Young gorillas over the age of two that lose their mothers are still able to survive, and even thrive as relatives and group members rally round to ease the loss, a ground-breaking new study in Rwanda has revealed.
The study, published in the journal eLife, uses data gathered over more than half a century in Rwanda’s famous Volcanoes National Park. It stems back to work carried out by the late renowned primatologist Dian Fossey.
In all 59 mountain gorillas, orphaned between the ages of 2-8, were studied in the period between 1967 and 2019.
It found that being orphans did not affect the gorillas’ ability to survive or reproduce or gain social status over their lifetimes, unlike in other mammals including other primates.
“What we found in this study is that despite all of this care that is provided by a mother, young gorillas, once they’re over the age of two can still survive and thrive without their mothers being there,” said lead author of the study, Robin Morrison.
“It seems to be because they are part of this much larger, cohesive group, which is full of their siblings and a dominant male that is sometimes their father and sometimes isn’t,” she told RFI.
“All of them seem to strengthen their bond with these orphaned gorillas.”
Most of the gorilla orphans included in the study lost their mothers when they migrated to other groups (as female gorillas often do), or died from natural causes. The park is tightly-guarded, meaning that few of the gorillas in the study had been orphaned by poachers.
Surprising and intriguing
The research showed that dominant male gorillas take young orphans into their nests each night, even when those orphans aren’t their own offspring. This “isn’t what you’d necessarily expect in primate society,” said Morrison, who is a postdoctoral researcher with the Dian Fossey Gorilla Fund.
She cited a case in 2017 when four mothers abandoned their young and left a group whose dominant silverback male got sick and died. Kubaha, an 18-year-old male gorilla took charge of the group and the four orphaned gorillas.
In other primates, like chimpanzees, the loss of a mother can be catastrophic to the surviving youngsters.
Young chimps need their mothers to protect them and to teach them complex foraging techniques, such as the use of simple tools to crack open nuts or fish termites out of their nests.
Matthew Zipple, a behavioural ecologist at Duke University, in the US, described the latest Rwandan study as “surprising and intriguing”.
“In primates our expectation is that the loss of the mother should have strong, negative effects on offspring survival,” Zipple, who was not part of the study, told RFI.
“This study is in contrast to results from other species of primates, where we see that early maternal death has multifaceted effects on offspring outcomes, including delayed reproduction, reduced fertility and survival in adulthood,” he said.
Zipple co-authored a study published last year that looked at the effects of maternal loss on offspring in seven species of primates, including baboons, monkeys, chimpanzees and gorillas. That study too saw no apparent negative effects of maternal loss on gorilla offspring.
Morrison said her team’s study wouldn’t have been possible without the data gathered by hundreds of scientists working over the years at the Karisoke Research Centre run by the Dian Fossey Gorilla Fund.
“There’s been 50 years of brilliant Rwandan researchers collecting this data and working to produce this long-term science that we just couldn’t do unless we were able to study the same population over such a huge time period,” she said.
Some of the data was obtained by combing through old notebooks, deciphering other scientists’ handwriting and analysing the findings across time.
“It’s really wonderful to combine what we’re seeing now with what other people saw 50 years ago,” she said.