This project focuses on measuring behavioral responses of elk to the risk of predation by wolves, and
determining the consequences of behavioral responses for elk physiology, demography and population
dynamics.
Using custom-built GPS collars, we have collected 18,317 unbiased locations for adult elk in 4 drainages
of the Greater Yellowstone Ecosystem. These data reveal significant changes in use of the landscape on
days that wolves are present within a drainage, relative to days when wolves are absent. Changes in
landscape use are driven primarily by changes in the behavior of bulls, while cows show less response.
This sex-difference is as predicted, because analysis of 51 wolf kills on our site reveals that bulls and
calves are taken significantly more often than predicted if prey were selected at random, while cows are
taken less often than expected (x² = 39.21, df = 2, P < 0.001).
Herd sizes also shift on days that wolves are in a drainage, dropping significantly (F = 33.48, P < 0.0001)
from a 95% CI of 18.8 - 23.2 elk/herd in the absence of wolves to 7.9 - 13.4 elk/herd in the presence of
wolves. This suggests that grouping in response to predation risk does not benefit elk (for example via
the 'many eyes' effect). Instead, the bulk of our data suggest that behavioral responses by elk are aimed
at reducing the likelihood of being detected by wolves, rather than altering the outcome following
detection. This interpretation is reinforced by data on herd size in relation to distance to obstructive
cover. In general, herd size increases as distance to cover increases (ANOVA, F = 13.22, P < 0.0001).
However, there is a significant interaction between the distance to cover and the presence of wolves in
their effects on herd size. Aggregation far from cover occurs only when wolves are absent. When wolves are
present, elk remain in small herds at all distances to cover (interaction from ANOVA F = 4.73 P = 0.0028).
This pattern strongly suggests that aggregation on open (grass) plant communities is a foraging response
during periods of low predation risk, rather than an antipredator response during periods of high risk.
The behavior of individual elk also responds to the presence of wolves within a drainage. Using scan
sampling to record 10,642 observations, contingency tables reveal that the presence of wolves within a
drainage on the day of observation is associated with an increase in vigilance (x² = 113.78, df = 1, P < 0.001),
with a decrease in foraging (x² = 16.64, df = 1, P < 0.001) and a decrease in movement
(x² = 29.46, df = 1, P < 0.001). The proportion of time bedded did not change
significantly (x² = 0.10 , df = 1, NS), probably in part because this was not a common
behavior.
These data show that elk behave differently on days that wolves are present within their drainage, but
they do not demonstrate that behavioral responses carry costs. We are less advanced in analyses of the
costs of antipredator responses, but we have recorded reduced rates of pregnancy (61-85% for a low
predation site and 21% for a high predation site) for herds exposed to wolf predation, while pregnancy
rates remained high (84-100%) on two nearby sites with little wolf activity. Data from more years are
needed. Trends in population size (by aerial total count) and recruitment (from ground and aerial
classification counts) suggest that predation (both direct and indirect effects) may be altering elk
demography and dynamics. Recruitment in early winter has been at or below 20 calves:cow on 5 of 6 winters
with data since colonization by wolves, compared to 1 of 13 winters prior to wolf colonization (Fisher's
exact test, P = 0.12). Similarly, population size has been below 1500 elk in 6 of 7 winters since
wolf colonization, compared to 16 of 41 winters prior to wolves (Fisher's exact test, P = 0.17).
Recall that factors other than wolves contribute to these patterns.