Testosterone, territorial response, and song in seasonally breeding tropical and temperate stonechats

Abstract

Background: Testosterone facilitates physiological, morphological, and behavioral changes required for breeding in male vertebrates. However, testosterone concentrations and the link between its seasonal changes and those in reproductive behaviors vary greatly among species. To better understand the impact of tropical and temperate environments and life history factors on this variation, we have compared testosterone, territorial behavior and song performance across sequential stages of the breeding season in males of 16 closely related taxa of East African tropical and West European temperate stonechats (Saxicola spp), which all breed during a short breeding season, but differ in migratory behavior, seasonal territory-acquisition and pace of life.

Results: We found that generally, the profiles of testosterone and territorial behavior were similar across latitudes. African stonechats with a slow pace of life had equally high peak testosterone concentrations and responded as aggressively to an intruder as European stonechats with a fast pace of life. However, song performance at the beginning of the breeding season was lower in African than in European stonechats. The differences in song performance were not associated with variation in testosterone levels between tropical and temperate stonechats.

Conclusions: The results suggest a very similar role for testosterone as a mediator of high intensity territorial aggression during the fertile period of females in tropical and temperate stonechats, which all are highly seasonal, locally synchronous breeders. A potential explanation may be high risk of extra-pair copulations which has been associated with synchronous breeding. Interestingly, an association was not consistent for song performance. Our data suggest that song performance can be disassociated from peak testosterone levels depending on its role in breeding behavior. Despite similar testosterone levels, European males, which early in the breeding season acquire territories and mates, showed greater song performance than African stonechats, which maintain year-round territories and pair-bonds. Taken together, our study comparing related taxa of old world songbirds suggests that short breeding seasons may be a major selective force for high peak testosterone levels during breeding regardless of latitude and pace of life, but that particular behaviors, in our case song, can be uncoupled from peak testosterone levels.

Keywords: Breeding season length; Migratory behavior; Song; Songbirds; Steroid hormones; Temperate; Territorial behavior; Testosterone; Tropical.

Fig. 1

Testosterone concentrations (ng/ml) of African and European male stonechats in relation to breeding phase and population. Points and error bars (means and 95% credible intervals) represent African (white fills) and European (black fills) populations. Letters indicate the location of each population and full location names are displayed together with sample sizes in the legend

Fig. 2

Testosterone concentrations (ng/ml) of African and European male stonechats during detailed breeding stages of the first brood. Especially in African stonechats, testosterone concentrations peaked during nest-building and were lowest when stonechats were feeding nestlings. Bars and error bars represent back-transformed posterior means and their 95% credible intervals. Dots represent data points of individuals from different populations. Populations were included as random intercepts in the models

Fig. 3

Latency to attack (min) of African and European male stonechats in response to simulated territorial intrusions during different stages of the breeding season. Male stonechats attacked the decoy faster during nest-building and parental stages than during pre-nesting and incubation. Males that did not attack the decoy were assigned a latency of 60 min. Bars and error bars represent back-transformed posterior means and their 95% credible intervals. Dots represent data points of individuals from different populations

Fig. 4

Song traits of African and European stonechats with a) song rate, b) element rate per song, and c) peak frequency. European stonechats sang with a higher song rate, element rate, and peak frequency during the mating period than African stonechats. Filled points and error bars represent back-transformed posterior means and 95% credible intervals. Sample sizes are displayed below error bars and are similar for element rate and peak frequency. Open symbols represent data points of individuals from different populations