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.

Figure 1.

Partial regression plot showing inverse relationship between baseline corticosterone concentrations (Cort0; ng ml⁻¹) and breeding season length (months). Data are displayed as unstandardized residuals after controlling for body mass in a linear regression model. The figure shows the largest dataset used in this study. Larger symbols denote own original dataset, smaller symbols refer to published data. Environments are coded as: open circles, mesic; filled circles, tropical; crosses, arid; stars, cold (arctic or alpine).

Figure 2.

Interspecific variations in concentrations (mean ± s.e.; ng ml⁻¹) of (a) baseline corticosterone, (b) stress-induced corticosterone and (c) testosterone, in males from species breeding in different environments. Sample sizes are given in bars; significant differences between environments as derived from Tukey's HSD post hoc tests are indicated by horizontal lines and asterisks.

Figure 3.

Partial regression plot showing positive relationship between maximal corticosterone concentrations (MaxCort; ng ml⁻¹) and survival rate (%). Data are displayed as unstandardized residuals after controlling for body mass in a linear regression model. The figure shows the largest dataset used in this study. Larger symbols denote own original dataset, smaller symbols refer to published data. Environments are coded as: open circles, mesic; filled circles, tropical; crosses, arid; stars, cold (arctic or alpine).