- Mountain gorilla populations have grown steadily in recent decades, thanks largely to intensive conservation efforts in Rwanda, Uganda and the Democratic Republic of Congo.
- But the species’ entire population is confined to protected parks in these countries, with limited room to expand, and as the population has grown, so too has population density.
- A new study that tracked the incidence and intensity of parasitic infections across the mountain gorilla’s range suggests that greater population density correlates with greater susceptibility to parasites and other health problems.
Mountain gorillas, one of the world’s endangered apes, appear be facing a fresh health threat after a successful conservation campaign saved them from looming extinction, a new study says.
Thanks to intensive conservation efforts, the population of mountain gorillas (Gorilla beringei beringei) has risen to more than 1,000, up from 620 in 1989, according to the International Union for Conservation of Nature. As a result, in 2018 the organization changed the conservation status of mountain gorillas from critically endangered to endangered.
However, the habitat available to mountain gorillas has not expanded in tandem with population growth. The entirety of the species is confined to parks in Rwanda, Uganda and the Democratic Republic of Congo. Bounded by human settlements, the gorillas are unable to expand beyond these protected areas, leading to increasing population density.
The study, led by biologist Klára J. Petrželková of the Czech Academy of Sciences, found that as the mountain gorilla density increased, populations have become more susceptible to health problems.
After gastrointestinal diseases were reported in the primates, researchers examined the prevalence of two parasitic worms (helminths) — strongylids and tapeworms — and found the pattern of infection appears to be influenced by the age, location and group size of the gorillas.
The findings, which the authors of the report say point to possible “side effects” of the conservation success, are expected to guide future conservation efforts and academic studies on gorillas.
Patterns and Drivers
The study represents the first species-wide survey of parasite infections across the mountain gorilla’s entire range. “The study covered both mountain gorilla populations and was done across different seasons in order to uncover the drivers and patterns of helminth infections,” Petrželková said.
Working in collaboration with the authorities in Rwanda, Uganda and DRC, an international team of researchers tracked strongylid and tapeworm infections by looking for their eggs in mountain gorilla feces. They collected samples from night nests and samples from individually identified gorillas during dry and rainy seasons in 2018, and studied how age, sex, group size, season and location affect infections. They also paid attention to the vegetation type, gorilla subpopulation size and social structure in a given area.
The study found more infections (seen through higher amount of eggs of worms in gorillas’ feces) in areas that had more reported incidents of gastrointestinal disease, indicating that infections from parasitic worms pose a severe health risk to gorillas.
The researchers also found that, in some areas, gorillas from smaller family groups had more tapeworm and strongylid eggs present in their feces. According to the study, this is likely because smaller groups experience more stress caused by factors such as increased intergroup clashes and attacks by external males, making them more vulnerable to health problems.
Critically, the researchers said the results indirectly suggest that “high growth rates of gorilla subpopulations in some areas in the last 40 years can be linked to high strongylid infection intensities found in these areas today.”
The prospect of a conservation accomplishment introducing a new challenge for the primates is a learning curve for conservationists and the academic community alike.
“Increasing numbers of the highly-endangered mountain gorilla are a source of optimism and something to celebrate. But, as conservation scientists and wildlife managers, we have much to learn,” said Joanna Lambert, a conservation biologist and professor of environmental studies at the University of Colorado Boulder, who did not take part in the research. “The data presented indicate that what is happening on the ground with endangered animals is an evolving story with unexpected consequences.”
Limitations of the study
Further study is required to reach definitive conclusions, Petrželková said. While the study indicates a link between high gorilla populations and parasitic infections, area-specific data on current population densities is too limited for the link to be certain.
Furthermore, the researchers found that adult males, the subgroup most frequently diagnosed with fatal gastritis, did not show higher levels of parasitic worm infections than adult females.
Nonetheless, the findings so far provide important insights into understanding mountain gorilla populations within their confined habitat, Anna Behm Masozera, director of the Kigali-based International Gorilla Conservation Programme, told Mongabay.
“While the study puts forward a number of areas for further study, the conclusions point to the importance of safeguarding their remaining habitat from infrastructure development and mitigating human impact,” she said.
“There are no easy answers to the conservation challenges that face mountain gorillas in the decades to come, and we can look to what we know has been an enabling environment for the recovery of the mountain gorilla: informed and integrated, collaborative and transboundary action and monitoring. This study provides both important information and furthers the argument that a comprehensive action plan for mountain gorillas will be a precursor to effectively and collaboratively tackle the many challenges ahead.”
Lambert said the study reveals a “clear and actionable path” for conservation practitioners and wildlife authorities, and that it suggests that gastrointestinal parasites should be carefully monitored.
“The good news is that such monitoring is relatively inexpensive and can be done non-invasively, with no harm to the animal. Moreover, animals infected with parasites can be treated with highly effective medicines well-known to wildlife veterinarians,” she said in an interview.
- Petrželková, K. J., Uwamahoro, C., Pafčo, B., Červená, B., Samaš, P., Mudakikwa, A., … Modrý, D. (2021). Heterogeneity in patterns of helminth infections across populations of mountain gorillas (Gorilla beringei beringei). Scientific Reports, 11(1), 10869. doi:10.1038/s41598-021-89283-4
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