were determined by using the Tukey–Kramer multiple comparison test. All statistical analyses were completed by using Excel (Microsoft, Redmond, WA), with significance being associated with a P value of less than 0.05.

Results

Differences in reproductive performance, as measured by number of litters born, average litter size, average number of animals weaned, and weanling survival did not differ (P = 0.59; Table 1) between feeding methods. Weanling survival of pups from all 3 feeding groups was 100%.

Table 1.

Reproductive parameters for gerbil feeding groups (n = 10 monogamous pairs per group)

		No. of pups born per litter		No. of pups weaned per litter
Feeding method	No. of litters born	Mean	SD	Mean	SD
Wire-bar lid	14	7.21	1.71	7.21	1.71
J-feeder	15	7.45	1.55	7.45	1.55
Cage floor	17	7.76	1.20	7.76	1.20

For all groups, 100% of the pups born were weaned.

After 1 wk (at 5 wk of age), both male and female gerbils fed on the cage floor weighed significantly more (P < 0.001) than gerbils fed through the wire-bar lid (Figures 1 and 2). After 3 wk (that is, at 7 wk of age), male gerbils fed from J-feeders also weighed significantly more (P < 0.0001) than gerbils fed through the wire-bar lid. Female gerbils fed on the floor or by J-feeder weighed significantly more (P < 0.0001) than gerbils fed by using wire-bar lids after 1 wk (at 5 wk of age). Once a statistically significant difference in body weights developed between feeding groups, the differences continued for the remainder of the study (that is, through 8 wk of age). Body weights of both male and female gerbils fed from the cage floor and by using the J-feeder were statistically indistinguishable from one another throughout the study period.

Figure 1.

Weekly body weights (mean ± 1SD) of male gerbils fed from the cage floor, J-feeders, and wire-bar lids from 4 to 8 wk of age. Significant (P < 0.001, ANOVA) differences were found between mean body weights for cage floor and wire-bar lid groups (a) and between mean body weights for J-feeder and wire-bar lid groups (b).

Figure 2.

Weekly body weights (mean ± 1SD) of female gerbils fed from the cage floor, J-feeders, and wire-bar lids from 4 to 8 wk of age. Significant (P < 0.0001, ANOVA) differences were found between mean body weights for cage floor and wire-bar lid groups (a) and between mean body weights for J-feeder and wire-bar lid groups (b).

Discussion

Specific requirements for the care and husbandry of gerbils used in research and testing are limited in the regulatory and guidance material governing the use of research animals in the United States. For example, the language of the AWR, enforced by the US Department of Agriculture, does not define precise housing conditions for gerbils. Other rodent species used in research (for example, hamsters, guinea pigs) each have a subsection of the AWR describing minimal standards of their care, but requirements regarding the care and husbandry of gerbils are included within a broad subsection of the regulations titled “Warmblooded Animals Other Than Dogs, Cats, Rabbits, Hamsters, Guinea Pigs, Nonhuman Primates, and Marine Mammals.”^1,2 The relevant regulatory language describing food delivery methods in this section is as follows:

Food, and food receptacles, if used, shall be sufficient in quantity and located so as to be accessible to all animals in the enclosure and shall be placed so as to minimize contamination. Food receptacles shall be kept clean and sanitary at all times. If self-feeders are used, adequate measures shall be taken to prevent molding, contamination, and deterioration or caking of food.²

Similar to the AWR, the Guide also lacks specific guidance or recommendations regarding the use of feed receptacles for gerbils. When discussing feeders, the Guide recommends that they should be designed to allow ease of access to feed, that group-housed animals are provided sufficient feeding points to minimize competition, and that feeders should minimize the opportunity for contamination of feed with urine and feces.¹³

The rationale for feeding most research animals from a feed receptacle elevated off the cage floor is straightforward. For many animals, feed provided on the floor of the cage, pen, or enclosure, may be subject to wastage, be rendered unpalatable by contamination with the animals’ wastes, or appear unacceptably soiled to caretakers. However, this philosophy of feed presentation to research animals need not be applied equally to all species used in a research setting, particularly for animals originating from arid environments. As a desert animal, the physiology of the gerbil prioritizes conservation of water and therefore little liquid escapes in waste.^5,14 Consequently, food placed on the floor of a gerbil cage is unlikely to become soiled with urine, in contrast to what is expected with other common research rodent species such as mice, rats, or guinea pigs. Contamination of feed with fecal material may be less of a concern for rodents than for other species. In some cases, it is theorized that the presence of fecal matter nearby signals to conspecifics that a food is safe for consumption.¹¹ In addition, rodents generally are acknowledged to be sporadically coprophagic,^9,15 and questions of the significance of contamination of feed with fecal matter become of less importance in light of their normal behavior.

Recognition of the natural behaviors of captive hamsters (Cricetus and Mesocricetus spp.) allows for an exception to the elevated feeder guideline, and feed usually is provided on the cage floor for these animals. Although a citation is not provided, the AWR states that hamsters can be fed on the cage floor because they will remove feed from any feeding apparatus and pile it on the cage floor consistent with their natural hoarding behavior.^6,12,19 Feeding hamsters from inappropriately spaced wire-bar feeders has a negative effect on growth and reproduction.¹² Although we did not present the data, our initial observational study indicated that gerbils, like hamsters, remove food from the J-feeders and then use the J-feeders as play objects and sleeping places. The interactions with J-feeders appeared to be less consistent than those we have noted in hamsters, with some groups of gerbils removing only a portion of available feed from the J-feeder.

Although the weight of gerbils fed by using J-feeders did not differ from those fed on the floor, gerbils fed from a wire-bar feeder weighed significantly less than the other 2 feeding groups. The relatively shorter, broader muzzle of the Mongolian gerbil when compared with mice or rats could be responsible for this phenomenon, and some authors recommend a wider spacing between bars than is used for mice and rats if gerbils are fed through bar feeders.¹⁷ Alternatively, gerbils may not be able to maneuver their body to allow efficient feed intake from standard wire-bar lid feeders. Gerbils fed by using wire-bar or J-feeders were not deprived of necessary nutrition as was evidenced by their successful breeding, delivery, and nursing of offspring. Neither did feeding by wire-bar lids result in loss of body weight in animals through 8 wk of age. Rather, gerbils fed from wire-bar lids failed to gain weight at the same rate as animals fed through J-feeders or on the floor. Regardless of feeding method, all stock or breeding gerbils on study appeared bright, alert, and active, with no signs of ill health.

Providing feed for gerbils on the cage floor also may serve as a means of environmental enrichment and allow them to display species-specific naturalistic behavior. In the wild, gerbils live in extensive burrow systems and forage widely for seeds and other plant matter.³ Foodstuffs are brought back to the burrow system and cached.^4,21 Feeding on the floor provides opportunities for expressing the naturalistic behaviors of foraging and food hoarding.^{7,10,16,17,20} In captivity, gerbils may exhibit stereotypic behaviors, including a digging behavior often performed in the corners of cages, and bar chewing. However, when compared with feeding from wire lids, feeding from the floor did not decrease the incidence of bar chewing.¹⁸

Feeding gerbils from the cage floor did not adversely affect the gerbils, as measured by observations of general health, reproductive parameters, and rate of weight gain. Feeding gerbils through a wire-bar feeder did not allow for the same rate of growth as did feeding from the floor or a J-feeder. It must be acknowledged that the average weights of gerbils in this study are heavier than those of wild gerbils, regardless of feeding method.²¹ This pattern is to be expected, given that the weight of wild gerbils varies based on season,²¹ possibly related to both temperature and availability of food, and captive gerbils are both kept at a constant temperature and continuously fed. Feeding through a wire-bar feeder designed for rats or mice may serve as a form of dietary restriction for gerbils. This study was not designed to determine the outcome, whether positive or negative, of dietary restriction in gerbils.

On the basis of the findings of this study, we conclude that the method of feed presentation affects the rate of growth of gerbils but does not affect reproduction. Feeding gerbils from the floor of their cage or from a J-feeder resulted in similar growth curves, and these feeding methods appear to be acceptable practices. Feeding of gerbils from a standard wire-bar lid reduced the rate of weight gain in comparison to the other feeding methods studied. The long-term effects of feeding gerbils from wire-bar lids should be examined further.