The enigma of the snowshoe hare population cycle: explaining the low phase through stress and maternal programming

• Main Author: Sheriff, Michael J.
• Language: English
• Published: University of British Columbia 2010
• Online Access: http://hdl.handle.net/2429/23369

The snowshoe hare cycle has been fundamental to the development of ecological theory for more than half a century. Though these cycles have been intensively studied for over 70 years it is still unknown what mechanism causes young born in the decline to have much lower fitness than those born in the increase and peak phase and why the hare population remains low for 2-5 years after the decline phase even though the predator populations have collapsed and there is ample vegetation. My doctoral studies investigated how the risk of predation affected maternal stress and whether this would result directly in a decline in reproduction and indirectly, through maternal effects, in a decline in offspring physiology and fitness. To study the impact of predator-induced maternal stress I used a natural monitoring study and an experimental manipulation. In the former, I examined hares throughout the cycle and found that their stress levels were directly related to predator pressure, being greatest during the decline. During the low phase stress levels remained elevated at levels similar to those found at the peak when reproduction starts to decline. Lastly, I found that the variation in the length of the low phase (2-5 years) was related to the rate of loss of hares during the decline phase. In the experimental manipulation, pregnant hares were exposed to a simulated predator for 1 min. every other day for the last third of gestation. I found that an increase in predator-induced maternal stress resulted in a decline in litter size, birth weight, and birth size. Furthermore, these offspring had a compromised stress-axis resulting in higher baseline stress levels and an enhanced stress response. This occurred both at weaning and when the offspring were adult size. My results show that hares are highly sensitive to predation risk and that maternal stress results in a decrease in reproduction and also compromises their offspring’s stress physiology. These results support the hypothesis that the low phase of the population cycle is the result of the impact of inter-generationally inherited maternal stress caused by the high risk of predation during the decline.