Measuring the benefit of a risk-induced trait response: vigilance and survival probability

Defensive traits are hypothesized to benefit prey by reducing predation
risk from a focal predator but come at a cost to the fitness of the prey.
Variation in the expression of defensive traits is seen among individuals
within the same population, and in the same individual in response to
changes in the environment (i.e., phenotypically plastic responses). It is
the relative magnitude of the cost and benefit of the defensive trait
which underlies the defensive trait expression and its consequences to the
community. However, whereas the cost has received much attention in
ecological research, the benefit is seldom examined. Even in a defensive
trait as extensively studied as vigilance, there are few studies of the
purported benefit of the behavior, namely that vigilance enhances
survival. We examined if prey vigilance increased survival and quantified
that benefit in a natural system, with white-tailed deer (Odocoileus
virginianus) experiencing unmanipulated levels of predation risk from
Florida panther (Puma concolor coryi). Deer that spent more time vigilant
(as measured by head position using camera trap data) had a higher
probability of survival. Indeed, an individual deer that were vigilant 75%
of the time were more than three times as likely to be killed by panthers
over the course of a year compared to a deer that were vigilant 95% of the
time. Our results therefore show that within-population variation in the
expression of a defensive trait has profound consequences to the benefit
it confers. Our results provide empirical evidence supporting a long-held
but seldom tested hypothesis, that vigilance is a behavior that reduces
the probability of predation and quantified the benefit of this defensive
trait. Our work furthers an understanding of the net effects of a trait on
prey fitness and predator-prey interactions, within-population variation
in traits, and predation risk effects.