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

Abstract

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.

Menopause occurs late in life in the captive chimpanzee (Pan troglodytes)

“…We used the definition of menopause proposed for women by Burger (1999); i.e., 12 months without menstrual cycles. While the application of the 12-month criterion to chimpanzees is somewhat arbitrary, we chose it rather than the average interbirth interval, used by others (e.g., Nishida et al 2003) because the latter criterion does not seem appropriate for captive apes with controlled breeding. Table 2 lists the chimpanzees included in this extended analysis, and indicates the name of each chimpanzee and "age of record," and whether the chimpanzee exhibited menopause ("1") or not ("0").…”

“…Our observations are consistent with recent data on production of live offspring in older female chimpanzees. For example, although fertility in wild chimpanzees declined steadily through adulthood, they gave birth as late as their 50th year-essentially up until the maximum life span in these populations (Nishida et al 2003;Emery Thompson et al 2007). Captive chimpanzees have also been documented to give birth to healthy infants at the advanced maternal age of 49 (Puschmann and Federer 2008) and 56 years (Ross 2009).…”

Female parity, male aggression, and the Challenge Hypothesis in wild chimpanzees

“…Nulliparas, however, are invariably recent immigrants who experience a prolonged period of infertility after moving into their new communities. As a result, these females cycle consistently, sometimes for several years, before giving birth for the first time (Goodall 1986;Boesch and Boesch-Achermann 2000;Nishida et al 2003;Sugiyama 2004). Because many cycles do not represent legitimate reproductive opportunities, males do not compete for nulliparas as vigorously as they do for older, parous females (Muller et al 2006).…”

“…Individual females mate during discrete estrous periods where they develop sexual swellings that last about 12-13 days (Furuichi and Hashimoto 2002). Moreover, females give birth only once every 5-6 years (Goodall 1986;Boesch and Boesch-Achermann 2000;Nishida et al 2003;Sugiyama 2004), leading to an operational sex ratio that is skewed heavily toward males (Emlen and Oring 1977). As a consequence, males compete vigorously to obtain mating and reproductive opportunities with estrous females who are available only rarely (Boesch et al 2006;Inoue et al 2008;Wroblewski et al 2009;Newton-Fisher et al 2010).…”

Reproductive parameters of female orangutans (Pongo pygmaeus wurmbii) 1971–2011, a 40-year study at Tanjung Puting National Park, Central Kalimantan, Indonesia

“…Capuchin females had significantly shorter IBIs during very wet periods, when there was a higher availability of food resources (Fedigan et al 2008). For chimpanzees (Pan troglodytes), female life history data from Taronga Park Zoo demonstrate that this naturally breeding population of captive chimpanzees had a significantly earlier mean age at first birth and a significantly shorter mean IBI than chimpanzees living in the wild at Mahale or Gombe (Nishida et al 2003;Littleton 2005). These differences are thought to be associated, in part, with differences in the nutritional status of the two populations (Littleton 2005); captive females at Taronga likely experienced better nutritional conditions than their wild conspecifics, and thus a higher rate of reproduction.…”