Play for prey: do deer fawns play to develop species-typical antipredator tactics or to prepare for the unexpected?

Abstract

Animals ranging from mammals to fishes and even invertebrates play. Although this behaviour has been shown to improve the physical condition and survival of juveniles, we do not know exactly how these benefits are achieved. The motor training hypothesis suggests that play helps animals develop their motor skills. The self-handicapping hypothesis suggests that animals build cognitive and emotional skills to prepare for the unexpected, by using play as a way to practise losing and regaining postural control. We conducted focal observations and recorded videos of play to examine the specific form and timing of locomotor play in mule deer, Odocoileus hemionus, and white-tailed deer, Odocoileus virginianus, fawns, so as to test predictions associated with these two hypotheses. Consistent with the motor training hypothesis, play peaked early in life (≤3 weeks), which should coincide with development of the cerebellum and motor skills. These closely related species differ in antipredator tactics, so we predicted that they should display different motor patterns during play if it serves a motor training function. Although some characteristics reflected species-typical antipredator tactics (e.g. more ‘signal bounds’, leaps with long and high suspension, in white-tailed deer; more social play in mule deer), both species engaged in a similar amount of fast travel and similar rates of turns, traits that we expected to differ in line with their antipredator tactics. Consistent with the self-handicapping hypothesis, fawns of both species displayed high rates of nonfunctional manoeuvres, which were similar in form. However, these manoeuvres did not become more common as the fawns aged as expected if these help to develop the prefrontal cortex and cognitive skills. Our results suggest a refinement and blending of both hypotheses. Juveniles may play to develop similar motor skills rather than species-typical antipredator tactics, with nonfunctional manoeuvres to further promote the development of cognitive and motor skills during the early juvenile period.