Reproductive morphology and status of female Hawaiian monk seals (Monachus schauinslandi) fatally injured by adult male seals.

Abstract

Female Hawaiian monk seals at Laysan Island in the Northwestern Hawaiian Islands seasonally risk aggressive mating attempts by groups of adult male monk seals. These attacks, which also target immature female and male seals at a lower frequency, result in injuries that are often fatal and are termed mobbings. This study was undertaken to assess the reproductive status of nine female seals that died after mobbing attacks and to obtain basic morphological data of reproductive tracts from ten females. Reproductive morphology of the seals indicated that the lengths of the uterine body and both uterine horns were significantly shorter in nulliparous than in parous seals. Seven of the nine seals were periovulatory, on the basis of gross morphology of the ovaries at death. The ovaries of the other two seals possessed immature follicles. Histological studies of the vagina and uterus confirmed the reproductive status of the seals. When the reproductive status at the time of first injury was estimated, all seals were in the follicular phase of the oestrous cycle. At least four of these seals were estimated to be in oestrus at the time of their first injury, and seven of the seals sustained at least one injury during the estimated period of oestrus (2-6 days). These results support the hypothesis that most adult female Hawaiian monk seals that die following an attack by male monk seals are periovulatory, and that the majority of the attacks occur during oestrus.