Breeding site choice fails to explain interannual associations of female grey seals

Consistent, widespread fine-scale preferences for breeding sites may favour the emergence of intrasexual associations and hence sociality. Pairwise association was defined for a terrestrially breeding phocid seal in which intraseasonal colony mixing is limited and mothers spend only 3 weeks ashore breeding. Interannual associations may occur between pairs of breeding female grey seals at North Rona, Scotland because of pupping site fidelity, 90% return rates and limited variability in parturition dates that individual mothers show. Associations can occur when mothers are found together irrespective of location, or because of joint preferences for locations. Mothers were sedentary and were unlikely to interact if they are separated by >20 m. Using spatial and temporal association criteria, 160 of 176 of known mothers had intraseasonal associations with at least one other mother, with 14 of 126 (11.1%) of associations repeated in 2 years. The likelihood of female association in 2 years as a result of site fidelity, parturition date variation and pupping site quality was modelled. Interannual association between mothers that changed their pupping sites by up to the median observed pupping site fidelity (<40 m) was indistinguishable from our models predictions. However, the number of mothers that showed interannual association after displacements of >40 m from their previous years pupping sites was almost five times greater than the model predicted. We conclude that active association between adult female grey seals is not determined solely by habitat preference, and argue that examination of simple sociality in temporarily aggregating species can contribute significantly to socioecological models of social evolution.Communicated by F. Trillmich