A field demonstration of the costs and benefits of group living to edible and defended prey

Abstract

Both theoretical and laboratory research suggests that many prey animals should live in a solitary, dispersed distribution unless they lack repellent defences such as toxins, venoms and stings. Chemically defended prey may, by contrast, benefit substantially from aggregation because spatial localization may cause rapid predator satiation on prey toxins, protecting many individuals from attack. If repellent defences promote aggregation of prey, they also provide opportunities for new social interactions; hence the consequences of defence may be far reaching for the behavioural biology of the animal species. There is an absence of field data to support predictions about the relative costs and benefits of aggregation. We show here for the first time using wild predators that edible, undefended artificial prey do indeed suffer heightened death rates if they are aggregated; whereas chemically defended prey may benefit substantially by grouping. We argue that since many chemical defences are costly to prey, aggregation may be favoured because it makes expensive defences much more effective, and perhaps allows grouped individuals to invest less in chemical defences.

1. Introduction

The ability of many animals to move around their habitats has led repeatedly to the evolution of defensive grouping behaviours that protect individuals from predatory threats [1]. Grouping can bring several different survival advantages. Predators may, for example, satiate quickly when feeding on a localized group of large prey, and hence leave many individuals untouched [2]. This causes a dilution of per capita attack risk, so that individuals may have higher average survival if they aggregate together than if they disperse throughout the habitat. Grouping can lead in addition to more effective and efficient anti-predator vigilance [3], enabling individuals to undergo other activities such as foraging more safely than if they were dispersed. Defensive aggregation may also enable group members to use rapid, chaotic movements to confuse oncoming predators, reducing their capacity to target individual prey [4]. The role of defensive grouping in animal life cannot be underestimated. Only when animals live together do we see the evolution of complex social behaviours including heightened levels of communication, cooperation and cheating.

Though defensive grouping is common, it is not the ideal response for all animal prey species. In particular, animals that are much smaller than their predators may often have higher survival if they lived a solitary life than if they aggregated [5]. The reason for this is that on discovering a set of relatively small prey, predators could often consume all or a large proportion of a group before reaching satiation. Here there is no dilution of per capita risk. Similarly, when the prey is an egg or larval form it may be vulnerable to parasitoid enemies and small but numerous invertebrate predators such as ants [6–9]. Large numbers of aggregated, vulnerable, undefended prey can then be killed if they are easily detected. Hence we have a general expectation that vulnerable prey species will live in a dispersed, inconspicuous state; lacking social interaction and often deploying camouflage.

A common way, however, that prey can avoid vulnerability is through the use of mechanisms to repel predators, i.e. ‘secondary defences’ that act during an attack. Secondary defences can take many forms including physical defences such as spines and dense hairs, and chemical defences such as noxious secretions and internally stored poisons [10]. The survival value to individuals of chemical defence may be enhanced by group living, because predators may rapidly reach their tolerance for the prey toxin and cease attacks early [10]. Group living is therefore common in chemically defended insect prey, which are also often conspicuously (aposematically) coloured [11]. Furthermore, phylogenetically controlled analyses show a strong association between conspicuousness and aggregation in lepidopterans [12].

Laboratory-based empirical experiments can show the benefits of grouping in defended insect-sized prey. In laboratory settings, various vertebrate animals have been used as predators (for an extensive set of references, see review in [11]) as well as invertebrates (such as ants, spiders and bugs [6]). These experiments show increased protection for chemically defended prey from aggregation. Unexpectedly however field investigations show mixed survival benefits from aggregation by chemically defended prey. Some studies show positive associations between individual survival and group size [7,8,13], but others show no effect on survival over a range of group sizes [9,14]. Absence of benefits in some studies may be explained by density-dependent reductions in individual investment in toxicity with increasing group size [15]. Hence it is important to establish whether in the absence of these density-related effects, grouping per se is beneficial to chemically defended prey in the field [16]. It is similarly important to establish in the field whether grouping is sufficiently costly for prey that lack secondary defence to cause them to be solitary and asocial.

2. Material and methods

The experiment took place in two neighbouring fields near woodland (Sites 1 and 2 close to Lindale, Cumbria, September 2014). Artificial prey were pastry balls (ca 1 cm diameter; 110 g flour to 50 g lard, 10 ml water and of red or green Dr Oetker food dye). We presented the prey on flat ground along a transect at each site (figure 1), in a pattern alternating a spatial set of grouped prey (10 prey presented within a 10 cm diameter) and a spatial set of dispersed prey (in a 2 × 5 matrix), leaving a metre between grouped and dispersed sets. Colours alternated between groups and within sets of dispersed prey along the transect. On each day of presentation, we set out 14 sets of aggregated prey and 14 sets of dispersed prey at each site and the procedure was repeated for 3 consecutive days (in all 1680 prey were used). Each spatial set was identified with a unique code relating to position on the transect. Prey were presented around mid day, checked at 7.00 the next morning and checked and collected in at mid day 24 h after initial presentation. We recorded whether prey survived (untouched) or had been ‘killed’ (completely absent or bitten). Based on markings on rejected prey, small mammals and birds were likely predators.

Figure 1.

Distribution of prey types in the field. Prey in dispersed and grouped sets were the same size, but are not drawn to scale here. (Online version in colour.)

At Site 1, red prey were unpalatable and green prey were edible. At Site 2, we reversed this so that green was distasteful. Edible prey were dipped in water and dried. Unpalatable prey were dipped a solution of Bitrex, an aqueous solution of denatonium-benzoate, 2.5%w/v, (a bitter-tasting but colourless and odourless, non-toxic substance) and dried. Variation in colour of the Bitrex-treated prey did not have much influence on the results (below), but it did enable the experimenters to distinguish the edible from unpalatable prey with certainty.

To analyse the results, we used a generalized mixed model in R (lme4 [17]) for a binomial variable (prey survival), using chemical defence (presence or absence), colour of chemically defended prey (which correlates with site) and dispersal status (grouped or dispersed) as fixed factors. The identification number of each spatial set was used as a random factor to control for spatial non-independence. Raw data are available in the electronic supplementary material.

3. Results

Using AIC evaluation, the best available model is the full factorial model (chemical defence × colour × dispersal status). As table 1 shows, there is a very highly significant effect on prey survival from the interaction between edibility and dispersal status (p < 0.001, figure 2). The nature of the interaction can be seen by looking first at edible prey; here grouped individuals survive less well than dispersed individuals (p = 0.014), thus aggregation heightens the probability of ‘death’ for undefended prey. By contrast, aggregation greatly increases survival in the chemically defended prey (p ≪ 0.001). Note that there is neither indication of any interaction between chemical defence status and colour of the unpalatable prey (p = 0.21), nor is there a significant main effect of colour of chemically defended prey; hence the red (potentially) warning coloration of the bitter-tasting prey has no effect on overall predation, nor on the assessment of chemical defence.

Table 1.

Results of GLMM analysis of the data. Colour refers to whether the unpalatable prey was red or green; dispersal refers to whether prey are dispersed or grouped; edibility refers to whether prey are unpalatable or edible. Bold text and * indicates significance at p < 0.05, and *** indicates significance at p < 0.0001.

	estimate	s.e.	Z-value	p-value
(intercept)	−1.9942	0.3082	−6.471	9.72 × 10−11***
edibility	0.8214	0.4123	1.992	0.0463*
dispersal	−0.3866	0.4472	−0.865	0.3873
colour	−0.3701	0.4444	−0.833	0.4050
edibility × dispersal	2.4054	0.5955	4.039	5.37 × 10−5***
edibility × colour	0.7435	0.5914	1.257	0.2087
dispersal × colour	−1.3728	0.7557	−1.817	0.0693
edibility × dispersal × colour	1.4312	0.9374	1.527	0.1268

Figure 2.

The effect of aggregation and edibility status on prey survival. Means with error bars indicate 2 SEM. Defended prey were made unpalatable using a Bitrex solution, see Material and methods.

There is a marginally significant main effect of edibility status on prey survival (p = 0.05, because chemically defended prey were taken less than edible prey). There is also a marginally non-significant interaction between colour of chemically defended prey and aggregation status (p = 0.07). Looking at the datasets separately for each site, the pattern is similar to figure 2 but grouping does not harm edible prey as much at Site 2 where the red prey were edible (the difference between mean proportionate survival of dispersed and aggregated solitary prey is only 0.03, whereas it is 0.08 at Site 1).

4. Discussion

This is (to our knowledge) the first simultaneous field test of the benefits of grouping in chemically defended prey, and conversely the excessive costs of aggregation for edible prey. We found that unpalatability increased survival in dispersed prey from 15 to 30%. However, addition of a grouped distribution had a profound benefit, increasing survival of defended prey to nearly 70%. It is likely that having attacked two or three bitter-tasting baits in an aggregation, a predator rejected the rest—was effectively satiated—and moved on. When prey were solitary and defended however the predators were more likely to sample individual prey, only learning over successive encounters that a colour was associated with a bitter taste in that locality. When predators encountered groups of edible prey they were more likely however to eat them all, hence the dispersed state with a lower likelihood of detection favours edible prey.

Chemical defences are often costly [18,19] hence aggregation may often be an effective way of increasing the net benefit of investment in defences; maximizing protection from survival for the investment made. This has two implications. First, where there are high costs of generating or acquiring effective toxins, the additional protection from aggregation could be necessary to make toxicity profitable. Second, costs of defensive toxicity may lead to density-dependent reductions in individual investment. One explanation for the lack of expected survival benefit from grouping in some studies is that individuals may negate additional protection from grouping by reducing investment in their own costly toxins [15]. Hence they maintain some level of protection from group membership and (lower) toxicity but have more resources available for growth and reproduction. Consistent with this we note that larvae of the large white butterfly (Pieris brassicae) do show a negative relationship of toxin production with group size [15] and also a lack of an association between group size and individual survival in the wild [9]. By contrast, the eggs of the butterfly Battus philenor show no density-dependent toxicity relationships (indicating that mothers do not reduce toxin investment in eggs that form larger groups), and they do show strong benefits of aggregation in the field [7]. The B. philenor example may be more like that in our experiment with toxin investment regardless of group size, whereas the P. brassicae example may have an additional group-dependent reduction in defence investment.

We have shown then that wild predators can behave in a way that favours aggregation in chemically defended small prey and crucially favours dispersal in edible non-defended prey. Though we focused on individual benefits, it is apparent that many insect aggregations involve family members, for example where larvae from a clutch live together before dispersal. Hence we suggest that, having demonstrated the clear benefits of group living for defended prey, there is now an excellent opportunity to integrate kin selection theory into models of toxin evolution [18,19].

Data accessibility

Electronic supplementary material contains the raw data.

Author contributions

E.A.M.C. and H.E.R. designed and executed the experiment and wrote the manuscript; M.P.S. designed the experiment and wrote the manuscript. All authors approved the final version of the manuscript.

Competing interests

The authors have no competing interest.

Funding

We thank the School of Life Sciences, University of Liverpool for financial support.