- Recent research published on the Ngogo chimpanzees of Kibale National Park shows that female chimps don’t always leave their birth group in search of mating partners, which is considered the norm in most chimpanzee societies.
- Moving to a different social group is a strategy to avoid inbreeding, but to their surprise, researchers found that despite staying back, sexually mature females still manage to steer clear of relations with male relatives. How they do so remains a mystery.
- Primatologists who studied chimpanzees in Tanzania’s Gombe National Park also found that some females choose to remain. In that case, human disturbance and the lack of access to other chimp groups could be responsible for this behavior.
- However, the Ngogo population is large and lives close to other chimp troops, so the chimps have opportunities to leave but choose not to. This raises questions about why, if there are opportunities to breed successfully in their birth groups, do female chimpanzees leave at all?
Puberty comes with its challenges for Homo sapiens, but for female chimpanzees it presents a particular quandary: how to avoid mating with your male kin.
It’s not easy, especially on home turf. Chimpanzees (Pan troglodytes) embrace promiscuous mating; during sexual swellings, female chimpanzees mate with almost all eligible males in their group. This means their daughters run a high risk of encountering brothers, half-brothers, and even their own fathers when they reach adolescence.
“With humans, you know who your dad is, you know who your paternal brothers are, because we’ve got paired bonds [sticking with one sexual partner] marriage, language, and all these things,” said Kevin Langergraber, a primatologist at Arizona State University and co-author of a recent paper in the journal Royal Society Open Science. “In chimps, it can get complicated.”
One strategy to prevent inbreeding is straightforward: leave. Get out of your birth group and seek partners elsewhere, or what’s known as dispersal. Among social mammals, it’s common for young males to strike out and start their own broods. But in chimpanzees, like in many human societies, it’s the females who emigrate in search of mates.
This isn’t always the case, however, according to findings by a team led by Lauren C. White, who did the work while she was a postdoctoral candidate at Germany’s Max Planck Institute for Evolutionary Anthropology. The team, which included Langergraber, traced kinship bonds among chimps in the Ngogo group in Uganda’s Kibale National Park.
“Over the years, we noticed something weird going on. A lot of the females were staying and reproducing,” said Langergraber, who is co-director of the Ngogo Chimpanzee Project and has followed the group for more than two decades. In fact, half the females (18 of 36) included in the study who were born in Ngogo stayed in the group after reaching 13 years, the age at which dispersal normally happens.
This behavior led the scientists to wonder if females staying back led to an uptick in inbreeding, which can lower the fitness and survival chances of both individual offspring and the group as a whole. They found that it didn’t.
White and her colleagues could minutely trace the Ngogo group’s kinship links through advanced DNA analysis. Researchers usually rely on chimpanzee fecal samples to get genetic samples, because darting these endangered apes to collect blood or tissue samples is invasive and unwise.
While earlier investigations could only extract sparse information from a few microsatellite markers (signatures in the gene that help build a unique profile of individuals), the new study relied on more than 200,000 such markers. This shed light on which individuals were related and how much genetic material they shared.
Anne Pusey, a leading primatologist who wasn’t involved in the Ngogo research, praised this use of more powerful genomic techniques. “This is fantastic and gives them a more accurate measure of the relatedness of potential mates,” she said in an email to Mongabay.
The work did suffer from a drawback common to DNA-based kinship studies: samples are usually taken from infants who are around 2 years of age. Chimp babies that died in infancy or were lost during pregnancy — something perhaps more likely among inbred offspring — aren’t accounted for.
Based on their findings, the team concluded that relatedness between actual parent pairs (who sired offspring) was lower than what it would be if the apes mated randomly. The females were clearly doing something to avoid sexual relations with their kin. What exactly that is remains a puzzle.
Female chimps are flexible
What researchers do know is that the Ngogo chimpanzees aren’t the only ones to buck the norm when it comes to dispersal. In Tanzania’s Gombe National Park, the site of Jane Goodall’s lifework, some females remained in their natal groups.
Pusey, who manages the Gombe Chimpanzee Project, noted that the findings from Ngogo are similar to what they saw in Gombe despite differences in the two populations. Gombe is a relatively small community heavily impacted by human activities and isolated from other chimp groups. On the other hand, the Ngogo troop is the largest known chimp group in Africa and is surrounded by other groups inhabiting the dense rainforests of Kibale National Park.
The study included natal females and chimps who had migrated from surrounding groups into Ngogo. These immigrant females had a lower risk of inbreeding compared to natal females, White’s team found. The former’s pool of potential partners is less likely to include kin. Staying back increases the risk of inbreeding. But when researchers looked at the parent pairs who actually sired offspring, it showed that female chimps, including those native to the group, had chosen unrelated partners.
It “strengthens the view that flexible female dispersal and additional means of inbreeding avoidance are general characteristics of chimpanzees,” said Pusey, who heads an eponymous lab at Duke University in the U.S.
Same group, different neighborhood
What made the Ngogo findings especially intriguing was that Ngogo females stayed despite having more options to emigrate to one of the many other chimpanzee groups in the region. One hypothesis for Gombe females remaining in their natal territory is that they have few options. They live in a heavily human-disturbed fragmented habitat. This isn’t the case for the Ngogo great apes, who inhabit relatively unscathed territory.
The other big mystery is how they sidestep inbreeding, given that they don’t maintain kinship records (that humans know of) or have access to 23andMe tests.
One explanation is that females move to a different sector within the same group, so they don’t end up hanging out with males related to them. “Males tend to sire offspring with females they spend time with,” Langergraber said. A female born in the western range of the population can, at 13, move to another neighborhood, he said.
That’s possible because chimpanzees live in societies where associations are fluid. Group members don’t always move as one. They can scatter into smaller factions for some time before getting back together as a unit. Within a shared social space, neighborhoods can exist.
Ngogo’s large population may give females more chances to mingle with unrelated males. There are more of them around, and moving to a different subgroup helps the females find those mates.
But if chimps can avoid the risk of inbreeding while remaining in their own groups, it begs another question: Why leave at all?
From the published research, Langergraber said it was difficult to say. It could be linked to the fact that alternative strategies don’t exist everywhere and may not be as effective as dispersal.
“I think that non-dispersing females at Ngogo were able to avoid inbreeding because of some unusual aspects of Ngogo,” he said. First, he noted that Ngogo, with around 200 members, is uncommonly large, so there are just more potential mates, who aren’t family, to choose from.
There’s another reason researchers are wary of dismissing dispersal as a means of preventing inbreeding. After all, though female chimps in Ngogo aren’t leaving their mothers’ group outright, they do appear to be practicing a “within-community form of dispersal.”
Pusey also noted that strategies to avoid inbreeding “don’t always work, especially in small communities,” and there’s evidence that “if there is no dispersal for long periods, general levels of relatedness will build up in the group, such that inbreeding depression will start to set in.”
Banner Image: Ngogo chimpanzees aren’t the only ones to buck the norm when it comes to dispersal. In Tanzania’s Gombe National Park, the site of Jane Goodall’s lifework, some females remained in their natal groups. Image by John Mitani.
Citations:
White, L. C., Städele, V., Ramirez Amaya, S., Langergraber, K., & Vigilant, L. (2024). Female chimpanzees avoid inbreeding even in the presence of substantial bisexual philopatry. Royal Society Open Science, 11(1). doi:10.1098/rsos.230967
Pusey, A. E., & Schroepfer-Walker, K. (2013). Female competition in chimpanzees. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1631), 20130077. doi:10.1098/rstb.2013.0077
McCarthy, M. S., Lester, J. D., Langergraber, K. E., Stanford, C. B., & Vigilant, L. (2018). Genetic analysis suggests dispersal among chimpanzees in a fragmented forest landscape in Uganda. American Journal of Primatology, 80(9), e22902. doi:10.1002/ajp.22902