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Weaned age variation in the Virunga mountain gorillas (Gorilla beringei beringei): influential factors

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Abstract

Weaning marks an important milestone during life history in mammals indicating nutritional independence from the mother. Age at weaning is a key measure of maternal investment and care, affecting female reproductive rates, offspring survival, and, ultimately, the viability of a population. Factors explaining weaned age variation in the endangered mountain gorilla are not yet well understood. This study investigated the impact of group size; group type (one-male versus multimale); offspring sex; and maternal age, rank, and parity on weaned age variation in the Virunga mountain gorilla population. The status of nutritional independence was established in 69 offspring using long-term suckling observations. A Cox-regression with mixed effects was applied to model weaned age and its relationship with covariates. Findings indicate that offspring in one-male groups are more likely to be weaned earlier than offspring in multimale groups, which may reflect a female reproductive strategy to reduce higher risk of infanticide in one-male groups. Inferior milk production capacity and conflicting resource allocation between their own and offspring growth may explain later weaning in primiparous mothers compared to multiparous mothers. Sex-biased weaned age related to maternal condition defined by parity, rank, and maternal age will be discussed in the light of the Trivers-Willard hypothesis. Long-term demographic records revealed no disadvantage of early weaning for mother or offspring. Population growth and two peaks in weaned age within the Virunga population encourage future studies on the potential impact of bamboo shoots as a weaning food and other environmental factors on weaning.

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Acknowledgments

We thank the Rwandan government and the Rwandan Development Board for their long-term support of the research and protection activities of the Dian Fossey Gorilla Fund’s Karisoke Research Center. This work was funded by a University of Chester Gladstone fellowship. Data derived from AF was funded and supported by University of Bristol and Stephen Harris. We are very grateful to Kelly Stewart for her valuable contribution to the weaned age dataset. We are indebted to all Karisoke research and field assistants who contributed to the data collection. We also thank Damien Caillaud and Donal Bisanzio for their advice during the process of statistical analysis. We would like to extend special thanks to both the anonymous reviewers and David Watts for their valuable contributions which improved this article significantly.

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Authors and Affiliations

  1. Department of Biological Science, University of Chester, Chester, CH1 4BJ, UK

    Winnie Eckardt & Alison W. Fletcher

  2. The Dian Fossey Gorilla Fund International, Atlanta, GA, 30315, USA

    Winnie Eckardt & Katie Fawcett

Authors
  1. Winnie Eckardt
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  2. Katie Fawcett
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  3. Alison W. Fletcher
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Correspondence to Winnie Eckardt.

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Ethical standards

This study was funded by University of Chester Gladstone fellowship. We declare that the study complies with the current laws of the country in which it was conducted. All applicable international, national, and/or institutional guidelines for the care of animals were followed. This article does not contain any studies with human participants performed by any of the authors.

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Communicated by D. P. Watts

Significance statement

Female reproductive rates are key in determining the viability of endangered populations. Using suckling observations from the Virunga mountain gorilla population spanning nearly 40 years, we investigated factors affecting variation in age at weaning, a principal determinant of reproductive rate. Our findings provide evidence that females in one-male groups that have higher risk of infanticide wean offspring earlier than those in multimale groups, that there is differential investment in males depending on age and reproductive experience of the mother, and that we should focus more on the availability of easily digestible foods (here bamboo) in relation to age at weaning. Improved understanding of such influencing factors enables us to more readily predict future population dynamics of these great apes as they grow toward their carrying capacity with continued change in their environment.

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Eckardt, W., Fawcett, K. & Fletcher, A.W. Weaned age variation in the Virunga mountain gorillas (Gorilla beringei beringei): influential factors. Behav Ecol Sociobiol 70, 493–507 (2016). https://doi.org/10.1007/s00265-016-2066-6

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  • DOI: https://doi.org/10.1007/s00265-016-2066-6

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