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Mountain gorilla reproduction slows with female transfers, study shows

  • Successful conservation interventions have helped mountain gorilla populations recover from 620 in 1989 to more than 1,000 today.
  • However, mountain gorilla habitat is not expanding, and a growing body of research indicates that increasing density comes with a price: More groups sharing territory leads to more frequent intergroup violence.
  • A new study finds that when intergroup contact increases, so do transfers of females between groups, leading to delayed reproduction.
  • The study also emphasizes that multiple factors, such as delayed reproduction and increased mortality, cascade to create a slowdown in population growth.

Mountain gorillas are currently the only nonhuman great ape whose population is not declining. Thanks to successful conservation interventions, the species’ population has increased from 620 in 1989 to more than 1,000 today, enabling the International Union for Conservation of Nature to change the species’ conservation status from critically endangered to endangered.

However, with mountain gorilla habitat confined to a few islands of protected area, a growing body of research shows that rising population numbers have come at a price.

Previous research showed that as their numbers increased and mountain gorillas (Gorilla beringei beringei) formed new groups within a fixed habitat, intergroup clashes became more frequent. That, in turn, correlated with a sharp increase in infanticide, higher mortality rates for adult males and a slowdown in population growth.

Now, a new study points to another factor dragging down population growth: When intergroup contact increases, so do transfers of females between groups, leading to delayed reproduction.

The study also emphasizes that all of these factors — higher encounter rates, more infant and male deaths, more female transfers — cascade. “All of this in combination means that the group fissions lead to higher mortality and slower reproduction resulting in a considerable slow in population growth,” Robin Morrison, postdoctoral researcher at the Dian Fossey Gorilla Fund and co-author of the study, told Mongabay by email.

Mother and baby mountain gorilla, part of the Sabyinyo group in Rwanda’s Volcanoes National Park. Increased infanticide as gorilla group density increases is a significant factor in slowing the species’ population growth. Image by Kwita Izina via Flickr (CC BY 2.0).

Slowed reproduction

Researchers analyzed detailed data on a subpopulation of mountain gorillas in Rwanda’s Volcanoes National Park that has been closely monitored by the Dian Fossey Gorilla Fund since 1967.

Using records on births, deaths, group composition and transfers, they examined the effects of social activities like group formation, breakups and encounters and their impacts on gorillas’ survival, reproduction and population growth rates. While this population grew rapidly between 1981 and 1993, the growth rate began to slow in 1994 and dropped further after 2007.

The study found that when groups split, especially if this happens in multiple nearby groups during a short time, this leads to higher group density, which leads to higher rates of intergroup encounters. At times, these encounters can be aggressive, leading to higher mortality in adult males and in infants who are frequently targeted by rival males.

The encounters also provide opportunities for females to transfer. This can happen when the only adult male of a group dies, or a female’s preferred male dies. High infant mortality may also prompt females to transfer groups, as it could mean the group’s dominant male cannot protect group members.

The researchers found that when females transfer they are slower to reproduce. A single transfer between groups increases a female’s interbirth interval by 7.5 months, while multiple transfers increase the interval between births by 1.5 years. (Females typically give birth around once every four years).

Further study is required to establish the precise reason female transfers result in delayed reproduction. Experts suggest it could be driven by a reduction in mating prior to transfer or during integration into a new group, although previous work suggests this is not the case in mountain gorillas. It could also be a result of prenatal loss, infertility or feticide.

The authors say the findings nonetheless show why it is important to understand factors that can influence rates of female transfers and the effects on population growth, for effective conservation of primates.

Mountain gorilla with baby, photographed in Rwanda. When females move between groups, reproduction is delayed, a recent study finds. Image by Ludovic Hirlimann via Flickr (CC BY 2.0).

Broader implications

Craig Stanford, professor of biological sciences at the University of Southern California, said the study provides lessons on what the natural population density and forest carrying capacity for gorillas should be, something that has been difficult to understand as a result of decades of poaching and habitat destruction.

“We are learning only now, as gorilla population density increases following years of effective conservation and protection from poaching, that there are important effects of increased population density on their population biology,” said Stanford, who was not part of the study.

The scale of data relied upon during the study makes the findings even more significant for conservation purposes, said Ian Colquhoun, associate professor of anthropology at the University of Western Ontario, Canada. “It was only efforts to compile long-term data on mountain gorillas that revealed the largely unexpected and unpredicted ‘downstream’ results and outcomes of mountain gorilla group density in a severely limited area of protected habitat.

“While an upper level of mountain gorilla population density in the Virunga mountain chain could have reasonably been predicted, the impact of this paper is the attention it draws to the multiple demographic ‘ripples’ that result from population growth and increased density of mountain gorilla social groups.”

Joanna Lambert, a conservation biologist and professor of animal ecology at University of Colorado Boulder, agreed.

“As conservation practitioners, a central goal is to increase total numbers of a species, but this must be done in concert with a second (arguably even more critical) central goal: protection of habitat,” she said.

“The results of this important study demonstrate unexpected impacts of increasing numbers of mountain gorillas — namely, that with more gorillas in a given area comes delayed female reproduction, higher male mortality and more infanticide. This is what happens when animal numbers increase in fragmented, degraded and declining habitat.”

Study co-author Morrison said while there should be no interference in the natural social dynamics of the population, something can be done to maximize the quality and size of the mountain gorilla’s available habitat.

“This research helps us understand what the consequences are when gorillas do not have the space they need to spread out,” she said. “We have seen group density peak around 2010-2015 but since then the groups have spread into regions of the park they were not using before, allowing group density to decrease with early signs that the rate of population growth is no longer slowing and may soon begin to increase again.

“As the mountain gorilla population grows in number we may soon reach a point where there is no longer room for groups to keep spreading out, with the higher rates of mortality and slower rates of reproduction we found in this study becoming the norm unless we are able to further expand their available habitat,” Morrison said. “This highlights the importance of protecting their remaining habitat as well as the conservation value of ongoing projects working to expand the protected area in which these gorillas live.”

Related reading:

For Africa’s great apes, even ‘best-case’ climate change will decimate habitat

Citations:

Caillaud, D., Eckardt, W., Vecellio, V., Ndagijimana, F., Mucyo, J., Hirwa, J., & Stoinski, T. (2020). Violent encounters between social units hinder the growth of a high-density mountain gorilla population. Science Advances, 6(45). doi:10.1126/sciadv.aba0724

Morrison, R.E., Hirwa, J.P., Ndagijimana, F., Vecellio, V., Eckardt, W. and Stoinski, T.S. (2022), Cascading effects of social dynamics on the reproduction, survival, and population growth of mountain gorillas. Animal Conservation. https://doi.org/10.1111/acv.12830

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