Mummified seals reveal ecological impact of ice change

Scientists are using the mummified remains of seals freeze-dried in Antarctica to examine the long-term effects of changing ice patterns on marine mammal ecology. Recent work, presented this week at the Society of Vertebrate Paleontology meetings in Dallas, Texas, examined over five hundred seal mummies collected from the Ross Sea region. Findings suggest that while some species maintain a similar ecology in spite of environmental change, others underwent significant shifts in diet.

Over the last 7,500 years, the area surrounding the Ross Sea has undergone dramatic environmental change. Once an open body of water; a large, land-fast ice shelves began to form there around 1000 years ago, transforming living conditions for the . This has given paleontologists Paul Koch and Emily Brault, from the University of California at Santa Cruz, a unique opportunity to study the long-term impact of changing ice conditions on mammal populations. "Studies of fossils let us see how species do or don't adapt to environmental shifts. Here, we are using that approach to explore the adaptability and vulnerability of different Antarctic seal species to less icy conditions in the near future" Explains Koch.

To understand the effects of ice expansion on the seals, Koch and colleagues from Santa Cruz and the University of Maine amassed a vast collection of seal remains, which were well preserved by the cold, dry conditions in the Dry Valleys of Antarctica. The hoard totaled over five hundred mummified animals, including about 380 crabeater seals, and 170 Weddell seals. Koch estimated the age of the specimens using radiocarbon dating, and using the degree of weathering of the carcasses. Colleagues at the Durham University confirmed species identification with ancient DNA analysis. Finally, the structure of carbon and nitrogen atoms in their bones was used as a window into their ecology. Comparing the ratios of heavy and light forms of these elements, known as isotopes, provided an indication of where seals were hunting and what they were eating.

The isotopes suggest that the response to changing ice conditions varies between seal species. The crabeater seals, which most commonly feed on small plankton such as krill, showed very little isotopic change over the past 2500 years, indicating their diet likely remained unaltered. However, the Weddell seals, that have a mixed diet of fish, squid and crustaceans, underwent a shift in isotopic values around 500 years ago. Likely this isotopic shift reflects a combination of changing diet of the seals and changing ecosystems in the Ross Sea associated with the expansion of the land fast ice shelves.

"It's becoming clear that big changes in the Ross Sea ecosystem occurred at the end of the Holocene." Concludes Brault. "We are beginning to understand what that meant for the organisms that lived and died there. For example, primary productivity may have decreased significantly, and predators may have adapted their foraging strategies to a changing environment."

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