Testosterone and Avian Male Reproduction

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

In many socially monogamous bird species, males sometimes engage in extra-pair copulations and sire viable offspring. Conventional wisdom has been that all males are continuously ready for such opportunities, but recent field studies of reproductive hormones have shown that testosterone (T) fluctuates greatly during the nesting cycle, peaking early during courtship/mating, then plummeting in tandem with a dramatic behavioral switch to care-giving activities (incubating and feeding nestlings). It has been proposed that males may experience reduced interest in or capacity for extra-pair matings when T is low, that they may then experience a physiological 'time-out' phase of their cycle. To test this, one sample of male house sparrows will be given time-release hormone implants (keeping T-levels high), while another sample serves as normal (cycling) controls. The resulting patterns of extra-pair paternity, as revealed by DNA fingerprinting, will allow assessment of what a male sacrifices by cycling. Conversely, the quality of parental care received by the nestlings of implanted vs. control males will show what is gained from cycling. Parallel experiments will calibrate the mechanism(s) responsible. This project will improve understanding of the social forces that promote and maintain monogamy, a relatively rare mating system in nature. It focuses attention on the costs and benefits available to males for investing heavily in one primary brood by evaluating the factors that constrain the pursuit of alternative mating partners.