Dominance rank differences in the energy intake and expenditure of female Bwindi mountain gorillas
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Socioecological models predict that contest competition for clumped foods can lead to higher energy intake and lower energy expenditure for higher-ranking individuals. Here, we examine the relationships between dominance rank and energy intake and expenditure of female mountain gorillas in Bwindi Impenetrable National Park, Uganda (Gorilla beringei beringei). Bwindi gorillas have weak dominance relationships, feed on nonreproductive plant parts throughout the year, and consume fruit when it is seasonally available. We used behavioral observations on one group of gorillas and nutritional analysis of their major food items to calculate energy intake rates and estimated energy expenditure. Using linear mixed models, we found a significant positive relationship between dominance rank and energy intake rates, due to higher-ranking females having faster ingestion rates, rather than consuming foods with higher energy concentrations. Lower-ranking females did not spend significantly more time feeding to compensate for their lower energy intake rates. Lower-ranking females spent significantly more time traveling than higher-ranking females, leading to a negative relationship between dominance rank and energy expenditure. The combined results revealed a significant positive relationship between dominance rank and energy balance. Higher-ranking females did not spend longer feeding on fruit than lower-ranking ones, and the relationship between dominance rank and energy intake rates was not stronger when fruit was available. According to socioecological models, these results suggest that contest competition may be occurring with both fruit and nonreproductive plant parts, which would be consistent with growing evidence that nonreproductive plant parts can be contestable.
KeywordsDominance rank Energy balance Energy intake Energy expenditure Mountain gorillas
We thank the Ugandan Wildlife Authority and the Ugandan National Council of Science and Technology for permission to conduct research in Bwindi Impenetrable National Park. The Institute for Tropical Forest Conservation provided logistical support. We would like to thank all the field assistants for their dedication in the field, especially Dennis Musinguzi, Beda Turyananuka, Emmanuel Tibenda, Ngamganeza Caleb, Byaruhanga Gervasion, and Edson Nzabarinda. Robert Barigira assisted in plant identification. We are grateful to Sylvia Ortmann and her lab for plant nutritional analysis and Jessica Ganas for her work on the nutritional analysis. We also thank Roger Mundry and Colleen Stephens for statistical advice. We thank Christophe Boesch, Cyril Grueter, and Jessica Rothman for helpful discussion. This project was funded by the Max Planck Society.
This study complied with all Uganda Wildlife Authority regulations.
Conflict of interest
The authors declare that they have no conflict of interests.
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