Award: OPP-1142108

The remains of thousands of seals are preserved in the McMurdo Dry Valleys and on the nearby beaches of the western Ross Sea in Antarctica. Specimens range from fur buried in gravel, to isolated bones, to mummies in different states of decomposition, to relatively 'fresh' carcasses exposed by glacial retreat. Fur samples more than 7000 year old have been found, but for mummies, specimens less than 2000 years old are more common. These remains provide an exceptional archive of biogeochemical and genetic data with which to study both changes in environmental conditions in the seas that surround Antarctica, and how animals in those seas responded to those changes. Through collections over two field seasons in the Dry Valley Region and nearby beaches, we assembled a large (>700 individual) collection of samples from four species: crabeater seals, Weddell seals, southern elephant seals, and leopard seals. Our primary tool for determining the age of carcasses was radiocarbon dating. To bolster our chronology, we explored if the weathering and breakdown of carcasses might provide a rough guide to specimen age. A carcass breakdown index did distinguish relatively young specimens (<500 years old) from older ones (>1000 years old). As expected, bone weathering in this frigid, hyper-arid setting was much slower than at lower latitudes. Yet bone weathering stage did not vary consistently with specimen age, perhaps because different bones from a carcass begin to experience the full brunt of the elements at different times, depending on when they are exposed as carcasses breakdown. Various lines of geomorphic, biological, and paleoceanographic data demonstrate that the Ross Sea had less land fast ice and perhaps less persistent sea ice than at present from about 8000 to 1000 years ago. Isotopic data from bulk tissues (bones and fur) and individual amino acids, as well as analysis of ancient mitochondrial and nuclear DNA, were used to explore how diet, habitat use, and population size for each species shifted as the Ross Sea environment changed over the Holocene. Isotopic data from seals suggest that there has been a baseline ecosystem shift in the Ross Sea over the last 1000 years, perhaps related to the increase in land fast and sea ice. The Ross Sea seems to have been more productive (or at least to have had more complete nutrient utilization) when less icy from the early Holocene to about 1000 years ago. During the prior baseline state, the Ross Sea large mammal community had a different composition. Elephant seals were present on the less icy beaches. As with modern elephant seals, foraging behavior varied among ancient individuals. Some animals foraged at high trophic levels on the Ross Sea continental shelf; others foraged off the shelf, far to the north in the Antarctic Convergence. Most Weddell seals foraged on the Ross Sea shelf on high trophic level prey. Leopard seals fed on high trophic level prey, albeit somewhat lower than Weddell seals. Crabeater seals had two different behaviors – some animals foraged in pack ice on low trophic level resources (as is the case today); others foraged on high trophic level resources on the continental shelf. Extended analyses of ancient genomic data are revealing a dramatic expansion of Weddell, crabeater, and leopard seal populations over the course of the Holocene. For Weddell seals, there seems to have been a relatively constant population size through much of the Pleistocene that was an order of magnitude smaller than the modern population. For the crabeater seals, the expansion seems to have extended throughout the Pleistocene, but most rapidly in the Holocene. For leopard seals, the expansion is stepped and more recent than in the other two species. Southern elephant seals show a very different pattern. They established an independent Antarctic population that grew to large numbers during the less icy period from 8000 to 1000 years ago. With the return of more icy condtions, genetic data suggest a decline in population size, and today the Antarctic colony has been extirpated. Understanding the ecological and demographic responses of Antarctic seals to environmental change will contribute to the more effective management of populations as they respond to anthropogenic climate change. The substantial changes in geographic range, habitat use, foraging behavior, and demography, especially in elephant seals and crabeater seals, reveal a sensitivity and adaptive capacity to environmental change that may be important as they confront rapid ecosystem change in the future. Last Modified: 02/22/2018 Submitted by: Paul L Koch