Photogrammetric estimation of mass fluctuation in female southern elephant seals at Marion Island

Abstract

The broad objective of the study is to examine mass change as related to other life history parameters of southern elephant seals Mirounga leonina at Marion Island. It has been argued that mass plays a fundamental role in their annual life cycle. This study investigates the reproductive investment and strategies of female southern elephant seals over a temporal scale using innovative sampling methods.

Long term population studies are of great interest to life history studies as they provide a unique insight to advance our holistic knowledge of population demography. Marion Islands’ intensive 28-year elephant seal mark recapture program provides such a foundation for demographic analysis to further our knowledge of a top predator in a changing environment. Together with advances in the field of photogrammetric measuring methods that allow mass estimation of large marine mammals, the opportunities arise to study southern elephant seal mass fluctuations over a temporal scale. Analysis between populations provided validation of the comparability of the method to physically weighed animals, over a temporal scale. Furthermore, differences between populations could be assessed, which are most likely a result of demographic and/or anthropogenic disturbances.

Previous field advances in identifying mother pup relation, and the ongoing execution thereof presented the prospect of identifying driving factors in age specific fecundity. Together with an accurate mass measuring method that can be applied over a temporal scale, advances were made in understanding specific age related parameters in pup survival. Findings suggest that older females are more successful and their reproductive success is essential for population growth.

Annually interrupted breeding is more common than previously thought in female southern elephant seals at Marion Island. Assessment here, of body mass between females that exhibit different breeding strategies, strengthens this argument. Mass gain for annually breeding females’ is shown to be marginally sufficient to sustain them for their obligatory fast. These females are surviving on the edge in terms of body mass requirements. Females with interrupted breeding schedules have greater mass at critical stages in their annual life cycle, which could be beneficial to future reproduction and survival. Perturbations in energy budget may be a factor resulting in different breeding strategies.