Corticosterone, testosterone and life-history strategies of birds

Abstract

Steroid hormones have similar functions across vertebrates, but circulating concentrations can vary dramatically among species. We examined the hypothesis that variation in titres of corticosterone (Cort) and testosterone (T) is related to life-history traits of avian species. We predicted that Cort would reach higher levels under stress in species with higher annual adult survival rates since Cort is thought to promote physiological and behavioural responses that reduce risk to the individual. Conversely, we predicted that peak T during the breeding season would be higher in short-lived species with high mating effort as this hormone is known to promote male fecundity traits. We quantified circulating hormone concentrations and key life-history traits (annual adult survival rate, breeding season length, body mass) in males of free-living bird species during the breeding season at a temperate site (northern USA) and a tropical site (central Panama). We analysed our original data by themselves, and also combined with published data on passerine birds to enhance sample size. In both approaches, variation in baseline Cort (Cort0) among species was inversely related to breeding season length and body mass. Stress-induced corticosterone (MaxCort) also varied inversely with body mass and, as predicted, also varied positively with annual adult survival rates. Furthermore, species from drier and colder environments exhibited lower MaxCort than mesic and tropical species; T was lowest in species from tropical environments. These findings suggest that Cort0, MaxCort and T modulate key vertebrate life-history responses to the environment, with Cort0 supporting energetically demanding processes, MaxCort promoting survival and T being related to mating success.

Variation in Testosterone and Corticosterone in Amphibians and Reptiles: Relationships with Latitude, Elevation, and Breeding Season Length

“…Similar geographic influences have been proposed for baseline (samples obtained immediately upon capture) plasma concentrations of glucocorticoids. Accordingly, in a comparison of tropical and temperate bird species, baseline corticosterone concentrations were negatively related to breeding-season length (Hau et al 2010). Further, peak plasma corticosterone concentrations, those elicited in response to capture and handling (typically 30-60 min after capture), in birds were positively related to latitude (Bó kony et al 2009).…”

“…More recently, however, geography has been suggested as an important large-scale factor that influences the social factors typically proposed to explain variation in plasma hormone concentrations in birds (Goymann et al 2004;Garamszegi et al 2008;Bó kony et al 2009;Hau et al 2010).…”

“…Across bird species, male testosterone appears to be negatively correlated with the length of the breeding season (Goymann et al 2004;Garamszegi et al 2008;Hau et al 2010). For vertebrates in general and birds in particular, breeding seasons are typically shorter at higher latitudes and elevations (Ricklefs 1969;Morrison and Hero 2003).…”

“…At baseline concentrations, the primary function of corticosterone is thought to be the regulation of uptake, storage, and mobilization of energy (Landys et al 2006 and references therein). Hau et al (2010) proposed that males of species with shorter breeding seasons experience greater reproductive intensity, requiring mobilization of more energy reserves on a daily basis and thus higher baseline corticosterone concentrations.…”

“…However, even within a sex and a vertebrate class, there is substantial variation. For example, comparative metaanalyses of bird species, probably the best-studied group in this regard, have reported ranges of both baseline corticosterone and breeding-season testosterone concentrations that span 2 orders of magnitude (Goymann et al 2004;Garamszegi et al 2008;Bó kony et al 2009;Hau et al 2010). Testosterone and glucocorticoids (corticosterone in reptiles, amphibians, and birds, cortisol in many mammals and fish) are the hormones that have received the most attention in comparative studies of free-living vertebrates.…”