The influence of snowfall, temperature and social relationships on sleeping clusters of Japanese monkeys during winter in Shiga Heights

Abstract

We studied Japanese monkeys (Macaca fuscata) of the Shiga A(1) troop at their sleeping sites in Shiga Heights, Japan, for 41 nights during 3 winters. Monkeys chose their sleeping sites in Japanese cedars and in deciduous broad-leaved forests on non-snowing nights and in Japanese cedar forests on snowing nights. We counted 399 sleeping clusters in which 2 or more monkeys remained in physical contact through the night and 43 solitary sleeping monkeys, though monkeys did not maintain physical contact with others in the daytime. We found 397 clusters on tree branches and 2 clusters on rocks. The mean size of huddling clusters was 3.06+/-1.22 SD. The cluster size (3.17+/-1.26 SD) at lower ambient temperatures between -7 and -4 degrees C was larger than that at higher temperatures between -2 and 4 degrees C (cluster size 2.88+/-1.13 SD). Most clusters were composed of kin. Females kept close to related females in the daytime and huddled with them at night. The highest-ranking male mainly huddled with his kin and his familiar females. Other males kept farther apart from each other in the daytime, probably to avoid social conflicts. Through cold winter nights, however, such males reduced inter-individual distances and huddled with other males. Japanese monkeys appear to recognize three types of inter-individual distances: an intimate distance less than 1 m, a personal distance of 1-3 m and a social distance of 3-20 m; they change their inter-individual distances according to social and ecological circumstances.

Coprophagy-related interspecific nocturnal interactions between Japanese macaques (Macaca fuscata yakui) and sika deer (Cervus nippon yakushimae)

“…However, all the sleeping clusters observed in this study were on the ground or on rocks and therefore vulnerable to disturbance by deer. Japanese macaques in the Shiga Heights, a snowy region, sleep in trees during the winter when the ground is covered with snow (Wada et al 2007). Although trees are available as potential sleeping sites, the Yakushima macaques do not use them for this purpose.…”

“…Although trees are available as potential sleeping sites, the Yakushima macaques do not use them for this purpose. This suggests that benefits of ground-sleeping for the Yakushima macaques, such as physical comfort in terms of thermoregulation by forming large clusters (Wada et al 2007), potentially outweigh the cost of disturbance by deer. Furthermore ground-sleeping seems to be safer than tree-sleeping, because falls do occur during the night (Anderson 2000).…”

“…A sleeping cluster of macaques was defined as a stable unit of macaques huddling while embracing or clinging together (Wada et al 2007). Because the number of individuals in the sleeping cluster occasionally changed during the night due to departures and arrivals, the size of the cluster was defined as the number of individuals counted when we observed interspecific interactions, or when the members of the cluster started moving in the morning if no interaction had been observed during the night.…”

“…Therefore, knowledge about their behavior in the vicinity of their sleeping site is important for understanding the diversity of primates' adaptations to their environment (see reviews by Anderson 1984Anderson , 1998Anderson , 2000. Previous studies on wild primates' sleep and sleeprelated behaviors at sleeping sites have mainly focused on their function regarding safety against predators (Hamilton 1982;Nakagawa 1999;Di Bitetti et al 2000;Ramakrishnan and Coss 2001), physical comfort (Stewart et al 2007;Wada et al 2007), and social behavior (Ansorge et al 1992;Takahashi 1997). These empirical studies emphasized that the ethology and ecology of sleep encompass several fundamental areas of behavioral biology (Anderson 1984).…”

Shape of, and body direction in, huddles of Japanese macaques (Macaca fuscata) in Arashiyama, Japan

“…Some primates conserve body heat via physical contact when the air is cold (Anderson 1998;Hanya et al 2007;Wada et al 2007). Because air temperatures are lower at night and diurnal primates have reduced body heat during the night (Honnebier et al 1992), these animals often huddle from evening until the next morning (Vessey 1973;Anderson and McGrew 1984;Ansorge et al 1992;Wada et al 2007).…”