For long-term monogamous species, it may be important to have demographic, morphological, physiological and/or behavioural compatibility within pairs. We used field data on wild populations of Dark-bellied Brent Geese, a species showing extreme mate fidelity, to test for (1) within-pair covariations of body measurements (body mass and size) and hormone levels (testosterone and T3) during winter and spring and (2) the impact of male seasonal hormone levels on the accumulation of body reserves by the female during the spring staging period. Here, we used testosterone as a proxy of aggressiveness and dominance and T3 as a proxy of male metabolism and activity. We observed a covariation of body mass within pairs, at least on the wintering grounds. Body sizes and plasma hormone levels of mates did not covary, neither in winter nor spring. We did not observe an association of female body mass with male hormonal levels.