Safety in numbers: The impacts of environment and demography on group size and vigilance in white-tailed deer

In response to the threat of predators, white-tailed deer (Odocoileus virginianus) alter their behavior to increase survival. Behaviors such as vigilance and group formation are potentially costly but can increase fitness if they help deer avoid predation. Our past research found that solo deer are more vigilant in open habitats and in areas with temporary human disturbance (THD). In this study, we tested whether group size, age, or sex affected vigilance. We also investigated whether group formation and group vigilance varied with habitat type or THD. We collected data from images captured by camera traps in Upstate South Carolina. Each observed deer was labeled vigilant if their head was higher than their back and not vigilant if their head was lower than their back. We hypothesized that adult deer would be more vigilant than juvenile deer and that females would be more vigilant than males. We also hypothesized that groups would be more vigilant than solo deer. Finally, we hypothesized that in groups, vigilance would be higher in more open habitats and in habitats with higher THD. In contrast to our hypotheses, we found that juvenile deer were significantly more vigilant than adult deer, and that males were significantly more vigilant than females. Juveniles were also more likely to be found in groups than adults, but males were in groups as often as female deer. As predicted, we found that groups of deer were significantly more vigilant than solo deer. Habitat type had no significant effect on the vigilance of groups; however, deer were more likely to be found in groups in open habitats. Deer that were observed in areas with high THD were significantly more vigilant than in areas with low THD; additionally, we found that deer were in groups more often in areas with higher human disturbance. Our results suggest that deer in groups may be less vulnerable to predation because they are more vigilant than solo deer and that group formation is most beneficial when deer are most vulnerable to predation. Furthermore, the benefits of vigilance may outweigh the costs for juveniles, as well as for males. Overall, vigilance and group formation in white-tailed deer are complex behaviors and influenced by many different environmental and demographic factors