Traditionally, the activity patterns of primates have been classified as nocturnal and diurnal, with the former recognized as the ancestral trait. Since cathemerality, i.e., active both day and night, was discovered in Eulemur (Lemuridae) in the 1960s, the evolutionary origin and mechanism of cathemerality have been explored as a key to understanding diurnalization in primates. To understand cathemerality in lemurs, this article reviews current knowledge and outlines future issues. Although several theories have hypothesized that cathemerality is an evolutionary disequilibrium condition, as a consequence of recent, incomplete diurnalization, the current analyses of phylogenetic history and eye morphology conclude that it is an adaptive strategy that originated in the common ancestor of Lemuridae around 46-20 MYA. In Madagascar, the days are generally long during the rainy season and short during the dry season. Cathemeral lemurs often increase their diurnal activities in the rainy season and nocturnal activities in the dry season. Chronobiological approaches that have explored the proximate mechanisms have clarified that the light-dark cycle controls the daily activity rhythms and the day-length cause seasonal shift of activity patterns as zeitgebers. In addition, moonlight has a masking effect that facilitates nocturnal activities. Ecological/ethological approaches have examined four hypotheses, as ultimate mechanisms: avoidance of predation risk, relief of interspecific competition, thermoregulation, and extension of feeding activities. However, there is evidence supporting and countering all four hypotheses. Therefore, cathemerality cannot be defined as an adaptive consequence of any single factor. Consequently, cathemerality is recognized as a flexible strategy for dealing with several factors in the harsh, unpredictable Madagascar environment. In the future, researchers need to examine flexible activities in response to other factors, such as habitat disturbance caused by humans, to explain complex mechanisms caused by compound factors, and to compare the activities of diurnal lemurs using ecological and physiological approaches.

Group-living primates are likely to spend a substantial amount of time grooming and resting with a small number of other group members. Such close and enduring relationships are regarded as affiliative. The properties of affiliative relationships are not fully understood and no consensus exists on how to quantitatively describe them. In this review, I explain the primate behaviors that are related to affiliative relationships and examine the means for using these behaviors to measure the relationship. Traditionally, affiliative relationships are defined by the frequency of proximity and grooming. Individuals with frequent proximity and grooming tend to perform altruistic behaviors for their partners without immediate return from them, groom each other in a reciprocal manner in the long-term, show distress and reconcile after agonistic interactions, and synchronize their behaviors with those of their partners. Thus, in addition to measuring the frequencies of proximity and grooming, these behavioral tendencies might be used as indices of affiliative relationships. Similarly, other questions concerning affiliative relationships remain unexplained. Some studies show that affiliative relationships increase reproductive success, but the mechanisms leading to fitness outcomes remain unclear. Although typically in primates, related individuals tend to form affiliative relationships, such relationships are also formed with unrelated individuals. Affiliative relationships could mitigate the negative effects of competition among individuals in large social groups and ensure that the individuals that form the relationship receive benefits from each other (e.g., agonistic support and collective mobbing). It is also unclear whether the affiliated relationships of nonhuman primates are equivalent to those of humans. Further research is necessary to elucidate similarities or differences in affiliative relationships between human and nonhuman primates.

We studied changes in the abundance of wild Japanese macaques (Macaca fuscata) in lowland Yakushima from 1991-1994 to 2013-2014. We quantified abundance based on the detection frequency during point observation. Data included 271 points censused between July and August from 1991 to 1994 and 58 points censused between July and September in 2013 and 2014. Abundance of Japanese macaques significantly decreased in northern and eastern areas, but not in the western and southern areas. Comparing the estimated population between 1991 and 1994 and the number of hunted macaques for pest control between 2007 and 2013, hunting pressure seems to be high in northern and eastern areas, low in southern areas, and absent in western areas. Conifer plantation is more widespread in eastern and northern areas, which may also explain the declining trend of the macaque population in these areas. The number of macaques hunted for pest control exceeded 1,000 each year following 2009 in Yakushima. Our results suggest that macaques are overhunted in Yakushima, so we recommend putting more emphasis on measures against crop-raiding other than pest control, in particular in eastern and northern areas.