Population declines in the snowshoe hare and the role of stress

Every 10 years snowshoe hare populations across the boreal forest of North America go through a population cycle, culminating in a decline lasting 4 or more years. We tested the hypothesis that snowshoe hares during the decline are in poor condition and less able to respond to challenges in their environment by examining the stress response of male hares. Three groups from February and May, 1991 (the second year of the hare decline in the Yukon), were compared: baseline hares were collected to obtain resting hormone levels; control hares were wild animals caught at randomly placed sites; and fed hares were wild animals caught on supplementary fed areas. The latter two groups were sequentially bled to examine their response to dexamethasone (DEX) followed by adrenocorticotropic hormone (ACTH). Trapping and handling were stressful to the experimental hares as the initial blood levels of total and free cortisol levels were higher (especially in controls), testosterone levels were lower, and glucose levels were higher in experimental hares than in baseline hares. Control and fed hares showed similar total and free cortisol responses, falling to low levels after the DEX injection and increasing rapidly in response to the ACTH injection. However, control hares were in worse condition than fed hares as indicated by the higher free cortisol levels and lower maximum corticosteroid-binding capacity (MCBC) in control hares. In addition, though testosterone levels fell in both groups in response to DEX, only the fed hares showed a large, transitory increase 30 min after the ACTH injection. An unexpected finding was a dramatic increase in MCBC levels 30 min after the ACTH injection in both experimental groups, but it was more pronounced in the fed group. We conclude that the pituitary-adrenocortical feedback system in hares from declining populations is operating normally and that they should be able to cope with acute, short-term stressors, but that they are in poor condition and are exposed to higher levels of free cortisol than fed hares in good condition.