In all mammals, newborns rely on their mothers to produce milk that provides them with nourishment as they grow. For highly social animals like primates, certain types of whales, and elephants, however, young animals also rely on their mothers to teach them the rules of socializing with their group long after they have stopped relying on her milk for food. If these animals lose their mother, they often face substantial risks of early death or lower social status and breeding opportunities if they survive into adulthood. Humans are a notable exception to this; human children over the age of two who lose their mother have similar odds of survival as other children because they are taken in by relatives or other caretakers. A recent five-decade long study from the Dian Fossey Gorilla Fund in Rwanda found that mountain gorillas are similarly able to protect young animals who have lost their mothers.
The researchers observed a group of gorillas for several decades. During this time, 59 young gorillas lost their mother before the age of eight. These are referred to as orphaned gorillas, although many had surviving fathers. They also studied 139 non-orphaned gorillas. The orphaned animals spent more time with other young gorillas in the troop as well as with the males, including the dominant male. While the handful of infants who lost a mother before the age of two generally didn’t survive, the others received enough care from the remaining adults in the troop that losing their mothers did not significantly impact their chances of surviving to adulthood.
It is common for mountain gorillas to “disperse” or leave their original group sometime after reaching adulthood. The female orphaned gorillas were somewhat more likely to disperse than their non-orphaned peers, particularly if they had lost their mother after infancy. For females, dispersion doesn’t generally affect how old she will be when she has her first offspring and, in fact, researchers found that the orphaned females had their first infant at slightly younger ages than non-orphaned females and the resulting baby was equally likely to survive infancy. This indicated that their reproductive success was not harmed by the loss of their own mothers.
While only a small number of orphaned males dispersed from their original groups, it did seem that orphaned males were more likely to do so than their non-orphaned counterparts. This could hurt their reproductive success since male mountain gorillas who leave their original troop to try to find female gorillas for their own troop tend to be father fewer gorillas during their lifespan. However male orphans who stayed with their initial troop did become dominant a number of times, possibly because of strengthened social relationships with the dominant male after the loss of their mothers.
This study shows mountain gorillas, like humans, can step in to protect their young from some of the consequences of maternal loss and subsequently protect their odds of survival and reproductive success. Future work can examine whether other social animals also have means of protecting orphaned young.
The first author Robin E Morrison is a postdoctoral researcher at The Dian Fossey Gorilla Fund International interested in gorilla social behaviour.
Managing Correspondent: Emily Kerr
Scientific Source: Social groups buffer maternal loss in mountain gorillas
Popular Press Source: Gorillas in the wild often adopt young orphaned apes