Much sexual selection theory is based on the idea that ejaculate is cheap. Since further details are unknown our aim was to determine the energy that primate males require for ejaculate production. We addressed this problem by measuring the energy content (in kJ) of ejaculates from Japanese macaques (Macaca fuscata) using standard bomb calorimetry. Then, we estimated the relative amount of energy that individuals require for ejaculate production by relating the net energy content of ejaculates to males' daily basal metabolic rate (BMR). Fresh macaque ejaculate contains 3.0 kJ ml(-1). Assuming a mean volume of 2.7 ml an average macaque ejaculate contains 8.1 kJ. Depending on the individuals' body mass (6-13 kg) and the number and volume of the ejaculates, macaque males are assumed to use between at least 0.8% and at most 6.0% of their BMR for ejaculate production per day during the breeding season. Even when regarding only the minimal energy investment of 0.8% of daily BMR for ejaculate production, clearly ejaculates come with some cost for primate males.

We have conducted long-term research on sympatric gorillas and chimpanzees in Kahuzi-Biega National Park since 1987. The demographic history of habituated gorillas has provided insights into their reproductive strategies. Infanticide by male gorillas, which has occurred frequently in the Virunga mountain gorilla population, had not been reported in Kahuzi for more than 20 years. However, soon after the large-scale killing of gorillas during a war in the late 1990s, it occurred three times within a few months. The infanticidal male might have discriminated between infants who were not his offspring and an infant whom he presumably sired based on past interactions with their mothers. At Kahuzi, births occurred most frequently during the period of ripe fruit abundance, and female Grauer's gorillas show longer inter-birth interval than female mountain gorillas in the Virungas. A comparison of reproductive strategies among different gorilla populations suggests that seasonal fluctuation in food abundance may lead to slow reproduction, whereas the potential pressure of infanticide may promote rapid reproduction. The reduced ranges and increased encounters between unfamiliar groups induced by large human disturbance, such as wars or conversion of their habitat to farmland, might have produced conditions leading to infanticide.

Feeding conditions, competitive regime, and female social relationships of Japanese macaques (Macaca fuscata) on Yakushima were compared between the two habitats at two different altitudes (coniferous forest, 1,000-1,200 m and coastal forest, 0-200 m). Fruit availability was higher in the coastal forest. There was no consistent difference in the frequency of agonistic interactions within a group during feeding between the two habitats. The coastal forest evoked stronger inter-group contest competition compared to the coniferous forest as evidenced by a higher inter-group encounter rate and a higher proportion of aggressive encounters to non-aggressive ones. Birth rate was higher in larger groups compared to smaller ones in the coastal forest, but did not differ in the coniferous forest. In spite of these differences in competitive regime, no variation in female social relationships was observed, such as direction and concentration on particular individuals in grooming, linearity in dominance rank, counter-attack, and support of juvenile kin during agonistic interactions. The present results indicate that the female social relationships of Japanese macaques are robust and do not change according to changes in the current environment.

Methods for the identification of the sex and species of individuals from samples non-invasively taken from humans and gorillas were established. Amplification of a segment of amelogenin (AMG), which is an X-Y homologous gene, using two pairs of primers from human AMG, revealed both X- and Y-specific bands. The possibility of sex identification was examined by typing the AMG gene using hair and fecal samples from captive western lowland gorillas (Gorilla gorilla gorilla) in Japan and hair samples from wild eastern lowland gorillas (Gorilla beringei graueri) in the Kahuzi-Biega National Park, Democratic Republic of Congo, which were sexed by direct observation. Species-specific bands of AMG in gorillas and humans were identified by restriction fragment length polymorphisms analysis. These tests could be used for sexing unidentified individuals of wild western and eastern lowland gorillas, and screening contamination of human DNA from non-invasively acquired samples.

The mass mortality of wild Japanese macaques ( Macaca fuscata Blyth) was observed in a warm temperate forest of Yakushima, southern Japan. Demographic changes of eight troops between August 1998 and August 1999 were studied and 56% of macaques disappeared from the five intensively studied troops. Mortality varied among troops: two troops became extinct, while another troop did not decrease in size. The rate of mortality of the other troops was between 33 and 80%. The variation in mortality among the troops was either the outcome of local concentrations of mortality or of intertroop competition. The rate of mortality decreased with increasing distance from the two extinct troops and with increasing troop size; these two factors could not be separated statistically. The direct cause of death was diagnosed as pneumonia for four out of five fresh carcasses. The fleshy fruit production in autumn 1998 was the lowest in 14 years, and macaques had relied on leaves earlier than in usual years. It was exceptionally hot and dry in the summer of 1998. The exceptionally poor fruit production and hot summer of this year, with the resulting shortage of high-quality foods, was consistent with the scenario that mass mortality was due to the poor nutritional conditions. However, the possibility that epidemics caused the mass mortality cannot be ruled out. Our findings proved that primates in a seemingly stable habitat experience fluctuations in demographic parameters under natural conditions.