Primates are unique among placental mammals in having trichromatic color vision, while most mammals possess dichromatic color vision. It has been hypothesized that the adaptive significance of trichromacy in primates is to detect reddish ripe fruits against a dappled foliage background. However, a behavioral advantage of trichromacy for fruit foraging has not been demonstrated in wild populations. The present paper reviews color vision status and utility of color vision in various primate species and describes recent advances in examining the significance of trichromacy. New World monkeys, which express high intraspecific color vision diversity due to an allelic polymorphism of the X-linked opsin gene, provide the excellent model to explore the significance of trichromacy for frugivory. The comparison of fruit foraging efficiency between dichromatic and trichromatic individuals in free-ranging black-handed spider monkeys (Ateles geoffroyi) inhabiting a Costa Rican dry forest did not reveal any trichromat advantage. This result is explained via the luminance contrast between fruits and background leaves, which cues the detection and selection of edible ripe fruits when fruits are viewed from close distances. In addition, sniffing behavior toward fruits was negatively correlated with luminance and blue-yellow contrasts, suggesting that monkeys use olfactory cues when vision alone is insufficient to select edible fruits. These results suggest that an advantage of trichromacy is not salient under natural conditions where many sensory cues are available. To understand the significance of trichromacy, it is necessary to evaluate how trichromacy benefits fruit detection over long distances. It is also important to observe various social feeding behaviors to examine alternate hypotheses, such as mutual benefit of association.

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Meat-eating and hunting behavior by blue monkeys (Cercopithecus mitis) was observed on several occasions in the Kalinzu Forest, Uganda. The monkeys ate three squirrels and a frog. Multiple members of a group comprising more than 10 individuals engaged in repeated simultaneous hunts on the ground for about 2 weeks following the first meat-eating episode. Blue monkeys in the Kalinzu Forest do not often eat meat; however, such behavior may occur intensively for short periods. It is possible that the initial predation provoked subsequent hunting attempts in the rest of the group. This "hunting craze" ceased after 2 weeks. The predatory tendency appeared to be motivated by some social factor, such as other members' success, because there was no evidence for the influence of an environmental factor.

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Laboratory of Physical Anthropology, Facul Muscular anatomy of the tail has rarely been studied in cercopithecoids. An important exception is Howell & Straus’ (1965) work on the rhesus macaque. However, tail muscular anatomy is only briefly described in their comprehensive work, which covers the whole body musculature. I investigated the origins and insertions of caudal musculature in five cadavers of the Formosan rock macaque (Macaca cyclopis) and two cadavers of the rhesus macaque (Macaca mulatta). The results of the rhesus macaque examination showed two major differences from the description of Howell & Straus (1965). In the present study, M. flexor caudae brevis was absent, and MM. iliocaudalis and pubocaudalis were found to insert more proximally than previously reported. The reason for these differences is not clear. However, intra-specific variation in tail length among regional populations of rhesus macaques may deserve consideration. Although the Formosan rock macaque and rhesus macaque do not show any significant differences in the origins of their tail muscles, they show great differences in the insertions of several muscles. Among the flexors, the insertions of three muscles (M. iliocaudalis, M. pubocaudalis, M. ischiocaudalis) were more distal in the Formosan rock macaques. Among the extensors, M.abductor caudae lateralis inserted into an additional distal caudal vertebra, and two muscles (M. extensor caudae lateralis M. abductor caudae medialis) became tendinous at more distal levels [by 1∼3 segment(s)] in the Formosan rock macaque. More distal insertions in the Formosan rock macaque are not surprising given its greater tail length. However, it is worthy to note that the difference is more marked in the extensors compared to the flexors. This may suggest that the extensors have a greater functional role in tail use among cercopithecoids.

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When we say something is ‘complex,’ we may define its complexity in terms of 1) the number of its components, 2) the number of kinds of components, 3) the number of connections among components, 4) the number of kinds of connections, or 5) strata of such connections. This means that we can only define and treat complexity by decomposing the thing to its components. Contradictorily, however, the most evident characteristics of complex systems, such as society, are not usually simple sum of their components. When ‘social complexity’ of primates or other animals is discussed, its presupposed components are, in most cases, individuals. In this paper, I aim to introduce the current situation in primatology where ‘social complexity’ society is often reduced into sum of individual behaviors, and then I will raise a problem in relation to individuals and the society. By doing so, I would like to relativize the predominant standpoint that easily presumes ‘autonomous’ individuals in a society. In the predominant standpoint, infants are often measured as pre-social existence because they are usually attached to their mothers and do not often actively perform social behaviors such as grooming. Thus they are placed peripheral to the social network and are often said ‘dependent’ to their mothers. However, it is evident that even infants have various means to interact socially with other individuals. Their social interactions may be different from adult conspecifics to some extent, but they are not pre-social or incomplete beings. Although we are tempted to view society as a sum of autonomous individuals or their behaviors, society has its own autonomy. In addition to this, it is essential to understand society in the ‘historical’ current, in which individuals and their relationships are always embedded.

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I studied huddling groups of Japanese macaques (Macaca fuscata) in the Arashiyama E troop at the “Arashiyama Monkey Park Iwatayama” in Kyoto, central Japan. Japanese macaques made physical contact with other individuals and formed huddling groups when air temperatures were low. The 99-101 adult females and 26-36 adult males in the study troop formed 345 huddling groups during 42 scan samplings in the winter of 2001, and 376 huddling groups during 52 scan samplings in the winter of 2002. The average size of huddling groups was 2.34 (range: 2-7) individuals in 2001, and 2.31 (range: 2-6) individuals in 2002. There was no huddling group of two males. Females more frequently huddled with females than with males. Two maternal kin related females huddled more frequently than unrelated females did. Mother-daughter pairs huddled most frequently. Two individuals usually huddled ventrally-ventrally, ventrally-laterally, and ventrally-dorsally. The distribution of huddling group sizes shows that the approaching individuals did not choose a particular size of huddling. However, the approaching individuals chose locations where they simultaneously contacted with two individuals 1.5 times more frequently than locations where they contacted with only one individual. This choice made the shape of huddling groups triangular and diamond-shaped more frequently than expected. By decision making of each individual, specific patterns emerged in the shape, composition, and position of each individual in huddling groups. As well as huddling behaviors, two and more primate individuals were involved in various social interactions. During the interactions, primates make their decision based on complex cognitive mechanisms and non-linear functions, compete and cooperate with the same opponents in their troop, and predict and manipulate the opponent’s behavior. These traits in social interactions among primates might make their society more complex and interesting.

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Chimpanzees form a multi-male/multi-female unit-group and the members of the group usually split into temporary parties. They sometimes exchange long-distance calls, pant-hoots (PH) between parties and utter PH in chorus within a party. Although most studies on PH have focused on the sociobiological functions for vocalizers or the referential functions, such studies based on individualistic or anthropocentric viewpoint do not tell us how chimpanzees interact with others by using PH. This study aims to analyze how the chimpanzees of Mahale connect their actions through PH and how they organize their social fields beyond visual contact with involvement of others in sight, by applying ethnomethodology and focusing on hearers’ actions. Case analyses based on the result of their usual interval for exchanging PH (10 sec.) revealed that both vocalizers and hearers practiced the same “call-answer” form. Utilizing this form, not only vocalizers could construct an auditory social field between parties but also non-answering hearers could observe the field. Even if PH hearers headed to the place from which they heard the PH, they sometimes restrained themselves from answering to hear whether or not the answer was uttered by other parties, and anew uttered PH to elicit the voluntary answer from the party with whom they tried to interact. Meanwhile, PH hearers sometimes answered immediately regardless of the contexts in which the PH was uttered. Employing these two hands, they would organize their auditory social fields. Once two parties constructed their auditory social field by exchanging PH, one could wait for next call from another or they could get to meet visually with repeating PH exchanges. On the other hand, PH hearers sometimes terminated or deferred those interactions, which hearers’ attitudes were supposed to generate the society in which the members could stay apart from each other even beyond PH distance. In conclusion, these hearers’ various actions show the sociality related to their dynamic fission-fusion society and engaging in non-referential interactions. When we say that chimpanzees’ society is complex, we may be often thinking of the process of interactions resulted from such their unique sociality.

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