Risky movements? Natal dispersal does not decrease survival of a large herbivore

Natal dispersal is assumed to be a particularly risky movement behavior as
individuals transfer, often long distances, from birth site to site of
potential first reproduction. Though, because this behavior persists in
populations, it is assumed that dispersal increases the fitness of
individuals despite the potential for increased risk of mortality. The
extent of dispersal risk, however, has rarely been tested, especially for
large mammals. Therefore, we aimed to test the relationship between
dispersal and survival for both males and females in a large herbivore.
Using a radio-transmittered sample of 398 juvenile male and 277 juvenile
female white-tailed deer (Odocoileus virginianus), we compared survival
rates of dispersers and non-dispersers. We predicted that dispersing deer
would experience greater overall mortality than philopatric deer due to
direct transfer-related risks (e.g., vehicular collision), indirect
immigration-related mortality attributable to colonization of unfamiliar
habitat, and increased over-winter mortality associated with energetic
costs of movement and unfamiliarity with recently colonized habitat. For
both male and female yearlings, survival rates of dispersers (male =
49.9%, female = 64.0%) did not differ from non-dispersers (male = 51.6%,
female = 70.7%). Only two individuals (both female) were killed by
vehicular collision during transfer, and over-winter survival patterns
were similar between the two groups. Although dispersal movement likely
incurs energetic costs on dispersers, these costs do not necessarily
translate to decreased survival. In many species, including white-tailed
deer, dispersal is likely condition-dependent, such that larger and
healthier individuals are more likely to disperse; therefore, costs
associated with dispersal are more likely to be borne successfully by
those individuals that do disperse. Whether low-risk dispersal of large
mammals is the rule or the exception will require additional research.
Further, future research is needed to evaluate non-survival
fitness-related costs and benefits of dispersal (e.g., increased
reproductive opportunities for dispersers).