This book is about the evolution of contest behaviour in animals. It covers both predictive theories for contest evolution and empirical evidence. There are several potential strategies for organising an edited book that collects together a diverse range of study systems and a rich body of theory. One would have been to invite authors to each write a chapter about their favourite concept. For example, there are several alternative theoretical explanations (models) for contestant assessment during agonistic interactions that appear frequently in the contest literature (these are introduced in Chapter 1 and detailed in Chapter 2) and MB, along with several other contributors to this volume, has been especially interested in using a particular study-species to test the key features of these models in order to investigate assessment rules and the possible functions of repeated agonistic signals. In other words, we are all interested in how the loser makes the decision to give up, and could each have contributed a chapter along similar lines, covering the relevant theory as well as detailing our own experiments. It soon became apparent, however, that there was a potential cost associated with this layout: as a result of the tight links between theory and experimental work described above, many authors would have wanted to write about the same concepts, albeit applied to different animals, leading to much conceptual repetition between chapters.

Our alternative, and adopted, strategy for organising this book has been to divide it into two main sections, the first dealing with general theory and the second comprising a series of chapters arranged by taxon (in the somewhat uncomfortably traditional ‘invertebrates to humans’ sequence). The link between the theoretical and empirical sections is a chapter on analysis of contest behaviour data. This includes recent advances in our understanding of the appropriate experimental design and analytical approaches for testing hypotheses about contest behaviour, with the aim of providing practical advice to those engaged in empirical contest research. As we see it, this scheme has two main advantages.

Summary

The past decade has seen a marked convergence between evolutionary models of animal contests and the analysis of interstate war, or ‘militarised interstate disputes’ (MID). Since James Fearon's landmark paper in 1995 on war as a bargaining problem, the literature on ‘rationalist’ approaches to modelling war has burgeoned and become increasingly sophisticated. It has moved from a ‘Costly Lottery’ approach (in which the decision to cease bargaining and fight is a game-ending move with a costly, probabilistic outcome) to a ‘Costly Process’ approach, in which states continue to accumulate information on relative strength and motivation while fighting, and use this to inform their strategic decisions about whether to continue fighting or revert to bargaining. The Costly Process approach has much in common with the evolutionary analysis of animal conflict, and may stand to gain from incorporating some of its theoretical insights and approaches. The actors in evolutionary models are in a very similar strategic situation to those of rationalist models: they are unitary actors with imperfect information who have a range of behavioural options to facilitate mutual assessment and may have incentives to resolve conflicts short of lethal combat. The concept of rational utility maximisation is analogous to the assumption that, over evolutionary time, natural selection has honed behaviour such that it represents a game-theoretic equilibrium. Most importantly, the expectation that signallers will misrepresent their capabilities and intentions means that costly, inefficient actions will usually be required to stabilise the reliability of the signalling system. We discuss two key evolutionary models of conflict, comparing them with recent Costly Process models of war and suggesting how they could stimulate new theoretical and empirical research.

Summary

In this chapter we outline and discuss statistical approaches to the analysis of contest data, with an emphasis on testing key predictions and assumptions of the theoretical models described in Chapters 2 and 3. We use examples from an array of animal taxa, including cnidarians, arthropods and chordates, to illustrate these approaches and also the commonality of many key aspects of contest interactions despite the differing life histories and morphologies (including weaponry) of these organisms. We first deal with the analysis of contest outcomes, a useful approach for determining which traits contribute to an individual's resource holding potential (RHP). Here we outline alternative statistical approaches that treat the outcome as either an explanatory (independent) variable or as the response (dependent) variable. In both cases, we treat a single contest as one ‘experimental unit’ and consider ways in which multiple measures taken from the same experimental unit should be accounted for in the analysis. Thus, we introduce paired and repeated measures approaches for contest data and also the calculation of composite measures. We then discuss more complex mixed models, which are particularly useful for dealing with multi-party contests when multiple individuals from the same group occur in more than one observation. Having established what factors influence RHP, one might then ask questions about the roles of information-gathering and decision-making during contests. These questions are prompted by the theoretical models of dyadic contests discussed in Chapters 1 and 2, and we consider the advantages and limitations of using analysis of contest duration to distinguish between ‘mutual-’ and ‘self-assessment’ type contests. An additional tool that we can use to address this question is the analysis of escalation and de-escalation patterns, and we thus shift the focus to within-contest behavioural changes.

Repeated patterns in animal contest behaviour research

Among studies of contest behaviour that have been conducted within the framework of evolutionary theory, one can discern distinct phases of activity that have been associated with developments in an underpinning body of theory. As recounted in Geoff Parker's Foreword to this volume, the initial period of intense activity that occurred in the early to mid 1970s involved the laying down of a fundamental body of theory. During this time, contest behaviour provided the original context for the biological application of evolutionary game theory (as opposed to economic game theory, from which it derives). Game theory still acts as a cornerstone for behavioural ecology research and it is testament to its explanatory power that the Hawk–Dove game, wars of attrition and other examples of ‘Evolutionarily Stable Strategy, or ESS, thinking’ (Davies et al. 2012) still dominate undergraduate curricula in the subject. These early models stimulated empirical studies that provided evidence for ESSs in contests in diverse study systems including scorpionflies (Thornhill 1984), butterflies (Davies 1978) and red deer (Clutton-Brock et al. 1979). Studies such as these provided the early foundation for the cross-taxon approach to contests that we have continued in this book.

Animal contests in nature

Next time you stand on a seashore and look carefully with your ‘zoologist's eyes’, you may be surprised at the high diversity of animal phyla that are present, even within a single intertidal rock pool. If you are patient and can stay still for a few minutes, another surprise in store is the preponderance of aggressive behaviour demonstrated by the intertidal fauna. Depending on which part of the world your rocky shore is in, you might observe some of the following: male Azorean blennies fighting over the nests that they need in order to attract females; pre-copula pairs of shore crabs with inter-male aggression over the ownership of recently moulted females, as these females are only receptive to sperm during a brief post-moult period; common European hermit crabs rapping in an attempt to evict an opponent from its gastropod shell; and, if you really have a lot of time on your hands, you might notice slow-moving sea anemones striking one another with special tentacles called acrorhagi, during disputes over space. Of course, aggressive behaviour is not restricted to intertidal marine animals. Take a walk in the woods and you could witness aggression over the ownership of territory; this is one of the reasons why male birds sing, why male butterflies perform many of their aerial displays and why armies of female worker wood ants try to kill individuals from a different colony. These examples illustrate two important points about aggression: first, animals will fight over a range of resources, when the ability to access those resources is a major constraint on fitness. In many cases this involves conflict over access to mates, as in the case of shore crabs. However, other resources such as territory, food and shelter are also contested, and influence the fitness of females as well as males. The second point is that aggressive behaviour is extremely widespread among animal taxa: these examples alone are drawn from three different phyla: chordates, arthropods and cnidarians.

Summary

Crustaceans have been used extensively to study aggression from a variety of perspectives. Traits that make them good models for studies of aggression include the possession of prominent weapons and a ready willingness to fight, the ease with which they may be obtained and their ease of maintenance in the laboratory. Furthermore, hard exoskeletons mean that equipment such as heartbeat sensors can be easily attached directly to the animals and they are very amenable to physiological investigation. Therefore a feature of studies into the contest behaviour of crustaceans is a strong link between ultimate ‘functions’ and proximate ‘mechanisms’. Given the very wide range of studies on crustacean aggression any review of this group could potentially be extremely broad in focus. Rather than attempt such a broad review, I focus on studies that have combined ethological data with data on underlying mechanisms in order to address questions that have arisen from the body of evolutionary contest theory described in Chapter 2. This has meant that some areas of research on aggression in the Crustacea, such as the neuroendocrine control of aggression and studies of social aspects of aggression such as dominance hierarchies, are outlined only briefly. I consider evidence for information-gathering and decision-making in respect of resource holding potential and resource value, and describe how studies of the underlying motivation to fight can provide useful insights into what information fighting animals might use when making strategic decisions. I also consider studies of agonistic signals, which in crustaceans include visual, tactile and chemical modalities, with a particular consideration of the extent to which ‘dishonesty’ might play a role in crustacean agonistic dealings. I then review physiological aspects of fighting in crustaceans, including studies based on post-fight assays of metabolites and by-products, studies based on ‘real-time’ non-invasive techniques and studies based on a functional morphology approach of investigating whole organism performance capacities. Finally, I suggest ways in which studies on crustacean contests could inform new theoretical models of contest behaviour and discuss the potential for applying approaches used to study crustaceans to the study of contests in other taxa.