Models of antipredator vigilance typically assume that individuals in groups scan for threats independently of one another.Recent theoretical developments indicate that group foragers should monitor the vigilance of companions and predict that vigilance should be synchronized when individuals who are less vigilant than their neighbours can be targeted preferentially.In the context of reducing the risk of being singled out,we predicted that synchronization should increase in smaller groups,in groups with proportionally more threatening conspecifics,and when individuals are closer together.We examined vigilance patternsin groups of Pere Davids deer (Elaphurusdavidianus) at the end of the rutting season.Overall,we documented a statistically significant synchronization of vigilance in this species.Level of synchronization increased in smaller groups and when inter-individual distances decreased but was not related to sex ratio,which was used as a proxy for the level conspecific threats.Results support the idea that synchronization of vigilance should be more prevalent in situations where failure to copy vigilance is more costly.The lack of effect of sex ratio probably reflects reduced sexual competition at the end of the rutting season.Our results document synchronization of vigilance in Pere Davids deer and highlight some biological factors that can modulate level of synchronization