Yesterday is Gone, Tomorrow has not yet Come – Compound-specific Stable Isotopes of Polar Bear Collagen over 2000 Years

Lara Horstmann1, Raphaela Stimmelmayr2, Matthew McCarthy3 1 College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, 2 Department of Wildlife Management, North Slope Borough, 3Institute of

Marine Sciences, University of California Santa Cruz

Photo: B. Rone Photo: R. Rockwell

Background

Methods We analyzed bone collagen of SBS bears obtained from subsistence harvests (2006-2016), University of Alaska Museum (1906-1971), and archeological digs (1850BP-1180BP) for bulk stable isotopes (SI), compound specific SI (CSI) of 12 amino acids (AA) using GC-IRMS (Fig. 2), fatty acids (FA) using GC-FID, and steroid hormones using LC/MS/MS.

Figure 1: Polar bear feeding on a beluga carcass on ice (left), and a caribou carcass on land (right). • Has dependence on terrestrial food webs increased in recent years? As a

result of diminishing sea ice habitat? • Do polar bears suffer energetic and physiological consequences by

turning to terrestrial prey?

We analyzed bone samples of Southern Beaufort Sea (SBS) polar bears over a 2000 year time period to answer questions about reliance on terrestrial food webs over time and physiological consequences of exploiting alternative prey.

Acknowledgements We thank the North Slope subsistence hunters, who graciously allowed us to sample their bears. We are indebted to the University of Alaska Museum Mammal Collection and A. Jensen for making historical and archeological polar bear samples available. This work would have not be possible without the assistance of J. Ruth, C. Heninger, E. VanDam, C. Clark, P. Charapata (UAF), T. Howe and M. Wooller (UAF-ASIF), S. Christensen (UCSC), B. Hagedorn (UAA-ASET), A. Jannash (Purdue University), and D. Ramey (NSB-DWM). This study was funded by CIAP, Grant F12AF01265. Samples have been analyzed under permit #MA 134907-1.

Figure 2: Chromatogram of an AA standard analyzed by GC-IRMS. AA identified in samples were Ala, Gly, Thr**, Ser, Val, Leu, Ile, Nle*, Pro, Asp, Glu, Phe, Lys).

Stable Isotopes Figure 3: SI ratios of polar bear bone collagen. Trophic level (δ15N) did not differ among time periods, while Suess corrected δ13C was significantly depleted in modern bears. This points to increased input of either terrestrial carbon or open-water phytoplankton in recent years.

Fatty Acids

Figure 4: PCA of carbon isotope ratios of 12 AA in polar bear collagen (94% of variation explained). Two groups emerge driven by a separation in PC1 due to differences in ∆13CGly-Phe and ∆13CVal-Phe. These isotopic proxies are effective discriminates of marine and terrestrial prey consumption (Webb et al. 2015), and are depleted in terrestrial consumers. About 50% of modern bears and one bear from 1970 show are distinct terrestrial signal. Three of these bears are also enriched in 15NThr, which is common in terrestrial food webs.

Figure 5: PCA of 60 FA in polar bear bone (88% of variability explained). Separation along PC1 is driven by differences in the relative proportions of the sum of omega-3 and omega-6 FA. Omega-3 FA are more abundant in the marine food web, while omega-6 FA are more common in terrestrial systems. Two of the bears identified as “terrestrial” (Fig.4, green triangle) also have unusual FA signatures.

Bone Cortisol

Figure 6: Validation of cortisol extracted from marine mammal bone (left) and isolated from modern, historic, and archeological polar bear bone (right). Mean bone cortisol concentrations are higher and more variable in bears identified as “marine” (Fig. 4) compared with “terrestrial” bears.

Terrestrial prey consumption by polar bears seems to increase, but without apparent elevation in stress response