JOURNALS - CONSERVATION PHYSIOLOGY

JOURNALS J

SEARCHING THE PAST FOR ANSWERS TO THE FUTURE: AN INNOVATIVE APPROACH TO STUDY AN ARCTIC ICON

CONSERVATION PHYSIOLOGY BY KIM BIRNIE-GAUVIN

Department of Freshwater Fisheries and Ecology, National Institute of Aquatic Resources, Technical University of Denmark, Silkeborg 8600, Denmark

We’ve all heard of the walrus; with its enormous tusks, the walrus is an icon in the Arctic realm. Unfortunately, we’ve also all heard of the decreasing sea ice cover and the general decline of the Arctic habitat, which is of course highly concerning for the future of the walrus population. What is less clear however is how the physiology of walruses might be affected by changes in the Arctic ecosystem. Does the physiology of walruses today differ from that of walruses several hundred years ago, presumably before humans had significantly altered the Arctic? This is precisely what Charapata and team explored in their recent study.

Using walrus bones collected over a period of approximately 3651 years, the team analysed the concentration of four steroid hormones (progesterone, testosterone, estradiol, and cortisol) to track changes in reproduction and stress over more than 3 millennia. The bones of 281 walruses were collected from marine mammal collections of universities and museums, and grouped into one of three time periods: archaeological (3585 – 200 calendar years before present), historical (1880 – 2006) and modern (2014 – 2016). The bones were pulverized into powder, and hormone concentrations determined using liquid chromatography/ tandem mass spectrometry. Because the objective of this study was in part to establish whether the changing Arctic ecosystem is affecting stress and reproduction in walruses, the team obtained annual estimates of sea ice cover in the Chukchi Sea from data centres – based on analyses of whaling records or from microwave sensor data – to relate hormone concentrations to.

The authors found that cortisol concentrations, a stress indicator, were similar between modern and archaeological samples, despite a loss in sea ice cover, which may suggest that walruses have developed a sort of physiological resilience to current conditions in the Arctic. However, walruses seemed to have high cortisol in historical samples from the 1950s and 1960s. This is likely because there was an exponential increase in the walrus population during that period and reproduction can be rather stressful for females. This is also corroborated by the finding that modern walruses had much lower reproductive hormone concentrations than walruses from that period of rapid population increase, and may actually suggest that the modern walrus population is at a relatively high number, possibly even at carrying capacity.

In the last 15 years, walruses have begun to use beaches instead of sea ice haulouts, possibly in response to climate change. This has resulted in longer foraging trips, reduced foraging activity, changes in nutrition, lower survival of calves, increased encounters with predators, and the introduction of new diseases. These concerning trends have landed walruses on the Vulnerable List since 2016, though the current population trend remains unknown according to IUCN. This study is the first of its kind, but offers a promising avenue to monitor long-term changes in marine mammal physiology, particularly as summer sea ice cover may be gone by 2050, and other stressors are on the rise.

Original paper: Charapata, P., Horstmann, L., & Misarti, N. (2021). Steroid hormones in Pacific walrus bones collected over three millennia indicate physiological responses to changes in estimated population size and the environment. Conservation physiology, 9(1), coaa135.