Video-recorded measures of blinking differ with group size in red deer Cervus elaphus and fallow deer Dama dama

The group vigilance hypothesis suggests animals within a group have a lower overall vigilance level than solitary individuals due to the collective responsibility of predator detection. Previous methods of studying vigilance have used head position as a proxy to indicate a state of vigilance, but it has been suggested that this method is too ambiguous, and the observer cannot accurately ascertain the exact behavioural state of the focal animal. Here, we consider blinking as a proxy for vigilance, as the eye-closure during a blink momentarily reduces the animals information about the environment, increasing the risk of unanticipated predator attack. We videoed semi-captive herds of red deer Cervus elaphus and fallow deer Dama dama, and quantified their blinking behaviour using behavioural logging software. We show that blink rate decreased with increasing group size in both species, suggesting that blinking may be tied with social vigilance rather than anti-predator vigilance. Similarly, the length of the inter-blink interval increased with increasing group size in red deer. We discuss the suitability of blinking measures for assessing vigilance behaviour.