Table 3.

Definitions of play signals

Play signal	Definition
Crouch-and-stare	Animal’s ventral surface is on/near ground and its limbs are fixed, while maintaining visual fixation on partner (Symons, 1978).
Dangle-and-stare	Animal stares at partner while hanging from an object by hind limbs (Levy, 1979).
Gamboling	Bobbing, high stepping gait in which forequarters and hindquarters are alternately raised (Symons, 1978). Often accompanied by rotation of the head (Sade, 1973).
Hide-and-peek	Animal hides behind an object and then peeks at partner, alternating the two behavior patterns.
Leg-peek	Animal stares at partner through its legs with the top of its head against the ground (Symons, 1978). Animal may hold its ankles or place forearms on ground.
Look-back	Animal’s body is oriented away from partner in a fixed position on all fours, while the head is turned toward the partner over the shoulder (Levy, 1979; Symons, 1978).
Play face	Relaxed, open mouth face, typically observed during play bouts (Levy, 1979).
Roll-onto-back -and-stare	Animal rolls and lies on its back and stares at partner (Levy, 1979).
Play threat (candidate signal)	Animal directs a lunge <2 body lengths towards another individual, ending the movement by hitting the ground, without facial expression.
Slap-and-play face (candidate signal)	Animal hits another individual’s body while simultaneously directing an open mouth face towards the individual.

Adapted from Yanagi & Berman (2014b).