Japanese macaques relax vigilance when surrounded by kin

Abstract

Vigilance behaviour in group-living animals helps to detect threats. Individual vigilance levels are thought to decrease as group size or the number of neighbours increases. However, the results of studies on the effects of group size or the number of neighbours are inconsistent, especially in primates. Since social vigilance (i.e. monitoring for group members) plays an essential role in group-living primates, discrepancies among studies may have been caused by differences in the quality of social relationships between group members. Using data from a provisioned group of Japanese macaques, Macaca fuscata , this study examined the effects of the quality of social relationships and the number of neighbours on vigilance levels. We found that the frequency of vigilance was lower when the majority of an individual's neighbours were kin than when kin were in the minority. Since Japanese macaques show strong nepotism and cope with social threats by forming coalitions between close kin, individuals may be able to reduce their vigilance levels by relying on the vigilance of potential allies. Furthermore, when the majority of neighbours were kin, the proportion of time spent on vigilance decreased as the number of neighbours increased. However, when kin were in the minority of neighbours, the number of neighbours did not affect vigilance levels. Overall, our results indicate that the quality of social relationships with neighbours and the number of neighbours interactively affect vigilance levels. • Several social factors affect individual vigilance levels in Japanese macaques. • Time spent vigilant decreased as the number of neighbours increased. • This effect of the number of neighbours exists only if the neighbours are kin. • Vigilance levels may be adjusted depending on the quality of social relationships.