Plastic endocrine regulation of year-round territorial aggression in tropical male spotted antbirds

Abstract

Studies investigating the hormonal regulation of aggression often focus on the role of the steroid hormone testosterone (T). These studies have generally found an association in temperate zone species between T and male aggression in a reproductive context. However, in most temperate zone species seasonal variation in reproductive context cannot easily be separated from concomitant seasonal changes in other variables, including territory location and partner presence. Therefore, we investigated the hormonal regulation of territorial aggression in a tropical bird that breeds seasonally, but remains mated and territorial year-round. Free-living male spotted antbirds (Hylophylax n. naevioides) displayed similar aggressive behavior in the non-breeding, early, and middle breeding seasons when exposed to a simulated territorial intrusion (STI). Plasma T concentrations after STIs were low and seasonally invariant, but plasma dehydroepiandrosterone (DHEA) was elevated during the non-breeding season. Simultaneous administration of pharmacological inhibitors of androgenic and estrogenic actions during the non-breeding season was less effective in reducing aggressive behavior of captive males compared with a previous study conducted during the breeding season. Hence, in male spotted antbirds endocrine mechanisms appear to vary with reproductive context, both in DHEA concentrations and in the importance of T for regulating aggressive behavior. Furthermore, combining the current data with previous findings suggests that short-term increases in T during aggressive interactions in this tropical species are dependent on the context and the intensity of aggressive stimuli, regardless of season.