The concept of net energy balance is familiar to all of us: we eat too many calories and we gain weight; we exercise a lot without changing caloric intake and we lose weight. Additionally, young people having a healthy and/or better diet will go through puberty and reach maturity at an earlier age than people whose lives require more energy expenditure and/or have lower caloric intake. Similarly, in wild animals, there is variation in what are referred to as 'life history variables', with this variation being driven by energetic trade-offs among growth, maintenance, and reproduction in differing ecological conditions. Namely, the more calories ingested and the fewer calories expended, the greater the net energy gain that an animal can devote to getting larger (or fatter) and to reproduction. When energy balance is positive, individuals can have faster growth, earlier reproduction, faster reproductive outputs, and larger adult body size.

Gorillas live in a wide variety of habitats and have large variation in their diets and distance moved per day (see table). For example, the well-studied mountain gorillas of the Virunga Volcanos live in a habitat with extremely high densities of herbaceous vegetation and almost no fruit. As a result, they can easily meet their nutritional needs and they do not move very much per day (low calorie expenditure). Their nearby neighbours in Bwindi Impenetrable National Park have slightly lower availability of herbaceous vegetation, their diet is about 15 % fruit, and they travel further per day than the Virunga mountain gorillas. There are even larger differences in food availability, diet, and movement patterns between western gorillas and mountain gorillas. Herbaceous vegetation occurs at much lower densities, the gorillas' diet is about 30 % fruit, resulting in longer travel distances per day.

Therefore, gorillas are interesting species to examine if differences in habitat, diet, and daily movement patterns lead to variation in reproductive parameters. In particular, we can predict that the Virunga mountain gorillas may have the greatest energy balance (net energy gain per calories expended), followed by Bwindi mountain gorillas, and then western gorillas having the least. However, this does not account for certain aspects of energy expenditure, namely the cost of climbing trees and thermoregulation (it is much colder in the Virunga Volcanoes than where western gorillas live).

To test these hypotheses, a study was conducted that compared life history traits for female gorillas from three populations: mountain gorillas in the Virunga Volcanoes (study groups monitored by the Dian Fossey Gorilla Fund), mountain gorillas in Bwindi Impenetrable National Park, and western gorillas observed at Mbeli Bai in the Republic of the Congo. Data from each study site was based on observations of over 300 gorillas spanning more than 25 years. Specifically, we compared age of dispersal from natal group, age of first reproduction, and interbirth interval.

Ecological, dietary, and life history variables for three populations of gorillas. From Robbins et al (2023) and references therein.

The results generally showed the earliest maturation and fastest reproductive rates for the Virunga mountain gorillas, followed by Bwindi mountain gorillas, and then the western gorillas at Mbeli Bai (see table). However, not all traits consistently varied along a fast-slow continuum. Bwindi mountain gorillas have interbirth intervals that are one year longer than Virunga mountain gorillas, which is consistent with a later age of weaning in Bwindi. However, the two populations shared a similar age at natal transfer, age at first birth and surviving birth rate. The Mbeli western gorillas had a later age at first birth than both mountain gorilla populations, but the interbirth interval for Mbeli was not different from Bwindi and it was longer than for the Virunga mountain gorillas. The age of natal transfers is the only trait that did not vary significantly among all three sites.

Those results suggest that there is more variation in traits that require meeting a threshold of energetic, physiological, or physical condition such as the age at first birth, interbirth intervals, and surviving birth rates. The results would suggest that the variance in energy supply has been highest for western gorillas at Mbeli, followed by mountain gorillas in Bwindi and then those in the Virungas. Those expectations are consistent with the differences in frugivory among the three populations, which has been used as a proxy for the variance in energy supply for gorillas.

This study is useful both for understanding how animals adapt to their environmental conditions as well as interspecific variation in these life history traits. Furthermore, these results indicate that a slower population growth rate would be expected for western gorillas compared to mountain gorillas and Bwindi gorillas would have a slower growth rate than Virunga gorillas. This can be estimated from the age of first reproduction, the rate of birth (interbirth intervals) combined with the last age of reproduction and infant mortality (which is roughly 30 % for both mountain gorilla populations and 50 % for Mbeli western gorillas). This has implications for conservation, in that populations with slower life history traits and growth rates would take longer to recover from a decline in population size. Lastly, this study emphasizes the value of long-term study sites for long-lived animals as well as the value of collaborations among different organizations.

Martha M. Robbins

Original article
Robbins, M. M., Akantorana, M., Arinaitwe, J., Breuer, T., Manguette, M., McFarlin, S., Meder, A., Parnell, R., Richardson, J. L., Stephan, C., Stokes, E. J., Stoinski, T. S., Vecellio, V. & Robbins, A. M. (2023): Comparative Life History Patterns of Female Gorillas. American Journal of Biological Anthropology 181, 564-674