Influence of a mother's dominance rank and her yearling's sex on the onset of the next reproductional cycle was studied in a free-ranging troop of Japanese macaques at Jigokudani Monkey Park, the Shiga Heights, Nagano Prefecture, Japan. Yearlings of lower ranking mothers were harassed more frequently by unrelated animals. No difference in the frequency of social interactions between a mother and her yearling was noted with regards to dominance rank or the offspring's sex. However, the onset of estrus in higher ranking mothers was earlier in the subsequent mating season. The present study suggests that local resource competition is not intense and aggression toward yearling macaques by unrelated animals does not affect interactions between a mother and her yearling. It is hypothesized that because higher ranking mothers were in good condition nutritionally, they exhibited estrus earlier.

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Teeth of primates must reduce the risk of the fracture under cyclic load during masticating various foods, through their lifespan. I examined the relationship between the shape of the molar teeth and both magnitude and distribution of stress under various loadings in African apes by finite elements stress analysis (FESA). I modelled the occlusal shape of the upper first molar of African apes (chimpanzee, bonobo and gorilla) in two-dimensional plane that dissects the tooth buccolingually passing the tips of two mesial cusps. The parameters of this simplified model for each African apes can effectively discriminate the species. The FESA was applied to the model.
Four types of load were applied to this two-dimensional model simulating mastication of the following four kinds of foodstuff: 1) large brittle 2) tender 3) soft and ductile 4) small brittle. No significant difference about the pattern of stress was observed in these three apes. However the maximum value of equivalent stress differed significantly. Under all type of loading, gorilla indicated the largest value of maximum equivalent stress. This result seems to reflect that the morphology of the gorilla's molar tooth is most specialised to “shear” fibre-rich diets among these African apes. Despite of the relatively large maximum equivalent stress, it is unlikely that gorilla has a high risk of the tooth fracture compared with chimpanzee and bonobo. Thus, it is suggested that the molar tooth of the African apes is designed highly safely against the mechanical loads during mastication.

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This paper reviews recent studies of reciprocal altruistic behaviours in non-human primates. Although altruistic behaviours such as alliances in agonistic interactions and social grooming are explained by the theories of kin selection or reciprocal altruism, evidence of reciprocity is scarce. Recent development of methods, however, may allow us to investigate reciprocity in quantitative and systematic ways. Model-based comparison of matrices of social interactions in a group are a powerful tool to investigate reciprocity at group level. Sequential analyses of social interactions reveal how monkeys do decision-making during the interactions with different partners in terms of reciprocity. Some computer simulations may give us an insight into false reciprocity in the case that human observers may assume the existence of reciprocity. It is also discussed whether reciprocal altruism could develop into true altruism which is reported exclusively in humans.

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鹿児島県屋久島の海岸線から1～2kmまでの地帯, 計127.26km²において, ヤクシマザル(Macaca fuscata yakui)の野生群の分布状態を調査した。その結果, 131群の存在が推定された。植生と群れの分布状況から, ヤクシマザルは自然の広葉樹林に強く依存する傾向が認められた。とくに自然植生が残っている西部海岸において群れの密度がもっとも高かった。センサスでは, 合計1,852頭のヤクシマザルをカウントした。推計では, 約2,000～3,850頭のサルが調査地域内に生息するものと思われる。

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A census of wild Yakushima macaque (Macaca fuscata yakui) troops was carried out in a 10.5km² area of 1, 020-1, 830m above sea level of Yakushima Island, Japan. In a 5km² area of the ecotone of broadleaf evergreen and conifer forests, troop sizes were larger (ca. 20 monkeys), and the home ranges were also larger. Troop density was about 0.7 troo ps/km². Some troops might have been provisioned by tourists. In a 5.5km² of conifer forest (1, 260-1, 830m a. s. l.), both troop size and home range size were smaller, and troop density was about 1.4-1.5tr oops/km². Fecal analysis suggests that they fed on more leaves of broadleave trees than the area of lower altitude.

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