Patterns in stress and reproductive hormones in salmon-eating grizzly and black bear populations

Background/Question/Methods

Stress and reproductive hormones provide valuable insight into life history trade-offs made under different physiological, environmental and social conditions. Though adaptive in some circumstances, chronically elevated steroid hormones such as cortisol and testosterone can negatively impact fitness. Hair, which integrates steroids over the period of hair growth, is emerging as a valuable tool for comparing long-term hormone levels within and among populations. Our investigation focuses on grizzly bears in coastal British Columbia (BC) where bears have evolved with abundant spawning salmon. Recent and dramatic declines in salmon might negatively affect bear health and ultimately fitness. Accordingly, we are investigating how availability of salmon, a spatially and temporally constrained resource, relates to hormone levels in hair. We collected hair samples from grizzly (n=37) and sympatric black bears (n=77) from hair snags across a 5000 km² area from 2009 to 2011. Additional samples were obtained from compulsory government sampling of hunted grizzly bears in coastal BC (n=38) and an interior population (n=42). We used genetics and stable isotope analysis to quantify the proportion of salmon in the diet of individual bears. Enzyme immunoassays were used to quantify testosterone, cortisol, and progesterone in hair.

Results/Conclusions

Testosterone was consistently higher in male compared with female grizzly bears (p=0.01); this trend was not evident in black bears (p=0.76). Across species, progesterone was higher in females than males (p<0.01). Progesterone was similar in coastal and interior female grizzly bears (p=0.68) but higher in coastal female black bears (p<0.01). Differences between sexes reflect expected endocrine trends; those between species might be due to differences in reproductive physiology or behaviour. Coastal grizzly bears ate significantly more salmon than interior bears (p<0.01) and had higher hair testosterone concentrations (p=0.01). Coastal male grizzly bears also showed more variable testosterone (p<0.01) and cortisol (p=0.048) compared with interior male grizzly bears. Cortisol and testosterone were correlated in black bears but not grizzly bears. These patterns in hormone levels and diet provide novel insight into physiological trade-offs that might be driven by resource availability and bear densities. Monitoring steroid levels in hair over time might provide insight into how populations respond to environmental change.