The quantitative and demographic features of infant-corpse-carrying behavior in Japanese macaques (Macaca fuscata) at Takasakiyama, southern Japan, have been studied over 24 years. More than 91% of the dead infants that were carried by their mothers were abandoned within a week. Mothers of all age classes exhibited this behavior and neither the carrying rate (number of carriers/number of deaths) nor the duration were significantly different between young and older mothers. The sex of the infant was not a decisive factor. Nearly 80% of all cases observed involved infants that had died within 30 days of birth. The oldest infant whose corpse was observed being carried had died at 253 days. The overall carrying rate was 15% when death had occurred within 253 days and 28.7% for infants that died within 30 days of birth. Most mothers whose infants had lived for more than a month abandoned the corpse soon after death. Some females persist in exhibiting behaviors performed towards live infants but the exact reasons for this are unclear at present.

In a wild-living, artificially provisioned population of Japanese macaques at Takasakiyama in southern Japan, nine sets of twins were recorded from 12,392 known deliveries over a 56-year study period. Recorded twinning frequency was 0.073%. During the first 28-year period, artificial food was given until macaques were satiated and population size increased rapidly. In the second 28-year period, provisioned food was restricted to about half of the former period's calorific content. Seven sets of twins were born in the first period and two sets in the second. Twining frequency in the two periods was 0.137 and 0.027%, respectively. In comparing studies of other Catarrhine primate samples, we hypothesize that twinning frequency is influenced by living conditions, and we suggest that living conditions should be carefully evaluated in studies of twinning frequency.

It has been suggested that the physical condition of a mother may affect her pattern of investment in her sons or daughters. In addition, when competition over local resources becomes intense, severe aggression among the philopatric sex leads to a higher mortality of the sex and a biased birth sex ratio toward the opposite sex in the low rank. Parental sex-biased investment has been studied in many animal species to test these hypotheses, but the results have been highly inconsistent. We examined maternal sex-biased investment in relation to dominance rank using data on infant growth, infant mortality, birth sex ratio, and delay of subsequent reproduction by rearing current offspring from a provisioned, free-ranging Japanese macaque (Macaca fuscata) troop at Takasakiyama, Japan. The results showed that there was no sex difference in infant body mass among offspring of high-and low-ranking females. Use of the logistic regression model to analyze infant mortality with several independent variables failed to show a statistically significant sex bias. Birth sex ratio did not differ significantly between high-and low-ranking females. Among high-ranking females, there was no significant difference in delivery rate in the next year between those that reared a son and those that reared a daughter. For low-ranking females, however, the delivery rate after rearing daughters was markedly lower than that after rearing sons. Thus, there was no evidence of statistically significant maternal male-biased investment. For low-ranking females, we found a delay in subsequent reproduction for mothers after rearing daughters and no sex difference in offspring mortality. These results suggest that the females increased their offspring's chances of survival, irrespective of sex, by postponing their subsequent delivery.

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