Within‐pair testosterone covariation and reproductive output in Greylag Geese Anser anser

In monogamous animals, reproductive success may vary considerably among pairs. To study this variation, we focussed on reproductive events and the circannual hormonal co‐ordination within pairs during the reproductive cycle. Testosterone was chosen as covariable for both sexes because of its mediator function between behaviour and physiology. In a flock of free‐living Greylag Geese Anser anser , individual faecal samples were collected weekly from 23 pairs over a complete annual cycle. From the faeces, equivalents of testosterone, oestrogen and other steroid hormones were analysed by enzyme immunoassay. In contrast to correlations between male and female testosterone, no correlations were found between oestrogen‐oestrogen or oestrogen‐testosterone. Therefore, only testosterone (T) is considered here. Sex‐specific differences in T were in amplitude rather than in the annual timing of maxima and minima. However, the annual timing varied to some degree between individuals within sexes. Therefore, we examined the degree of annual testosterone correlation (TC) within pairs. Pairs that nested had significantly higher TC over the year than pairs that did not. The higher the within‐pair TC, the larger the clutches and the heavier the eggs. Also, TC in the year investigated was positively and significantly correlated with the pairs' long‐term reproductive output. No correlation was found between TC and the duration of the pair‐bond, individual age, or age difference from pair partner. We suggest that TC is a measure of behavioural synchrony and, therefore, pair‐bond quality. We consider whether within‐pair TC results from mate choice.