Brain size as a driver of avian escape strategy.

After detecting an approaching predator, animals make a decision when to flee. Prey will initiate flight soon after detecting a predator so as to minimize attentional costs related to on-going monitoring of the whereabouts of the predator. Such costs may compete with foraging and other maintenance activities and hence be larger than the costs of immediate flight. The drivers of interspecific variation in escape strategy are poorly known. Here we investigated the morphological, life history and natural history traits that correlate with variation in avian escape strategy across a sample of 96 species of birds. Brain mass, body size, habitat structure and group size were the main predictors of escape strategy. The direction of the effect of these traits was consistent with selection for a reduction of monitoring costs. Therefore, attentional costs depend on relative brain size, which determines the ability to monitor the whereabouts of potential predators and the difficulty of this task as reflected by habitat and social complexity. Thus brain size, and the cognitive functions associated with it, constitute a general framework for explaining the effects of body size, habitat structure and sociality identified as determinants of avian escape strategy.

Sex differences in lizard escape decisions vary with latitude, but not sexual dimorphism.

Sexual selection is a powerful evolutionary mechanism that has shaped the physiology, behaviour and morphology of the sexes to the extent that it can reduce viability while promoting traits that enhance reproductive success. Predation is one of the underlying mechanisms accounting for viability costs of sexual displays. Therefore, we should expect that individuals of the two sexes adjust their anti-predator behaviour in response to changes in predation risk. We conducted a meta-analysis of 28 studies (42 species) of sex differences in risk-taking behaviour in lizards and tested whether these differences could be explained by sexual dichromatism, by sexual size dimorphism or by latitude. Latitude was the best predictor of the interspecific heterogeneity in sex-specific behaviour. Males did not change their escape behaviour with latitude, whereas females had increasingly reduced wariness at higher latitudes. We hypothesize that this sex difference in risk-taking behaviour is linked to sex-specific environmental constraints that more strongly affect the reproductive effort of females than males. This novel latitudinal effect on sex-specific anti-predator behaviour has important implications for responses to climate change and for the relative roles of natural and sexual selection in different species.

Pheomelanin-based plumage coloration predicts survival rates in birds.

Higher vertebrates synthesize two forms of melanin: eumelanin and pheomelanin. While the adaptive functions of eumelanin are diverse, those of pheomelanin, which is phototoxic and whose production consumes a key intracellular antioxidant (glutathione), are not clear apart from being involved in color patterns that confer concealment. The factors that have favored the evolution of pheomelanin thus remain a mystery, causing this pigment even to have been considered an "accident of nature." A recent hypothesis posits that pheomelanin has evolved because it represents an alternative mechanism to remove excess dietary cysteine, which can be toxic because of its oxidation. We tested for links between pheomelanin-based color and survival in both an intraspecific study of barn swallows Hirundo rustica and an interspecific study of 58 species of birds from North and Central America. As predicted on the basis that birds degrade excess dietary amino acids by transferring their amino group to uric acid synthesis, we found that under equal levels of uric acid in plasma, individuals or species with a higher intensity or greater proportion of plumage colored by pheomelanin (brown and chestnut coloration) had higher relative annual survival rates while controlling for the potentially confounding effects of age, sex, body size, and phylogenetic descent. Likewise, barn swallows with more intense pheomelanin-based coloration had higher prospects to survive the winter after controlling statistically for age, sex, body size, and level of uric acid. This supports the idea that pheomelanic traits evolve because of the removal of excess cysteine in nonstressful conditions, thus avoiding its toxic effects.