Evidence of polygyny from spatial patterns of hooded seals (Cystophora cristata)

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Abstract
Based on scant empirical data, the mating system of the hooded seal (Cystophora cristata) has been variously characterized as monogamous or polygynous. To evaluate the hypothesis that female hooded seals are clustered to a degree that would facilitate polygyny, we collected data on the spatial dispersion of female and male seals on the ice floes off the Labrador coast. While flying from a ship at the edge of the seal herd to a study site within the herd, we recorded each sighting of female seals as a "solitary female" or a "cluster of females" (using an approximate 10 body length radius to differentiate these conditions). The numbers of males near females were also recorded. Nearest-neighbor distances were obtained during on-ice transects. The frequency of nearest-female-neighbor distance classes peaked at 6–10 seal body lengths (one body length = 1.9–2.6 m) and then declined to distances of greater than 25 body lengths. About 40% of 357 females with pups (or 22% of 279 sightings of seals) were in clusters consisting of two or more mother–pup pairs; the maximal cluster size observed was five. The majority of females or clusters of females had a single male in attendance (54% of 245 sightings). Females in the central part of the herd were both clustered and attended by males more often than were females at the periphery. Observations of a few marked males suggested that some took up positions near additional females when their original female companions departed. These results are consistent with the hypothesis that the spatial pattern of hooded seals should facilitate polygyny.