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New study explores female gorilla mating patterns

In the Fossey Fund’s 55+ years of studying gorilla behavior, a lot of research has been conducted to study the mating patterns of male gorillas, since this can have important consequences for the population’s genetic structure and evolution. Unlike the other gorilla subspecies, mountain gorillas live in both single-male and multi-male groups, so mating behaviors are especially important to understand.

In single-male groups, one adult male benefits from all the mating opportunities, while in multi-male groups, the dominant male usually gets most of the opportunities, though our two decades of genetic research into paternity patterns shows that lower-ranking males get opportunities as well.

However, little was known about the mating patterns of mountain gorilla females – until now. Our scientists have just published the results of a study using 13 years of data on 71 female mountain gorillas and their analysis shows just how variable their mating behavior can be, especially within multi-male groups. The study is published in the journal Behavioral Ecology and Sociobiology.

Why mating patterns matter

Animal societies are shaped by their group compositions and relationships, and their mating systems tend to fall into a single category, either monogamous or those involving multiple partners. But some species have varying mating patterns and our significant database on the mountain gorillas has allowed our scientists to closely study this part of their highly flexible social system.

In mountain gorilla groups, previous research has shown that only about half of males and females leave the groups they were born into (called natal groups) once they reach sexual maturity. While males then either lead a solitary life or try to attract females to join them, females can join another group, often times transferring between multiple groups during their lives. 

Interestingly, females initiate the majority of copulations, which means that their choices and strategies really affect the population, explains Tara Stoinski, Ph.D., the Fossey Fund’s CEO and chief scientific officer. Stoinski is one of the study’s authors and a long-time researcher focusing on male gorilla behaviors.

What we’ve learned

The results of this new research show that mountain gorilla females adjust their mating patterns across their lifetimes, as the “costs and benefits” shift based on their opportunities and their stage of life. Almost half of the females moved between single-male and multi-male groups during the 13-year study. If they were in a single-male group, they mated with only one male, but if they moved to a multi-male group, they predominantly copulated with multiple males.  

“In this variable social system, when females transfer between groups they are not only choosing which group to join but also what mating system they will be a part of,” says lead author Robin Morrison, Ph.D. “This variability in mating patterns has interesting parallels to humans, who have also shown a wide variety of mating patterns across our evolutionary history and across cultures.”

And as gorilla females age, their rate of copulation drops as does their number of partners.  

“Our previous research has shown that female gorillas tend to have fewer infants as they get older, with bigger gaps between each offspring. Our findings suggest that it is not just that female fertility is declining but that they are also choosing to copulate less often, resulting in fewer infants being born,” adds Morrison.

Female gorillas’ mating patterns were also strongly influenced by their reproductive stage, the study found. In an unusual twist, females copulated the most when they were already pregnant, particularly in single-male groups. The researchers suggested that this may be due to competition between the females for access to the male, with copulations potentially strengthening their relationship with the male, who will serve as a critical protector of the infant once it is born, and limiting the chances for other females to become pregnant. 

After giving birth females mated only very rarely until their infant was around 2 years old. Females generally don’t cycle when they are lactating and this, combined with not copulating, may help ensure they don’t get pregnant when they already have a highly dependent infant.  

“I am delighted to have contributed to this interesting study that sheds more light on the mountain gorillas’ mating system,” says author Eric Ndayishimiye, Fossey Fund gorilla data and research officer, who is just embarking on his Ph.D. studies. “As an aspiring researcher, playing a part in this research was a huge opportunity for me to learn from senior researchers.”

You can read the full study here: “Female age and reproductive stage influence copulation patterns in mountain gorillas’ variable mating system”