A few years ago, four female mountain gorillas left home, abandoning not only their mate—a sick alpha silverback—but their infants, which were barely old enough to feed themselves. They may have sensed that their offspring would be safer with their ailing father than with new males that often kill infants from other groups. Still, most mammals abandoned by their mothers risk an early death, and researchers worried about the young gorillas.
Instead, the scientists got a heartwarming surprise. The juveniles’ uncle, a male gorilla named Kubaha, began to take care of them. “He let them sleep in his nest and climb all over him like a jungle gym,” recalls primatologist Tara Stoinski, chief scientist of the Dian Fossey Gorilla Fund.
Kubaha’s willingness to be a foster dad turns out to be surprisingly common in mountain gorillas, according to a new study. An analysis of 53 years of data on mountain gorillas at the Gorilla Fund’s Karisoke Research Center in Rwanda has revealed that when young mountain gorillas lose their mothers—and sometimes their fathers as well—they do not have a greater risk of dying or losing their place in the social hierarchy because the rest of the group buffers them from the loss.
“This paper was really surprising because we know that in primates and most social mammals that it’s really bad to lose your mother if you’re immature,” says behavioral ecologist Matthew Zipple of Duke University, who was not part of this study.
A study Zipple and 10 leading primatologists published last year found that young chimpanzees, baboons, and monkeys that rely on their mothers for support after they are weaned tend to die young if they lose their mothers at an early age. That’s because their moms feed and clean them, provide social support, and protect them from predators and attacks from unrelated males. Even if motherless apes survive to adulthood, they have lower social rank and produce fewer offspring. Other studies have documented the same dangers of losing a mother in social mammals such as killer whales, elephants, and hyenas.
But motherless mountain gorillas didn’t seem to suffer as much. Stoinski and her Gorilla Fund colleagues, including postdoc Robin Morrison, proposed that in mammals, such as gorillas, where mothers often disperse before their offspring mature, the social group has evolved to protect the infants from the ill effects of losing their mothers.
They tested this hypothesis in the new study by focusing on data on 59 gorillas between the ages of 2 and 8 who lost their mothers or were orphaned before they were fully mature. They then compared the survival of these animals across their lifetimes with the survival of 139 nonorphaned gorillas. They also compared their reproductive success and social rank as adults—and tracked who the infants spent the most time with.
Not only were the orphaned and motherless gorillas at no greater risk of dying, they also suffered no long-term effect on their ability to reproduce or on their social rank, the team reports today in eLife. Indeed, some motherless males eventually became the dominant male silverback of their group.
The study is “terrific,” says Duke primatologist Anne Pusey, who was not part of the work. The data come from one of the longest mammal field studies, she notes, and the number of orphaned gorillas is high enough to compare directly with data from young chimps. Those data show that chimps die young or suffer other ill effects if they lose their mothers because females don’t change groups often—and infants are more dependent on their care longer than are gorillas.
Now, researchers need to comb decades of data for bonobos and other species to see whether they, too, adopt motherless infants more often than believed, Zipple says. A study published last week found that two bonobo females adopted infants from another social group in the Democratic Republic of the Congo.
The findings suggest such altruistic behavior is not unique to humans—and that dads play an important role in primate youngsters’ lives, says Duke behavioral ecologist Susan Alberts, who was not part of this study. “Nonhuman primates often are really good dads,” she says. “This shows that paternal care goes very deep in our primate lineage.”