The volunteer’s dilemma, in which a single individual is required to produce a public good, predicts that individuals in larger groups will cooperate less frequently. Mechanistically, this could result from trade-offs between costs associated with volunteering and costs incurred if the public good is not produced (nobody volunteers). During predator inspection, one major contributor to the cost of volunteering is likely increased probability of predation; however, a predator also poses a risk to all individuals if nobody inspects. We tested the prediction that guppies in larger groups will inspect a predator less than those in smaller groups. We also predicted that individuals in larger groups would perceive less threat from the predator stimulus because of the protective benefits of larger groups (e.g. dilution). Contrary to prediction, we found that individuals in large groups inspected more frequently than those in smaller groups, but (as predicted) spent less time in refuges. There was evidence that individuals in intermediate-sized groups made fewest inspections and spent most time in refuges, suggesting that any link between group size, risk and cooperation is not driven by simple dilution. Extensions of theoretical models that capture these dynamics will likely be broadly applicable to risky cooperative behaviour.

AbstractRecognition of group members is an important adaptation in social organisms because it allows help to be directed toward kin or individuals that are likely to reciprocate, and harm to be directed toward members of competing groups. Evidence in a wide range of animals shows that responses to outgroups vary with context, suggesting that cues to group membership also depend on the social or environmental context. In termites, intergroup encounters are frequent and their outcomes highly variable, ranging from destruction of a colony to colony fusion. As well as genetic factors, nestmate recognition in social insects commonly relies on cues that are mediated by environmental factors such as food source. However, single‐piece nesting termite colonies share nesting material and food source with rival colonies (their wood substrate serves as both). In principle, the shared environment of single‐piece nesting termite colonies could constrain their ability to identify non‐nestmates, contributing to some of the variation seen in encounters, but this has not been investigated. In this study, we raised incipient colonies of a single‐piece nesting termite, Zootermopsis angusticollis, on two different wood types and conducted behavioral assays to test whether nestmate discrimination can be constrained by common environmental conditions. We found that non‐nestmates elicited higher rates of identity checking and defense behavior compared to nestmates, but there was no effect of wood type on the strength of behavioral responses to non‐nestmates. We also found that one key cooperative behavior (allogrooming) was performed equally toward both nestmates and non‐nestmates. These findings offer no support for the hypothesis that common wood type constrains the nestmate recognition system of single piece nesting termites. We suggest that where groups encounter each other frequently in a common environment, selection will favor discrimination based on genetic and/or higher resolution environmentally mediated cues.

Abstract


Advances in individual marking methods have facilitated detailed studies of animal populations and behavior as they allow tracking of individuals through time and space. Hemimetabolous insects, representing a wide range of commonly used model organisms, present a unique challenge to individual marking as they are not only generally small‐bodied, but also molt throughout development, meaning that traditional surface marks are not persistent.

Visible implant elastomer (VIE) offers a potential solution as small amounts of the inert polymer can be implanted under the skin or cuticle of an animal. VIE has proved useful for individually marking fish, crustaceans, and amphibians in both field and laboratory studies and has recently been successfully trialed in laboratory populations of worms and fly larvae. We trialed VIE in the single‐piece nesting termite Zootermopsis angusticollis, a small hemimetabolous insect.

We found that there was no effect of VIE on survival and that marks persisted following molting. However, we found some evidence that marked termites performed less allogrooming and trophallaxis than controls, although effect sizes were very small.

Our study suggests that VIE is an effective technique for marking small hemimetabolous insects like termites but we advocate that caution is applied, particularly when behavioral observation is important.

Groups of animals interact in a variety of different ways, from lethal conflict to cooperation and fusion. While intergroup conflict is relatively widely studied, the mechanisms and selective forces underpinning cooperation and group fusion are less well understood. The lower termites are a group of eusocial insects that displays a spectrum of intergroup relationships from violent battles to avoidance to colony fusion, making them a valuable model system with which to study intergroup interactions. In Chapter 2, I present a trial of a novel method of marking termites (Visible Implant Elastomer), finding that it causes slight changes to survival and behaviour associated with reproductive disinhibition. In the following chapters, I investigate two aspects of intergroup interactions in a single-piece nesting species of lower termite, Zootermopsis angusticollis: nestmate recognition and soldier caste ratio. In Chapter 3, I investigate the implications of colonies sharing similar nesting material on the ability of Z. angusticollis pseudergates to discriminate between members of their own and a different colony. I find that Z. angusticollis pseudergates discriminate between nestmates and non-nestmates but that this does not appear to be dependent on whether they encounter a non-nestmate raised on the same or a different wood type. Contrary to prediction, I also find that non-nestmates experience the same levels of cooperative allogrooming as nestmates. In Chapter 4, I use a theoretical model to examine the consequences of colony fusion on the sterile caste, the soldiers, at the 1 colony level. This model predicts that the reported increase in soldier number from some studies is supported under a narrow range of cost and benefit parameters, and that termites can benefit from fusion at the colony level under two scenarios: when fusion results in higher net benefit from soldiers, or when a colony can take advantage of the other’s soldiers. This suggests two pathways to the evolution of colony fusion, which might be applicable to lower termites of different ecological habits. Evidence from both Chapter 3 and Chapter 4 suggest that colony fusion in the lower termites could be driven by selection on pseudergates, which can gain direct fitness benefits from fusion. I suggest that this might be a general phenomenon and that the presence of conflict within a group can facilitate cooperation between groups across taxa.