Within polygynous breeding systems, intra-sexual selection between males generally leads to the formation of a dominance hierarchy. There has been debate over which factors favour the establishment of social dominance. Where some studies argue that body mass is the singular determinant of social rank, others show that rather than the males’ mass, it is their length. However most of these studies have been of males from terrestrial systems. In terrestrially breeding polygynous species, dominant males secure not only mating access to females, but also to areas of high value food resources. For semi-aquatic mammals, such as the southern elephant seal, social dominance does not confer access to food resources as the males fast while ashore in the breeding colonies. The males’ foraging choices made at sea, prior to the breeding haul outs, reflects a male´s diet prior to dominance establishment. We ask whether a male’s diet, body mass and/or length conferred an advantage in attaining social dominance. To reconstruct the diet of males of different social ranks we used bulk δ15N and δ13C values of serum collected from sexually mature male southern elephant seals (n=44) during the breeding season at King George Island, within the South Shetland archipelago. To identify which factors (diet-inferred from δ15N and δ13C values, body mass, and/or standard length) influenced male social rank, we conducted a series of mixed models and took a model selection approach. Socially dominant beachmasters had a larger body mass and significantly enriched δ15N values compared to less dominant males. The significant 15N enrichment (>3.4‰) of the dominant males, suggests that they fed on higher trophic prey. There was no difference in δ13C values of the males of different ranks suggesting that both dominant and subordinate males fed in similar regions. An interaction between diet (δ15N) and body size (both mass and length) explained the greatest variation between males’ social rank. This interaction between body mass and diet (δ15N) potentially infers a feedback, such that males of a larger body mass can make deeper/longer dives, gaining access to larger prey and so confers a higher energetic efficiency.