The influence of the starvation-predation trade-off on the relationship between ambient temperature and body size among endotherms.

John M. McNamara,Andrew D. Higginson,Simon Verhulst

doi:10.1111/jbi.12695

John M. McNamara, Andrew D. Higginson + Show 1 more

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https://doi.org/10.1111/jbi.12695

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Abstract

AimThe tendency for animals at higher latitudes to be larger (Bergmann's rule) is generally explained by recourse to latitudinal effects on ambient temperature and the food supply, but these receive only mixed support and do not explain observations of the inverse to Bergmann's rule. Our aim was to better understand how ecological variables might influence body size and thereby explain this mixed support.LocationWorld‐wide.MethodsPrevious explanations do not allow for the selective pressure exerted by the trade‐off between predation and starvation, which we incorporate in a model of optimal body size and energy storage of a generalized homeotherm. In contrast to existing arguments, we concentrate on survival over winter when the food supply is poor and can be interrupted for short periods.ResultsWe use our model to assess the logical validity of the heat conservation hypothesis and show that it must allow for the roles of both food availability and predation risk. We find that whether the effect of temperature on body size is positive or negative depends on temperature range, predator density, and the likelihood of long interruptions to foraging. Furthermore, changing day length explains differing effects of altitude and latitude on body size, leading to opposite predictions for nocturnal and diurnal endotherms. Food availability and ambient temperature can have counteracting selective pressures on body mass, and can lead to a non‐monotonic relationship between latitude and size, as observed in several studies.Main conclusionsOur work provides a theoretical framework for understanding the relationships between the costs and benefits of large body size and eco‐geographical patterns among endotherms world‐wide.

Highlights

The tendency for body size to increase with latitude was observed over 150 years ago (Bergmann, 1847), and is known as Bergmann’s rule
We use our model to assess the logical validity of the heat conservation hypothesis and show that it must allow for the roles of both food availability and predation risk
In order to assess the dependence of body composition on temperature in more realistic situations, we will assume in the following that either the rate of energy gain (c) or the susceptibility to predation (b) is affected by L and R. c may change with body composition if being more muscular means the animal can better compete with conspecifics or capture prey. b may increase with the ratio R to L if fat load impairs the evasion of predators

Summary

Introduction

The tendency for body size to increase with latitude was observed over 150 years ago (Bergmann, 1847), and is known as Bergmann’s rule. The original proposal relates to the need to conserve heat, because colder ambient temperatures might select for animals with lower surfacearea-to-volume ratios, which declines as body size increases (Bergmann, 1847; Peters, 1983). This does not necessarily mean that an animal should be as large as possible in cold conditions because the absolute rate of heat loss http://wileyonlinelibrary.com/journal/jbi doi:10.1111/jbi.12695 increases with size, meaning greater energy requirements (McNab, 1971; Ergon et al, 2004). In order to understand how the selective pressure to avoid death from heat loss will have influenced body size and composition we must consider the energetic tradeoff between the risks of starvation and predation (Lima, 1986; McNamara & Houston, 1987; Cresswell et al, 2009; Bennett et al, 2013)

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