Adult female survival, population trend, and the implications of early primiparity in a capital breeder, the southern elephant seal (Mirounga leonina)

Abstract

Adult female survival as a potential proximate factor responsible for observed changes in southern elephant seal Mirounga leonina populations was investigated. We compared the survival rate estimates from mark-recapture data for female elephant seals from the Marion Island population (using program MARK) for two periods (pre-and post-decline) during the past 15 years and with estimates from another population in southern Argentina, which had increased steadily during the same period. Survival of prime-age adult females increased significantly by 6.2% during the latter part of the decline at Marion Island, and the survival of adult females at the colony in southern Argentina was 3.2% greater than at Marion Island after the stabilization. We thereby demonstrated the importance of adult female survival in population regulation and emphasized the importance of monitoring adult females in order to understand population changes in southern elephant seals. In addition, we investigated whether reproductive expenditure early on in life reduces future reproductive potential in the population at Marion Island. We did this by estimating and comparing future survival and breeding probabilities of females primiparous at different ages. The future annual survival and breeding probabilities of females breeding at a young age, was similar to those from females primiparous at an older age. There was also no reduced survival in the year following first breeding in young or older first time breeders. Reproductive expenditure in young primiparous females therefore did not entail future fitness costs relative to older primiparous females, and we found no evidence supporting the existence of various life-history strategies in terms of age of primiparity within a population of southern elephant seals.

Evidence for density dependent population regulation in southern elephant seals in the southern Indian Ocean

“…Pistorius et al (2004) found no significant age-dependent variation in survival of parous southern elephant seal females from our study population. Age-parameters were consequently not considered in our present analysis.…”

“…Similar to other sites (Guinet et al 1999;Slip & Burton 1999), the Marion Island (46°54'S, 37°45'E; located in the southern Indian Ocean, approximately 2180 km southeast of Cape Town, South Africa; Fig. 1) population has now stabilized (Pistorius et al 2004). An intensive demographic study initiated on the small population at Marion Island in 1983 has yielded consistent mark-recapture data.…”

“…Population estimates were taken from Pistorius et al (2004). We also considered a model with survival maintained as constant over the period that the population was stable (1994 onwards; Pistorius et al 2004) and over the period that the population was declining (prior to 1994).…”

“…Furthermore, adult female survival increased with the stabilization of the population (Pistorius et al 2004), whereas juvenile survival was independent of the change in population growth rate (see Fig. 1 for first-year survival estimates; .…”

“…Several research papers have stemmed from this long-term data series detailing temporal patterns in survival (e.g. Pistorius et al 1999b;McMahon et al 2003;Pistorius et al 2004) and fecundity (Pistorius et al 2001).…”