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

This chapter reviews our current understanding of ungulate contest behaviour. Before proceeding to the meat and bones we offer a caveat: following Yeats’ question as to whether we can separate the dancer from the dance, we recognise that no single aggressive action employed by a contestant can be considered independent of all other actions. Nevertheless, for reasons of structure and economy we have presented an overview of the competitive process by portraying contests more as parts of their sum rather than vice versa. The reader should keep this in mind when considering the various sections on display behaviour, contest structure, assessment processes and opponent choice. Many ungulates vocalise and appear to show their body and weapon size to opponents during contests, and so we begin with a review of how ungulates communicate their competitive ability prior to physical confrontation. Following this, we consider factors that might affect how ungulates structure their fights: do body size, resource availability and the level of familiarity between opponents influence contest duration? What assessment processes might be driving the willingness of a competitor to engage in a fight and how might this influence contest structure and subsequent outcome? We conclude with a review of the literature concerning what factors might be involved in mediating the decision for one individual to escalate an interaction with a particular group member to fighting.

Introduction

Ungulates comprise approximately 257 species classified broadly within two different orders, the even-toed (order Artiodactyla) and uneven-toed (order Perissodactyla) ungulates. They represent the majority of large herbivores and, with the exception of the Antarctic, are currently resident in all continental regions. The numerous and diverse members of these orders and their wide geographic distribution is reflected in a complex range of social systems that extend from monogamous pair bonds to a variety of large group polygamous breeding systems. Despite the range and complexity of ungulate societies, competition to secure or defend access to resources is a common feature.

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.

his book on Animal Contests represents a landmark in evolutionary biology that is greater than its immediate title suggests. The adaptive interpretation of fighting behaviour in animals has been a catalyst in the study of evolutionary adaptation: first, it was influential in changing concepts about the mechanism of selection (from implicit group selection to individual selection), and second, it was the focus for shaping our understanding of frequency-dependent optimisation in biology through the evolutionarily stable strategy (ESS) approach. I regard it a great privilege and honour to have been invited by the Editors to write a Foreword, and I would like to use this opportunity to recount some of the history of these roles of contest theory in evolutionary biology, including my personal recollections of the events during that very exciting decade, the 1970s.

Before 1970

The way we now think of animal fighting behaviour owes most to the development of the first theoretical models of animal contests, developed in the 1970s. Before this time, with a few notable exceptions (e.g. Williams 1966, Lack 1968),most researchers in the disciplines of ethology and ecology routinely (and usually implicitly) applied group or species selection interpretations to what they saw. This ethos did not generally apply to evolutionary biologists or population geneticists, whose analyses were usually founded on principles derived from Darwinian natural selection.

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.

Summary

Both genders of spiders compete for a variety of resources. They typically use non-contact display, followed by increasingly escalated contact phases comprising touching and sparring and then escalated grappling and biting. Studies of spiders have been central to the understanding of assessment strategies and, for the most part, the data support self-assessment rather than mutual-assessment models. There is good evidence for effects of resource value and ownership on the conduct and outcome of these contests. Fights may have short-term consequences with respect to fatigue, but can have longer-term effects such as loss of appendages or death. Specific experience of winning or losing contests may also influence future encounters. Spiders have also been used in studies of the underlying genetic basis for variation in contest behaviour. Spiders have been the inspiration for motivational models of aggression and we propose a new two-dimensional model that uses cost and resource value as the major factors influencing motivational state and hence choice and duration of activities.

Introduction

Spiders present eminently tractable systems for the study of animal contests, both in field and laboratory settings, and have been the subjects in several seminal studies of aggression. While some taxonomic groups build webs and use them to capture prey and others do not, aggressive behaviour spans the taxonomic and foraging strategy distinction. It occurs in sedentary web-builders (e.g. orb-web spiders, money spiders, comb-footed spiders), sedentary ambushing species (e.g. crab spiders) and more cursorial hunting spiders (e.g. wolf and jumping spiders).

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.

Summary

Since the seminal work of Davies in the late 1970s, territorial male butterfly contests have offered an excellent system for the empirical scrutiny of contest theories, particularly residency-related game-theoretic principles. Because butterflies lack the obvious morphological traits usually associated with animal aggression, their extended and often spectacular aerial duels both defy simple explanation and provide unique empirical opportunities. Residency win rates often approach 100% in this group, which, coupled with the apparently ‘weaponless’ nature of butterflies and the non-contact nature of their disputes, provided the early impetus for tests of the ‘bourgeois’ resident-wins model of contest resolution. Subsequent work has emphasised how potential residency-related RHP asymmetries, including those relating to temporally variable biophysical parameters, such as body temperature, may instead contribute to high rates of residents winning. The balance of empirical work in this group, however, suggests that morphological and/or biophysical factors bear little relevance to content settlement. Contest participation is not obviously mediated by energetics, which contrasts markedly and interestingly with the aerial wars of attrition of other insects, such as odonates. More recent approaches to understanding butterfly contest resolution have led to an appreciation of how life history-level factors, such as ageing and changes in residual reproductive value, may influence aggressive motivation and subsequent levels of contest participation. These principles apply generally, thereby placing butterfly contests as a potentially important system for the empirical investigation of the broader life-historical context of animal aggression.

Summary

Contests between beetles have attracted the attention of biologists and natural historians for a long time. Contests almost always involve males competing for females or for access to resources crucial to female reproduction. With only a few exceptions, beetle contests typically involve non-injurious trials of strength. In this chapter we begin by reviewing the literature on beetle contests with an emphasis on how ecological conditions and resource availability have shaped the nature of contests in diverse beetle families. We then focus on two inter-related aspects of the biology of beetle contests that have received particular and recent attention: the evolution of alternative reproductive tactics and the origin and diversification of weaponry. We describe the nature of alternative reproductive tactics in a range of beetle families, and explore the developmental and physiological mechanisms that have mediated contest diversification within and between species. We then explore the evolutionary developmental biology of weaponry in beetles, focusing on the elaborate horns of scarabs. Specifically, we describe how horns are built during development, and explore the relative contributions of conserved developmental mechanisms and novel developmental properties to the origin and diversification of horns. We then highlight several instances in which developmental studies have assisted our understanding of the evolution and ecology of contest behaviour, such as the ancestry of alternative phenotypes and sexual dimorphisms, the costs of weaponry and constraints and biases in the diversification of beetle weapons. Combining rich morphological and behavioural diversity with a diverse genetic and developmental toolbox, beetles emerge as a powerful model system in which to explore the causes, mechanisms and consequences of contest behaviour in nature.

Summary

Studies of contests among amphibians are heavily biased towards acoustic contests in frogs: in these, males compete to be attractive to females or to defend territories required for some aspect of reproduction. While the calls of frogs are species-specific and appear to be highly stereotyped, these studies have revealed a high degree of plasticity that enables males to vary their calls in response to their immediate circumstances. Because calling is energetically expensive, males must face trade-offs between increasing their immediate calling effort and conserving energy for future mating opportunities. In some species, they also trade-off between repelling rival males and attracting females; this is because females are averse to aggressive calls. There is a great deal less known about contests among salamanders and caecilians, primarily because these animals are much harder to observe and because their primary means of communication, olfaction, is harder to manipulate experimentally than the acoustic signals of frogs. Despite many unanswered questions, a great deal has been learned about aggressive behaviour in amphibians, and these studies have made important contributions towards a general understanding of animal contests. This chapter discusses the diversity of issues related to contest behaviour in amphibians and highlights how these organisms remain fruitful subjects for future studies of animal contests.

Introduction

Compared with birds and mammals, amphibians are still a poorly known group of animals with one or two new species still being described each week: 20 years ago, 5399 species were listed (Halliday & Adler 2002) with the number currently exceeding 7000 (AmphibiaWeb). In part, this reflects the fact that amphibians are typically small animals that lead secretive lives and engage in observable behaviour for limited periods of time. As a result, our knowledge of the social behaviour of amphibians is fragmentary, with only a few species studied intensively. An encyclopaedic overview of amphibian behaviour is provided by Wells (2007).

Summary

Animal contests were the focal topic that brought game theory to the attention of behavioural ecologists, giving rise to evolutionary game theory. Game theory has remained by far the most popular method of deriving theoretical predictions ever since, although it nowadays coexists with other methods of analysis. Here I review the developments to date and highlight similarities and differences between models. There is a clear progression from simple two-player models with fixed payoffs to explicit tracking of fitness consequences in a population context. In many cases this development has helped to discover that some of the early predictions may have been misleading. Despite the large number of current models, there are still gaps in the theoretical literature: sometimes simplifying assumptions have been relaxed in one context but not another. I hope that by highlighting these gaps theoreticians will be provided with new research ideas, and empiricists will be encouraged not only to distinguish between existing models but to be able to point out assumptions that are essential for deriving a result yet may be violated in existing systems, thus directing new modelling in the most useful direction.

Introduction

Until the mid 1960s, animal contests were viewed using group selectionist thinking. Julian Huxley (1966) thought that ritualised fights evolved to limit intraspecific damage, partly based on Konrad Lorenz's (1964, 1965) ideas that species need to evolve mechanisms that limit aggression in species that possess dangerous weapons for other reasons, e.g. as adaptations for capturing prey. Following George C. Williams’ (1966) book Adaptation and Natural Selection: A Critique of Some Current Thought, however, biologists became aware of the need to distinguish between explanations that are based on benefits to the individual versus those that rely on benefits accruing to a group (or a species).

Summary

Work on birds has prompted some key theoretical developments in the field of animal contests. Birds have an unusually conservative morphology, which constrains the ability of many species to fight physically, but many birds have evolved adaptations to aid in conflict resolution. These include the display of bright colours, elaborate vocalisations and physiological mechanisms. Birds also have a relatively conservative life history (parental care is ubiquitous), which influences the nature and extent of contests: for most birds, competition begins at birth for access to food provided by their carers. Intense resource competition continues into their adult lives, especially as many birds are territorial and/or reside in social flocks of related and/or unrelated conspecifics. At breeding, competition for access to mates is particularly severe, with birds competing both overtly and more cryptically to gain fertilisation precedence. Here I consider the diversity of avian contests.

Introduction

Bird contests can be highly conspicuous. An example is the ferocious fighting that occurs between two unfamiliar (or staged) jungle fowl (Gallus gallus domesticus) where males may fight to the death using sharp-pointed spurs as weapons (Mench 2009). Many avian contests are, however, less physical. Under more natural circumstances, for example within the jungle fowl's mixed-sex flock, disputes between flock members are unlikely to escalate to physical fighting (Ligon et al. 1990, Collias & Collias 1996). Agonistic encounters generally involve stereotyped behaviours, a range of complex vocalisations, and variation in conspicuously coloured morphological traits (e.g. Ligon et al. 1990, Cornwallis & Birkhead 2008). Nevertheless, the lack of physical contests does not necessarily indicate an absence of competition. Indeed, birds compete for the same things that cause and define competition in other taxa. However, there are a number of general avian morphological and life-history adaptations that are likely to have influenced the nature of competition in birds.