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Ben Thornes

Behavioural Syndrome in a Solitary Predator is independent of body size

Wednesday 16th July 2014

…& Growth Rate.

Introduction

Empirical studies across a range of animal taxa are increasingly demonstrating the existence of personalities, where individuals within populations vary consistently in their behaviour over time. When individual behaviours are consistent or co-vary across situations or contexts, where a context is a functional behavioural category (e.g. feeding, mating, predator avoidance or dispersal), and a situation is the set of conditions at a particular time which can involve different levels along an environmental gradient (e.g. foraging behaviours in different habitats), it is referred to as a behavioural syndrome. Although individual consistency of single behaviours is considered to contribute meaningfully to the stability of the behavioural syndrome they comprise, repeated observations of individuals over time within situations or contexts are lacking in many studies. Despite this, and in conjunction with inconsistent methodologies employed to assess behavioural traits, behavioural syndromes are considered to be widespread. Furthermore, a focus on characterising behavioural syndromes in social or territorial species that show parental care or build bests, exhibit dominance hierarchies or other social structures, such as shoaling, has resulted in a paucity of studies in other species, such as in solitary apex predators. Yet characterising behavioural syndromes in ecologically-different species with contrasting behavioural life-histories should improve our understanding of the extent of behavioural syndromes and their ecological importance. For example, identifying behavioural syndromes in an apex predator may be particularly important for understanding their effect on trophic interactions and influence on prey fish communities.

Behavioural syndromes are temporally stable when the same association between different behaviours occurs at different stages in time. Temporal stability in behavioural syndromes suggests that individual behaviours may not be able to evolve independently and are therefore considered to be of particular evolutionary significance. Exploring the mechanisms involved in maintaining behavioural syndromes in animals has therefore recently received considerable theoretical attention, with a focus on state-dependency. Individuals differ consistently in a range of features or ‘states’, for example in morphology, physiology and even in aspects of their environment. State-dependent behavioural models are therefore based on the fact that an individual’s state influences the fitness costs and benefits of its behavioural decisions. As stable individual variation in growth rate has been reported in a variety of species with indeterminate growth, growth has also been suggested as a key factor in maintaining personality differences due to growth-mortality tradeoffs. Indeed, traits such as boldness, aggression and activity may correlate with higher growth rates, but these behaviours may also increase mortality through greater risk-taking. As few empirical studies have tested predictions derived from state-dependent models, this is now needed to further our understanding of behavioural syndromes.

Individual variation in risk-taking was originally used to define boldness in animals, which subsequently lead to a variety of interpretations on its measurement. Boldness has been measured, for example, by response to threatening stimuli, novel objects or food sources, predator inspection, latency to emerge from cover and foraging under predation threat. Although a consensus on the measurement of boldness is valuable for comparative purposes, a consideration of behaviours and related situations that would represent boldness in the species of interest in increasingly recognised, the fundamental differences between functionally different species should not be overlooked. To interpret results from studies using different tests for measuring the same personality trait, the specific context and methods should be considered.

Pike, Esox lucius, exhibit considerable growth differences in their wild populations, with size dimorphism already apparent in young-of-the-year (YOY), thus making it a strong model to test size- and growth dependent personality. Pike is a solitary and cannibalistic predator species that does not live in groups during any stage of its life. Cannibalism usually occurs between fish of different ages, but as considerable size variations occur within the same cohort, for example, among juveniles, individuals have been found to cannibalise on conspecifics 50-91% of their body size. In addition, pike are vulnerable to attack from conspecifics of similar size when handling prey. Due to the strong pressures from both intra- and interspecific predators on juvenile pike in their nursery habitats, an important trade-off has been suggested to occur between foraging activity in order to out-grow piscivores (due to piscivorous gape limitation) and anti-predator avoidance. We therefore consider that a measure of foraging behaviour across a gradient of risks is an appropriate indicator of boldness in this solitary apex predator. Foraging under risk of predation has been used as a measure of boldness in several fish species, but has recently been criticised as the behaviour measured might be motivated by hunger instead of boldness. To overcome this, equal starvation periods prior to measurements of foraging behaviour are commonly used to ensure similar hunger levels among the test animals.

Consequently, in the present study, the researchers determine the presence of a behavioural syndrome in juvenile pike by estimating the repeatabilities of individual foraging behaviours through time and across risk situations, including at different stages over time, and its relationship to individual state (i.e. body mass) and growth rate. The following hypotheses were tested: (i) individuals exhibit stable behavioural syndromes, shown as repeatable foraging behaviour within and across risk situations; and (ii) in high-risk situations, larger-bodied individuals consistently forage at higher rates compared to smaller individuals, and so have higher growth rates in conditions of abundant food.

Read the full, open access, article on PLOS ONE

Find out more about the author, Marina J Nyqvist, here.

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