title: The influence of the starvation-predation trade-off on the relationship between ambient temperature and body size among endotherms.
abstract: The 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.
  
  
  
  World-wide.
  
  
  
  Previous 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.
  
  
  
  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. 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.
  
  
  
  Our 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.
authors.affiliation: School of Mathematics University of Bristol Bristol BS8 1TW UK.
authors.forename: John M
authors.initials: JM
authors.lastname: McNamara
authors.affiliation: Centre for Research in Animal Behaviour College of Life and Environmental Sciences University of Exeter Exeter EX4 4QG UK; School of Biological Sciences University of Bristol Bristol BS8 1TQ UK.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Higginson
authors.affiliation: Groningen Institute for Evolutionary Life Sciences University of Groningen 9747 AG Groningen The Netherlands.
authors.forename: Simon
authors.initials: S
authors.lastname: Verhulst
keywords: Bergmann's rule; NPP; body composition; energy balance; fat storage; food shortage; heat conservation; latitude; resource availability; starvation resistance