Ranging behavior of Mahale chimpanzees: a 16 year study

We have analyzed the ranging patterns of the Mimikire group (M group) of chimpanzees in the Mahale Mountains National Park, Tanzania. During 16 years, the chimpanzees moved over a total area of 25.2 or 27.4 km², as estimated by the grid-cell or minimum convex polygon (MCP) methods, respectively. Annually, the M group used an average of 18.4 km², or approximately 70 %, of the total home-range area. The chimpanzees had used 80 % of their total home range after 5 years and 95 % after 11 years. M group chimpanzees were observed more than half of the time in areas that composed only 15 % of their total home range. Thus, they typically moved over limited areas, visiting other parts of their range only occasionally. On average, the chimpanzees used 7.6 km² (in MCP) per month. Mean monthly range size was smallest at the end of the rainy season and largest at the end of the dry season, but there was much variability from year to year. The chimpanzees used many of the same areas every year when Saba comorensis fruits were abundant between August and January. In contrast, the chimpanzees used several different areas of their range in June. Here range overlap between years was relatively small. Over the 16 years of the study we found that the M group reduced their use of the northern part of their range and increased their frequency of visits to the eastern mountainous side of their home range. Changes in home-range size correlated positively with the number of adult females but not with the number of adult males. This finding does not support a prediction of the male-defended territory model proposed for some East African chimpanzee unit-groups.     

Behavioral seasonality in Mahale chimpanzees

To analyze how the chimpanzees in the Mahale Mountains National Park, Tanzania, change their grouping pattern, activity budget, travel speed, and travel distance within an annual cycle, I divided 1-year data into four periods. The Mahale chimpanzees have the behavioral flexibility to adapt to various climates and exhibited at least three behavioral seasons. In the early wet season, chimpanzees formed a few, large parties, and spent much time feeding on insects and animal meat. In the early and late dry seasons, chimpanzees maintained party sizes as large as in the early wet season, and traveled distances similar to the early wet season, but spent the most time feeding and traveling within the year. By contrast, in the late wet season chimpanzee parties broke up into more numerous, small groups, and traveled slowly over shorter distances. Although time spent feeding and traveling were the same as that in the early wet season, time spent feeding on terrestrial herbaceous vegetation (THV) was the highest in the year. The results suggest that chimpanzees travel longer, faster, and farther in seasons when they form large parties.     

Extraordinarily low bone mineral density in an old female chimpanzee (<Emphasis Type="Italic">Pan troglodytes schweinfurthii</Emphasis>) from the Mahale Mountains National Park

We examined bone mineral density (BMD) of the femoral neck and lumbar vertebrae of four chimpanzee skeletons from Mahale Mountains National Park, Tanzania, and four captive ones, with a dual energy X-ray absorptiometer. The BMD of Wansombo, an old female chimpanzee from Mahale , was remarkably lower than the mean of the other six younger adult female chimpanzees and categorized as osteoporosis. Posture, locomotion, and trunk-sacral anatomy of chimpanzees may have prevented fractures in Wansombo, whose BMD was below human osteoporosis criteria. Electronic Publication     

Altruists are trusted based on non-verbal cues

The identification of altruists based on non-verbal cues might offer a solution to the problem of subtle cheating. Previous studies have indicated that the ability to discriminate altruists from non-altruists emerges during evolution. However, behavioural differences with regard to social exchanges involving altruists and non-altruists have not been studied. We investigated differences in responses to videotaped altruists and non-altruists with the Faith Game. Participants tended to entrust real money to altruists more than to non-altruists, providing strong evidence that cognitive adaptations evolve as counter-strategies to subtle cheating.     

Mahale chimpanzees: grouping patterns and cycling females

The social system of chimpanzees (Pan troglodytes schweinfurthii) is characterized by the fission-fusion of social groups. Several studies have reported that females are less gregarious than males. In the current study, adult female gregariousness depended on their reproductive state. Noncycling adult females (pregnant, lactating, or post reproductive) were observed in large bisexual parties less often than cycling adult females. On the other hand, cycling adult females were observed in large bisexual parties as often as males, regardless of their estrous state. More males were in parties that included cycling adult females with maximal genital swelling (estrous females) than in parties without them. Moreover, a bisexual party including more estrous females contained more males. These results suggest that large bisexual parties of chimpanzees are constructed by a dual mechanism. First, cycling adult females are attracted to parties that consist of the top ranking male and large numbers of adult and adolescent males. Second, adult and adolescent males that did not belong to parties originally are attracted by estrous females and join them. Thus, in Mahale Mountains National Park, Tanzania, bisexual parties of chimpanzees can be characterized as "parties for reproduction."     

Changes in the activity budget of cycling female chimpanzees

This study is a preliminary report on the time allocated to various activities by female wild chimpanzees (Pan troglodytes schweinfurthii) during their sexual cycle. Cycling females with maximal tumescence (estrous females) tended to spend more time moving than cycling females with quiescent sexual skin (anestrous females). Although there was no statistically significant decrease in any specific activity that corresponded to the increase in time spent moving, feeding time did decrease in four of the five females. The frequency of approach by females toward males and the frequency of approach by males toward females significantly increased when females were in estrus. Direct aggression by males occurred more frequently toward estrous females than toward anestrous females. The copulation frequency and the frequency of approach to males was not significantly correlated with the increase in time spent moving. There was a high but not significant correlation between the time spent moving and the frequency of direct aggression by males toward females. Mating effort, feeding competition, male aggression, and other possible reasons that might explain the increase in moving time are discussed. Am. J. Primatol. 46:157–166, 1998. © 1998 Wiley-Liss, Inc.     

Activity budgets of wild female chimpanzees in different reproductive states

Activity budgets of wild female chimpanzees (Pan troglodytes schweinfurthii) when lactating were compared with those during estrus and anestrus. Five anestrous females, seven estrous females, and three lactating females in Mahale Mountains National Park, Tanzania, were the subject of focal sampling. Time spent for feeding, resting, and moving was examined. For each of these three categories of activity, there was no significant difference of time spent for each activity between lactating females and either estrous or anestrous females. The females spent about 30% of their daytime feeding, about 40% resting, and about 30% moving. One of the reasons for the lack of difference might be the type of parties that females attended when they were observed. In most cases, the females were seen in large bisexual parties. Although lactating females are expected to need more nutrition than cycling females and this might affect their activity budget, the party type they attended might also affect the budget. Received: January 17, 2000 / Accepted: June 22, 2000     

“Intentional” control of sound production found in leaf-clipping display of Mahale chimpanzees

This study reports three observed episodes involving leaf-clipping behavior of wild chimpanzees. In the first episode, an estrous female moved toward the source of a leaf-clipping sound. In the second, an estrous female escaped from a possessive male by intentionally controlling her production of sound. The third episode involved a male producing a leaf-clipping sound and then concealing the act when a dominant male approached. These episodes might suggest that chimpanzees can control the production of the sound intentionally, and understand that other chimpanzees also comprehend the causal relationship between the production of sounds and the occurrence of subsequent events.     

Facial resemblance of Japanese children to their parents

Facial resemblance between parents and their children could be an indicator of genetic relationship, and selective pressure could bias the resemblance of appearance. We assessed the degree of resemblance of 38 Japanese children (3–6 years old) to each of their parents using photographs. We asked nonrelatives to assess which of the parents each child resembled, manipulating indications of the sex of the children. Variance in the degree of resemblance between the children and their fathers was very large. Although the basic facial appearance of each parent can be reflected in each child with 50% probability, the children did not equally show the facial characteristics of each parent at the individual level. The indication of sex had no significant effect on the assessment of resemblance. On the other hand, a questionnaire given to the assessors revealed that, as children, they tended to be said to resemble the opposite-sex parent. This result indicates that alleged resemblance does not reflect an actual condition but rather might have cultural meaning. Electronic Publication     

Demography,female life history,and reproductive profiles among the chimpanzees of Mahale

Demography provides critical data to increase our understanding of the evolution, ecology, and conservation of primate populations. The chimpanzees of the Mahale Mountains National Park, Tanzania, have been studied for more than 34 yr on the basis of individual identification and standardized attendance records. From this long-term study, we derived the following demographic data: The major cause of death was disease (48%), followed by senescence (24%) and within-species aggression (16%). Fifty percent of Mahale chimpanzees died before weaning. The median ages of female life history variables were: first maximal swelling, 10.0 yr (n = 5); emigration, 11.0 yr (n = 11); and first birth, 13.1 yr (n = 5). The median period of adolescent infertility was 2.8 yr (n = 4) when calculated from the age at immigration to that at first birth. Female fecundity was highest between 20 and 35 yr of age, with an annual birth rate of 0.2. Twenty-six females that were observed from a young age (10-13 yr) to death at various ages (15-40 yr) gave birth to an average of 3.9 and weaned an average of 1.4 offspring. Twenty-five females that were observed from middle age (18-33 yr) to death in older age (31-48) gave birth to an average of 2.7 and weaned an average of 2.0 offspring. The post-reproductive lifespan for female chimpanzees was defined as the number of years that passed from the year when the last offspring was born to the year when the female died, minus 5. Twenty-five percent of old females had a post-reproductive lifespan. The interbirth interval after the birth of a son (x = 72 mo) tended to be longer than that after the birth of a daughter (x = 66 mo). The extent of female transfer, which is a rule in chimpanzees, is influenced by the size and composition of the unit group and size of the overall local community.