Proximate and ultimate causes of dispersal in vertebrates vary, and relative importance of these causes is poorly understood.  Inter- and intrasexual dispersal cues reduce inbreeding and mate competition, respectively.  Further, emigration cues may affect dispersal distance, as inbreeding avoidance dispersal is often farther than dispersal to reduce competition.   Although multiple causes of natal dispersal may exist within vertebrate populations, field-based studies examining this possibility are lacking, especially for large mammals.  We radio-marked 396 juvenile male white-tailed deer (Odocoileus virginianus) in 2 sites in Pennsylvania, monitoring natal dispersal probability and distance over a 3-year period when large-scale demographic manipulation decreased density of adult females and increased density of adult males.  Using a Kaplan-Meier survival model to estimate interval-specific dispersal probabilities, we found 95 - 97% of dispersal occurred during two 12-week periods:  spring, when yearling males still closely associate with related females, and preceding fall breeding season, when yearling males associate with other breeding-age males.  Following demographic changes that reduced potential for inbreeding (i.e., decreased density of adult females) and increased potential for mate competition (i.e., increased density of adult males), proportion of annual dispersal during spring decreased from 0.35 (± SE = 0.05) to 0.21 (± 0.03); proportion of annual dispersal in fall increased from 0.33 (± 0.09) to 0.48 (± 0.04); and in all years, 0.30 to 0.32 remained philopatric.  Spring dispersal distances (9.0 ± 0.6 km) were significantly greater than fall dispersal distances (5.1 ± 0.3 km), suggesting that inbreeding avoidance dispersal requires greater distance than mate competition dispersal  when opposite-sex relatives are philopatric and populations are not patchily distributed.  Thus, inbreeding avoidance and mate competition appeared ultimately to cause white-tailed deer dispersal but were proximately cued by different social mechanisms that elicited different responses.