Eyeblink rate as an indicator of concentration on grooming in Japanese macaques (Macaca fuscata)

In primates, social relationships with a high frequency of social grooming are referred to as "affiliate relationships," "friendship," "bonding," and are described as involving positive emotion. However, the psychological state during social grooming has not been fully understood. In this study, I focused on blinking as a behavior that reflects psychological state during grooming in Macaca fuscata, a nonhuman wild primate, and examined for the first time whether the blink rate reflects concentration on social grooming. To test this hypothesis, I focused on ectoparasite removals during social grooming with the following three predictions: (1) blinks are synchronized with breakpoints (when monkeys can release visual attention) of mouthing ectoparasites during grooming, (2) the blink rate is lower during grooming (when requiring visual concentration) than during resting (when no visual concentration is required), and (3) the lower the blink rate during grooming (supposedly in high concentration), the higher the frequency of ectoparasites removals (outputs of the task). I recorded the faces of 10 wild female Japanese macaques on Kinkazan Island, Miyagi Prefecture, with video, while they were resting and grooming, and recorded the timing and frequency of eyeblink and ectoparasite removals. All three predictions were supported, suggesting that the blink rate during grooming can be an indicator of concentration on grooming in nonhuman primates. Whether grooming concentration is associated with an affiliative relationship with the grooming partner is an issue that should be carefully examined in future studies, but eyeblink studies may provide new perspectives for understanding the psychological state of individuals during grooming in a noninvasive manner.