Historical dynamics of Leopard Seal (Hydrurga leptonyx) and Southern Elephant Seal (Mirounga leonina) populations in the Southern Ocean

Abstract

The history of the Pinnipeds living in Antarctic waters has been shaped by periods of climatic changes and anthropogenic impact which have affected their population dynamics. In this study, molecular genetic data were used to test hypotheses about the impact of environmental change and the mechanisms generating changes in the population dynamics of two Southern Ocean Pinniped species. The genetic diversity of contemporary Southern Elephant Seal populations was investigated in order to assess the demographic history and the degree of connectivity between the only continental colony and the three closest island colonies. For Leopard Seals, ancient DNA in comparison with modern samples provided even greater resolution on how the dynamics of a population changed in the Antarctic region through time and how this could be related to climate change. The Leopard Seal data provided an estimate of the whole mitochondrial genome mutation rate that was higher than previous phylogenetic estimates, but consistent with other estimates incorporating ancient DNA (including that calculated earlier for the Southern Elephant Seal). The Leopard Seal showed an expansion that occurred from 7,500 to 2,500 YBP, overlapping with two major periods of climatic change. For the Southern Elephant Seal, all sub-Antarctic island colonies could be considered as a single population, whereas the mainland population (the Argentinean colony) was genetically differentiated from the island colonies and had a significantly lower effective population size. The divergence of the continental colony from the island colonies occurred during the Holocene. Each species showed transitional changes during the Holocene, but while the Leopard Seal population expanded, the Southern Elephant Seal populations diverged, founding a new colony on the mainland. The broader implications for understanding historical biogeography in marine systems are discussed.