Reproduction in female Atlantic walruses ( Odobenus rosmarus rosmarus ) from north‐west Greenland

Abstract The reproductive tracts of 152 female Atlantic walruses Odobenus rosmarus rosmarus were analysed. The material was collected between 19 January and 19 November in 1977–78 ( n = 44) and 1988–91 ( n = 108) from the subsistence hunt by Inuit in north‐west Greenland (i.e. in the eastern parts of the North Water polynya in northern Baffin Bay). Seventy‐six per cent of the walruses were sexually mature. Oestrus females ( n = 23) were found between 19 January and 25 June. Based upon ovarian anatomy, females may ovulate several times during oestrus but ovarian anatomy could not be used to determine if an individual was pregnant. The youngest female that had given birth was 5 years old, indicating that some female Atlantic walruses become sexually mature at 4 years of age. The oldest immature female was 8 years old, and the oldest animal that ovulated for the first time was 11 years. Age‐related growth in the reproductive tract indicated that sexual maturity was reached between 6 and 8 years of age. Based on reproductive history of 93 individuals (0–12 years old) the average age of attainment of sexual maturity (i.e. first ovulation) was 6.1 years (95% CI: 5.2–7.1 years). Of 81 mature females collected during May–November, 34.6% were pregnant, 43.2% postpartum and 22.2% inactive. These proportions did not differ from a model assuming a triennial reproductive cycle. Growth in foetal length and mass with time indicated that implantation occurred between 29 June and 11 July or between 26 June and 5 July, respectively. Mean birth date based on observations of 28 newborns was 20 June (range 4 February–11 November).