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

In monogamous animals, reproductive success may vary considerably among pai rs. To study this variation, we focused on reproductive events and the circ annual hormonal co-ordination within pairs during the reproductive cycle. T estosterone was chosen as covariable for both sexes because of its mediator function between behaviour and physiology. In a flock of free-living Greyl ag Geese Anser anser, individual faecal samples were collected weekly from 23 pairs over a complete annual cycle. From the faeces, equivalents of test osterone, oestrogen and other steroid hormones were analysed by enzyme immu noassay. In contrast to correlations between male and female testosterone, no correlations were found between oestrogen-oestrogen or oestrogen-testost erone. Therefore, only testosterone (T) is considered here. Sex-specific di fferences in T were in amplitude rather than in the annual timing of maxima and minima. However, the annual timing varied to some degree between indiv iduals within sexes. Therefore, we examined the degree of annual testostero ne correlation (TC) within pairs. Pairs that nested had significantly highe r 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 inve stigated was positively and significantly correlated with the pairs' long-t erm reproductive output. No correlation was found between TC and the durati on of the pair-bond, individual age, or age difference from pair partner. W e suggest that. TC is a measure of behavioural synchrony and, therefore, pa ir-bond quality. We consider whether within-pair TC results from mate choic e.