POSTPARTUM DYNAMICS OF REPRODUCTIVE HORMONES IN GRAY AND HOODED SEALS

Abstract

The hormonal changes associated with lactation typically inhibit ovulation and estrus to prevent concurrent pregnancy and competition for maternal energy resources. However, some animals such as kangaroos and pinnipeds undergo estrus during lactation (Boyd 1991a, Muths and Hinds 1996). Females of the larger phocid species, such as the gray seal (Halichoerus grypus) and hooded seal (Cystophora cristata), fast throughout a brief lactation period (16 and 4 d, respectively), while producing large volumes (4 and 10 kg d , respectively; Iverson et al. 1993, Oftedal et al. 1993) of extremely fat-rich milk (40%–61%; Oftedal et al. 1988, Iverson et al. 1993). Females of these species wean their pups abruptly, typically within 24 h of mating (e.g., Boness and James 1979). Previous studies of lactating phocid females show a typical mammalian hormone cascade (e.g., Daniel 1981, Raeside and Ronald 1981, Boyd 1983, Reijnders 1990, Boyd 1991a, Pietraszek and Atkinson 1994), however these accounts have largely been limited to captive, sacrificed, or few animals. To further elucidate postpartum hormone dynamics in relation to lactation stage, we investigated individual longitudinal profiles of estradiol 17s and progesterone in free-ranging gray seal females throughout lactation and compared these to hooded seal females sampled both near parturition and weaning. Gray seal mother-pup pairs (197.0 6 6.02 kg) were studied during the reproductive seasons of January 1996 (n 1⁄4 6) and 1997 (n 1⁄4 12) on Sable Island, Nova Scotia, Canada (438559N, 608009W). Each female was captured on the day of parturition and a blood sample (10 ml) was immediately collected from the extradural vein. Females were weighed to the nearest 0.5 kg, marked, released, and similarly resampled on 5, 10, and 15 d postpartum (dpp), based on an assumed 16d lactation length (Boness et al. 1995). One female was sampled on 6 instead of 5 dpp due to a severe winter storm, but was included with the 5 dpp averages. Another female prematurely terminated lactation after 5 dpp and was not sampled further. Finally, two females weaned their pups overnight prior to 12 and 15 dpp and, although sampled at 15 dpp, were removed from reported 15 dpp averages.