The costs and benefits of being fat or skinny

on the trade-off between between being fat & slow & energetically buffered and lean & fast & prone to starvation. There could be several alternative explanations for their finding, but maybe they control for some of these.

Mass-dependent predation risk and lethal dolphin–porpoise interactions

R. MacLeod, C.D. MacLeod, J.A. Learmonth, P.D. Jepson, R.J. Reid, R. Deaville, G.J. Pierce

Proc. Roy. Soc. B Online: Tuesday, August 14, 2007

Abstract: In small birds, mass-dependent predation risk (MDPR) is known to make the trade-off between avoiding starvation and avoiding predation dependent on individual mass. This occurs because carrying increased fat reserves not only reduces starvation risk but also results in a higher predation risk due to reduced escape flight performance and/or the increased foraging exposure needed to maintain a higher body mass. In principle, the theory of MDPR could also apply to any animal capable of storing energy reserves to reduce starvation and whose escape performance decreases with increasing mass. We used a unique situation along certain parts of coastal Britain, where harbour porpoises (Phocoena phocoena) are pursued and killed but crucially not eaten by bottlenose dolphins (Tursiops truncatus), to investigate whether a MDPR effect can occur in non-avian species. We show that where high levels of dolphin ‘predation’ occur, porpoises carry significantly less energy reserves than would otherwise be expected and this equates to reducing by approximately 37% the length of time that a porpoise could survive without feeding. These results provide the first evidence that a mass-dependent starvation–predation risk trade-off may be a general ecological principle that can apply to widely different animal types rather than, as is currently thought, only to birds.