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Researchers have uncovered the first evidence of long-lasting social relationships with “possible benefits” between chimpanzees and gorillas in the wild.

The study, published recently in the journal iScience, documents social ties between individual chimpanzees and gorillas in the Nouabalé-Ndoki National Park in the Republic of Congo.

These relationships with “possible benefits” have persisted over long periods and across different contexts, say researchers, including those from Washington University in the US, drawing from over 20 years of observation.

“It has long been known that these apes can recognise individual members of their own species and form long-term relationships, but we had not known that this extended to other species,” study co-author Crickette Sanz said in a statement.

Citing an example of one such relationship between the chimpanzees and gorillas in the region, scientists say they have observed, on several instances, one individual traveling through a group of the other species to seek out another particular individual.

They assessed several possible benefits from such interspecies interaction, including protection from predation, improved foraging options, as well as other social benefits from information sharing.

Researchers say these social interactions cannot be narrowed down as a behaviour to reduce threats because they did not find significant evidence of the chimpanzees and gorillas banding together to decrease leopard or snake predation attempts.

Instead, they say enhanced foraging opportunities could be a more important reason for them to associate with each other.

The study found co-feeding at the same tree represented over a third of the interspecific associations that they documented.

Another 18 per cent of observations of the two species teaming up involved the apes foraging in close spatial proximity but on different foods.

Scientists found at least 20 different plant species targeted by the apes during co-feeding events.

Some social relationships between members of the two different species also persisted over several years, the study found.

On several occasions, researchers say young gorillas and chimpanzees sought out particular partners to engage in bouts of play, with these interactions likely providing unique development opportunities beyond the individual’s social, physical and cognitive competencies.

“The strength and persistence of social relationships that we observed between apes indicates a depth of social awareness and myriad social transmission pathways that had not previously been imagined,” study co-author Jake Funkhouser said in a statement.

While the interspecies interaction seems to offer some benefits to the apes, scientists say the social exchanges also have their risks, especially in disease transmission between the two species.

Citing an example, researchers say a wave of ebola that emerged 20 years ago wiped out a third of the world’s chimpanzees and gorillas.

In the study, they found several potential modes of cross-species pathogen transmission, including via direct physical contact between individuals during play and aggression.

“Gorillas were observed feeding on fruit mesocarps that had been fed upon and discarded by chimpanzees. Also, gorillas were observed foraging on fruits and figs in the leaf litter under areas where chimpanzees had foraged, urinated, and defecated,” scientists write in the study.

A better analysis of such co-feeding behaviour between the two apes along with food availability data can help in modeling potential areas of disease spillover events, researchers say.

They say the findings can also shed more light on the likely interactions between early human ancestors.

“Despite more than 60 years of chimpanzee and gorilla research, there is still much to be learned about these fascinating apes – the main challenge at this time is to ensure the conservation of these endangered species so that such opportunities exist for future generations,” Dr Sanz said.

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