Birth spacing patterns in humans and apes

Abstract

Comparative studies of birth interval dynamics in wild primates suffer from several problems of analysis and interpretation: (1) the data are always right-censored, (2) sample sizes are usually small, (3) the distribution of birth intervals is expected to be non-normal, (4) early offspring mortality is a confounding variable, and (5) differences in life history (e.g., presence or absence of menopause) can complicate interpretation of the results. A survival analysis designed to minimize these problems is applied to published data on wild chimpanzees and gorillas from Gombe and Virunga Parks, respectively, and to new data on wild orangutans from Tanjung Puting National Park and on a human population, the Gainj of highland Papua New Guinea. According to this analysis, the estimated median birth interval (when the offspring whose birth opens the interval does not die within the interval) is 43.3 +/- 1.0 months for the Gainj, 45.5 +/- 1.2 months for gorillas, 66.6 +/- 1.3 months for chimpanzees, and 92.6 +/- 2.4 months for orangutans.

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

“…For analysis of age at first reproduction, we used a MannWhitney U test to compare first-and second-generation We used a log-rank Mantel-Cox test to look for a significant difference in the IBIs of first-and second-generation mothers. We also used a log-rank Mantel-Cox test to look for a significant difference between the IBIs of the Camp Leakey sample as a whole (first-and second-generation mothers) and those of wild orangutans at Tanjung Puting National Park, as published by Galdikas and Wood (1990). The sex ratio at birth was analyzed by conducting a binomial test comparing the ratio of males to females in our sample with the expected ratio (0.5).…”

“…Wild orangutans (genus Pongo) have the slowest life histories of any primate taxon (Galdikas and Wood 1990;Knott 2001). With their long interbirth intervals (IBIs), late age at first reproduction, and long life expectancy, orangutans demonstrate extreme values on the spectrum of mammalian life histories (Galdikas and Wood 1990;Knott 2001;Wich et al 2004).…”

“…With their long interbirth intervals (IBIs), late age at first reproduction, and long life expectancy, orangutans demonstrate extreme values on the spectrum of mammalian life histories (Galdikas and Wood 1990;Knott 2001;Wich et al 2004). Studying these values and the ecological contexts within which they occur is useful for testing theories that seek to explain the evolution of life history patterns among primates and other mammals (Purvis and Harvey 1995;Kappeler and Pereira 2003;Sibly and Brown 2007).…”

“…Wild Sumatran orangutans, who live in more fruit-productive forests Wich et al 2011) and experience less nutritional stress (Wich et al 2006) relative to Bornean orangutans, have the latest ages at first reproduction and the longest IBIs of all orangutans (Wich et al 2004). Within Borneo, there is some evidence that Pongo pygmaeus morio populations experience the highest degree of nutritional stress and have the shortest IBIs Morrogh-Bernard et al 2009), while P. p. wurmbii experience a mid-range level of nutritional stress and have mid-range IBI lengths (Galdikas and Wood 1990;Knott 2001) relative to P. p. morio in the east and Sumatran orangutans. Wich et al (2004) argue that these intertaxon variations can be explained by the ecological life history hypothesis, which speculates that increased levels of high-quality food in orangutans' diets allow for reduced overall mortality, which thus leads to a slower life history and the allocation of energy towards growth and maintenance rather than reproduction.…”

“…Kuze et al (2008Kuze et al ( , 2012 showed that these orangutans demonstrated a low mean age at first reproduction, a low ratio of males to females at birth, and a high rate of infant mortality relative to wild orangutan populations, such as those found at Gunung Palung and Tanjung Puting (Galdikas and Wood 1990;Knott 2001;Knott et al 2009). Interestingly, however, Kuze et al (2008Kuze et al ( , 2012 found that the IBI of rehabilitated orangutans at Sepilok (6.2 years) was comparable to that of wild populations of the same subspecies (P. p. morio) at Kutai National Park (6.1 years, as reported in Wich et al 2004;.…”