Inter-annual breeding and body mass correlates in southern elephant seals

Abstract

The number of times an individual reproduces in its lifetime is a critical life history trait which influences lifetime reproductive success and conceivably survival. Females have to allocate finite resources to maximize reproduction and survival. Furthermore they can alter their reproductive behaviour to maximise productivity in response to energy demands. During their reproductive lifetime (~1 to 20 years) more than half of the female southern elephant seal (Mirounga leonina) population at Marion Island breed intermittently (IBFs) by skipping a breeding season at some stage during the course of their lives. During the course of this study (2006 - 2013), on average 5.5 ± 0.7% of adult breeding-age females are not breeding in any given year. We photogrammetrically weighed annual breeding females (ABF) at the beginning of the breeding season (n = 70), and at both the beginning and end of the annual moult (n = 44). Similarly IBFs were photogrammetrically weighed after skipping a breeding event, at the beginning of the breeding season (n = 23) and at the beginning and end of the annual moult (n = 8). IBFs that skipped reproduction, returned significantly larger in absolute mass (ABF = 471 ± 56kg; IBF = 568 ± 87kg) and body length (ABF = 2.321 ± 0.106; IBF = 2.506 ± 0.123) presumably having 'freed up' resources to invest in somatic growth. By abandoning reproduction in a specific season and investing in somatic maintenance and growth, IBF consequently insuring the production of a similar sized offspring to ABF in the following year. By avoiding reproduction intermittently IBF's potentially survive to an extended reproductive age.