HORMONAL DYNAMICS DURING MATE CHOICE IN THE NORTHERN PINTAIL - A TESTOF THE CHALLENGE HYPOTHESIS

In previous mate choice experiments, we found no relationship between dominance rank and pairing success in male northern pintails, Anas acu ta. Once chosen by a female, however, males became aggressive, initiat ed fights with higher-ranked males and quickly established dominance. In the present study, we tested a variation of the 'challenge' hypothe sis, that the behavioural stimuli associated with acquiring and defend ing a mate induce an increase in testosterone level, which in turn fac ilitates aggressive behaviours required for males to establish dominan ce. We measured plasma hormone levels (testosterone, dihydrotestostero ne, luteinizing hormone and corticosterone) before and after mate choi ce in two experiments in which males competed for a single female (exp eriments 1 and 2) and in a control experiment in which no female was i ntroduced (experiment 3). We used groups of either three adult males ( experiment 1) or one adult and two yearling males (experiments 2 and 3 ). Contrary to expectation, in experiment 1, plasma levels of corticos terone increased significantly and testosterone levels decreased in ch osen males following mate choice. The magnitude of change in corticost erone was positively correlated with the rate of aggression by males. Chosen adult males in experiment 2 showed similar patterns of hormone change (corticosterone increase and testosterone decrease), although n ot all changes were significant. Hormone levels of unchosen yearlings in experiment 2 and control adults and yearlings in experiment 3 showe d no changes. The results are consistent with the hypothesis that beha vioural stimuli associated with successful pair formation induce a tra nsitory increase in circulating levels of corticosterone, which in tur n mediates the behavioural response of increased aggression leading to the establishment of dominance following mate choice. A short-term in crease in corticosterone may be adaptive in this situation because it would mobilize energy stores needed by the: male to defend the new pai r bond and establish dominance. (C) 1997 The Association for the Study of Animal Behaviour.