Testosterone Reduces Promiscuity of Female Blue Tits (Cyanistes caeruleus): An Experimental Study

In many animal species, extra-pair copulations (EPCs) are common and can increase fitness in both sexes. In males, EPCs can increase total reproductive output, whereas in females benefits of EPCs can be indirect through improving the genetic quality of their offspring. Males and females of many vertebrates show an increase in levels of the hormone testosterone (T) during the mating period. In males, T plays an important role in regulating mating behaviour including increasing their EPC rate. While much is known about the role of T in male mating behaviour, the role of T in female reproduction remains unclear. To study the influence of T on extra-pair paternity rates in females in a field setting, we created three experimental groups of female blue tits (Cyanistes caeruleus): treated with either T, flutamide (Flu; an androgen receptor blocker) or empty implants before egg laying. Subsequently, we scored the number of extra-pair offspring (EPO) in their broods. We also assessed the attractiveness of females treated with either T or Flu to males in mate choice trials in the laboratory. The overall proportion of EPO was lower for the T-implanted group compared with the control group, whereas Flu had no effect. Given that males did not show a preference for Flu- vs. T-treated females in the mate choice trials, it appears less likely that the reduction in EPO in the T-implanted females was due to a reduction in their attractiveness. T levels may have negatively influenced EPO rate by affecting female within-pair and/or extra-pair mating behaviour. Future behavioural studies should investigate how elevated T levels reduce the number of EPO.