The fear of unseen predators: ground squirrel tail flagging in the absence of snakes signals vigilance

Rather than fleeing from predators, many animals exhibit conspicuous displays that are thought to deter attacks by advertising quality (physical capabilities), or awareness of the predator. However, prey commonly exhibit the same antipredator display when predators are absent. Signaling in this context could either dishonestly indicate predator detection, or honestly indicate vigilance, but no study has tested these hypotheses experimentally. We tested the vigilance advertisement function of tail flagging, a signal given by California ground squirrels (Otospermophilus beecheyi) to rattlesnakes (Crotalus oreganus), by simulating snake strikes and quantifying strike avoidance behaviors in signaling and non-signaling squirrels. We removed the effect of snake presence on signaling by simulating strikes 1) in the presence of a rattlesnake, 2) with no snake present, or 3) in an area where squirrels had recently encountered a snake that was no longer present. Squirrels tail flagged most when interacting with a snake. Squirrels also signaled in areas of recent snake encounters, and in this context, they responded faster to simulated strikes and were more likely to leap vertically or horizontally away from the attack. Tail flagging rate was also related to squirrels’ body displacement time and flee modality. Squirrels rarely signaled or exhibited evasive leaping with no snake present. These results indicate that displays in the absence of predators can honestly signal vigilance, or the readiness to avoid an attack, even if predators are undetected. Such antipredator displays may be widespread as many species could benefit from advertising alertness when uncertain of the location of predators.