Effect of sex and ontogeny on the trophic ecology of Southern Ocean fur seals

Abstract

The biology of apex predators provides a valuable means of monitoring changes within the Southern Ocean. Knowledge of these animals is often incomplete as relatively little is known about the ecological roles of many marine predators within these ecosystems. They inhabit remote and poorly accessible habitats, making research difficult or nearly impossible. Two partially sympatric species of fur seals, Antarctic fur seals Arctocephalus gazella (AFS) and Sub-Antarctic fur seals A. tropicalis (SAFS), inhabit the Southern Ocean. Substantial differences exist in morphology and life history between the sexes of both species. Information on the influence of intrinsic factors such as sex, ontogeny and individual variation on their trophic ecology and their role as apex predators is limited. Certain parts of the populations (mostly adult breeding females) are more intensively studied than others, i.e., juveniles, sub-adults, adult males and non-breeding females. This study attempts to better understand the intra- and interspecific variation in the trophic ecology of Southern Ocean fur seals.

After death, the teeth of an animal provide a record of its life history. Species, sex and age class are necessary for it to be scientifically useful and we show that external morphological features of upper canine teeth can deliver this information. Upper canine teeth can be used to age AFS and SAFS. External annular ridge counts provide a quick and approximate age estimate, but need confirmation by internal dentinal growth layer group counts. Seven external upper canine tooth measurements are used to determine sex within species and to distinguish between the two species. The seven upper canine tooth measurements considered in this study are easily repeatable and could be used in other species as well. The island of origin cannot be determined, possibly due to the small sample sizes. This method provides for the use of upper canine teeth in determining age, sex and species for specimens of unknown provenance.

Fine-scale dentine sampling of annually deposited growth layer groups provides longitudinal data for the entire lifespan of an animal. Carbon and nitrogen stable isotopes are used to determine the effects of sex and ontogeny on the trophic ecology of both species. Little difference in both δ13C and δ15N is observed within species, between sexes within species, per age category or among age categories within species. Only SAFS neonates and pups exhibit significantly higher δ15N from sub-adults and adults. Considerable overlaps in δ13C between the sexes within species suggest foraging within isotopically similar areas. Isotopic niche width for δ13C is similar between species, but SAFS exhibits a larger niche for δ15N than AFS. AFS females appear to utilise a wider geographical area and trophic niche than AFS males, with the latter being more specialised on an individual and population level. SAFS females show less variability in foraging range compared to SAFS males, but potentially include a wider range of prey.

These results link with the biology of the two species. SAFS females are more restricted spatially due to longer lactation periods which necessitates their return to a dependent pup. AFS females wean their pups within four months of age and are able to travel further during the inter-lactational period. Females from both species are more limited in their ability to exploit the vertical oceanic environment since they are not able to dive as deep as males, possibly restricting their ability to access certain prey species.