Ecological correlates of cortisol levels in two bat species with contrasting feeding habits

Abstract

The immediate release of adrenal glucocorticoids can be crucial for an animal’s survival when facing a stressor, but constantly elevated or exceptionally high glucocorticoid levels are usually detrimental for health. Although baseline and maximal secretion of glucocorticoids are regulated within narrow ranges within species, plasma glucocorticoid levels vary largely across vertebrates. We asked what ecological factors affect baseline plasma cortisol levels (CortI) and maximum levels (CortMax) following a physiological challenge through administration of adrenocorticotropic hormone (ACTH). Specifically, we studied whether seasonal fluctuations in food abundance correlate with the capacity of cortisol increases in two phyllostomid bat species with contrasting feeding habits: the sanguinivorous vampire bat (Desmodus rotundus) and the frugivorous short-tailed fruit bat (Carollia perspicillata). Both species coexist in habitats with various levels of seasonality (dry and rainforest). On a seasonal basis, resource abundance is more stable for vampire than for fruit bats, but previous studies suggested that daily foraging success may vary more for vampire than for fruit bats. CortI and CortMax varied seasonally in C. perspicillata from dry and rainforests, with the exception of CortMax in rainforest bats. Although we expected food availability to be stable year-round for vampire bats, we found CortI and CortMax of vampires to be higher during the rainy season than during the dry season. Also, we found CortMax to be higher in vampires from the rainforest than in those from the dry forest. CortMax of vampires were among the highest measured for a free-ranging mammal; a pattern that could be related to the species’ vulnerability to starvation. We conclude that food availability modulates cortisol levels in free-ranging species that face seasonally fluctuating resources; in species, however, that benefit from food which is constantly abundant, other factors than food may become more important in modulating cortisol levels.