The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Weaning in the Virunga mountain gorilla (Gorilla beringei beringei) – factors causing variation in weaned age


1Biological Science, University of Chester, UK, 2Dian Fossey Gorilla Fund International

Saturday 1:30-1:45, 200ABC Add to calendar

Offspring age at the termination of direct nutritional support by the mother, here referred to as ‘weaned age’, is an important measure of maternal investment. Delayed weaning can reduce the mother’s future reproductive success, while premature weaning can reduce the offspring’s chances of survival and future reproductive success. Thus, weaned age is crucial in understanding female reproductive strategies. This study aimed to investigate the impact of offspring sex, group size, presence of silverbacks, as well as maternal age, rank, and parity on weaned age variation in the Virunga mountain gorillas. The status of nutritional independence in 69 offspring was monitored using suckling observations from 1973 to 2012. A Cox-regression was used to model weaned age and to examine its relationship with covariates. Preliminary analysis suggests that offspring in one-male groups are weaned at an earlier age than offspring in multi-male groups which may indicate a female strategy to reduce the higher risk of infanticide that is present in one-male groups. Offspring were also weaned earlier the older their mother which may reflect an overall increase in fertility with age. In addition, sons of high-ranking mothers were weaned earlier than daughters, consistent with previous findings on inter-birth interval and suckling frequency. The bimodal nature of weaned age distribution in mountain gorillas and the need of future studies on the impact of environmental factors on the timing of weaning will be discussed.

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