Hare Trigger 
Toxins produced by heavily browsed plants may play a 
critical role in regulating the ten-year snowshoe hare cycle 
by John P. Bryant 
In the spring of 1911, the naturalist 
Ernest Thompson Seton was traveling 
north from Winnipeg, Manitoba, to be- 
gin exploration of the Slave River region 
in subarctic Canada. As seemingly end- 
less miles of dense black spruce forest 
and fire-scarred shrubland slid silently 
past his train window, he noted in his 
journal that “last year had been ... a 
year in which the ever-fluctuating popu- 
lation of Northern Hares had reached 
its maximum . . . nine-tenths of the 
bushes in sight . . . had been barked at 
snow level. . . . But the fact that we saw 
not one rabbit shows that the ‘plague’ 
had appeared, had run its course, and 
nearly exterminated the species in this 
particular region.” 
Thus Seton described the crash phase 
of the ten-year cycle of the snowshoe 
hare (Lepus americanus) — the most 
striking of the cyclic fluctuations that 
characterize grouse and small mammal 
populations of the North American and 
Eurasian subarctic regions. 
Seton and other naturalists observed 
long ago that during the peak phase of 
the cycle — when hares are most abun- 
dant — hare populations frequently ex- 
haust their winter food supply. Recent 
work has confirmed this observation and 
suggests that the key to what triggers 
the hares' decline may well lie in the 
interaction of the hares and their food. 
This interaction, however, appears to be 
more complex than a simple depletion of 
the hares’ winter browse supply. My 
research and that of my associates at the 
Institute of Arctic Biology at the Uni- 
versity of Alaska indicates that woody 
plants respond to severe hare browsing 
during the peak of the cycle by produc- 
ing toxic metabolites and that this re- 
sponse may play a significant role in the 
regulation of the hare cycle. 
Answers to the question of what 
drives the snowshoe hare cycle are far 
from uniform. Ecologists have been 
studying the cycle for more than fifty 
years, and most feel the problem has 
still not been satisfactorily resolved. 
Certain aspects of the cycle, however, 
are well established. Snowshoe hares 
have enormous potential for rapid in- 
crease in numbers (in favorable habi- 
tats, hundredfold increases have been 
recorded during a five-year period). At 
the low phase of the cycle, the hare 
population declines to vanishingly low 
numbers. In one area, for example, hare 
numbers were estimated at 2,000 per 
100 hectares during the peak of the 
cycle and as low as 20 per 1 00 hectares 
at the crash. The period of the cycle 
may vary from region to region and 
from cycle to cycle (it ranges from 8 to 
12 years, with about a 9.6 year mean), 
but within a region, hare populations 
rise and fall with a high degree of 
synchrony. 
Many candidates for the mechanism 
that drives the cycle have been pro- 
posed — from weather patterns and 
cyclic wildfires to increased aggression 
among the crowded hares and increased 
predation and disease. Several of these 
theories provide partial answers, but un- 
til the recent work on the interactions of 
the hares and vegetation, I believe none 
has pinpointed the factors that trigger 
the decline or determine the period of 
the cycle. 
Weather, for example, may perturb 
the cycle, and thus modify the ampli- 
tude of its oscillations, but it does not 
appear to cause it. Detailed studies have 
not revealed a simple, consistent relation 
between boreal forest weather patterns 
and the periodicity of the cycle. 
The snowshoe hare gets its name 
from its large, hairy hind feet, 
which facilitate travel across 
snow. Its powerful hind legs enable 
the hare to flee from danger with 
long leaps and considerable speed. 
Pat Powell. Aperture 
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