Endogenous testosterone is not associated with the trade-off between paternal and mating effort

Males may face a trade-off between caring for offspring and pursuing additional matings. In birds, the androgen testosterone has been suggested to be a key proximate mediator in this trade-off for several reasons. At the population level, high testosterone is typically associated with the period of intense male-male competition over females, whereas low testosterone is associated with the period of paternal care. In addition, males with experimentally elevated testosterone during provisioning feed their young at a lower rate than control males. Nearly all studies observing these patterns, however, ignore the tremendous variation in endogenous testosterone concentration that exists within a population of males, even during the same breeding stage. Because selection acts at the level of individual, this variation has to be taken into account when studying proximate mechanisms mediating the paternal and mating effort trade-off. Studying barn swallows we here show that, within males, testosterone concentrations were not higher around the fertile period of the social mate than during nestling feeding. More importantly, 30% of males showed no decrease in testosterone concentration between these 2 periods. Further, male feeding effort was not related to testosterone concentration during feeding. These results indicate that, at least in barn swallows, endogenous testosterone is not a key mediator in the trade-off between paternal effort and mating effort. Our results also stress that to understand how selection has shaped temporal testosterone profiles and action, it is crucial to study the relationship between testosterone and traits contributing to fitness at the level of the individual.