Competition and sex-age class alter the effects of group size on vigilance in white-tailed deer Odocoileus virginianus

Increased group size is predicted to dilute predation risk for individuals and increase predator detection at the group level. Individual vigilance tends to decrease with group size for many species. However, this pattern varies across species, context, space, and time. We explored the effects of group size on vigilance behaviors of white-tailed deer (Odocoileus virginianus) in relation to season, sex-age status, group composition, diminishing food returns, and intraspecific competition. We used classical behavioral observation methods and camera traps to study deer behavior. Sex-age class, season, intraspecific competition, and diminishing food returns interacted with group size to shape vigilance behaviors in deer. During spring, the effect of group size was essentially non-existent, and during winter, vigilance patterns exhibited a non-linear relationship with group size. Subadult deer benefited most in terms of increased foraging and decreased vigilance from the presence of 1–2 conspecifics, likely a maternal family group. This effect diminished in the presence of additional conspecifics (≥3), apparently as a function of contest competition. Individual deer spent less time at a site in areas with greater intraspecific abundance; however, in the presence of conspecifics, the relationship was reversed. Our research suggests that maternal family groups play an important seasonal role in vigilance behaviors of deer. Our study demonstrates the complex effects of group size in white-tailed deer. Group size effects are generally considered to be in response to changes in predation risk; however, our work supports a growing body of evidence that group size effects may also be influenced by intraspecific interactions.