1. In terrestrially breeding phocid seals males are significantly larger than females with their large size conferring advantage in male-male competition, and increasing their ability to fast during the breeding season, prolonging tenure and hence mating opportunities. 2. For aquatically breeding seals, the opportunity to feed during the breeding season may offset the need for large size and the ability to fast. 3. Individual differences in male breeding success, behaviour and mass changes for Weddell seals (Leptonychotes weddellii) at Turtle Rock, McMurdo Sound (77.727S, 166.85E) during 1997, 1998 and 1999 were recorded. 4. Males were tracked under the ice through each breeding season using an acoustic array (n = 15). Mass changes were measured for 30 males, all of which were successfully genotyped and were aged between 6 and 20 years (mean 13.7). 5. Territory use was dynamic, with some males spending most of the time either at or near the surface, others diving deep and others switching from regular diving to near surface behaviour. 6. Rate of mass loss varied more than three-fold (mean 2.1 ± 0.53 kg day-1, range 0.0 to 4.1) as did mass-specific loss (mean 0.53 ± 0.23). 7. Maximum dive depth for the individuals also varied dramatically (10 to 518 m) and was inversely related to the rate of mass-specific loss, suggesting that the deep diving males may offset the costs of breeding by foraging. 8. Males that stayed near the surface successfully sired pups as did males that continued to dive. Both strategies therefore seemed to be equally successful. 9. Foraging during the breeding season may be a male strategy that may potentially prolong tenure, but appears to be facultative rather than obligative.