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Size Matters

Silverback Giraneza in a strut stance characterized by tight muscles in the neck, back, legs, and arms and the widening of the arms and legs.

Science confirms: Size matters for mountain gorillas

For male mountain gorillas, one thing is certain – size matters, especially when it comes to the ladies. A study involving Fossey Fund researchers finds a correlation between physical traits of male gorillas, their mating success and their dominance rank in social groups.

“There’s no doubt bigger is better,” says Eric Ndayishimiye, a Fossey Fund researcher and coauthor of the study published in the journal Animal Behaviour. “In mature males, we specifically found that the height of the gorilla’s crest, or elongated skull bone, and the breadth of his back typically indicate a higher rank in the gorilla group.”

The Fossey Fund researchers, working with scientists from the Max Planck Institute and The George Washington University, developed a composite measure of the crest and back. It correlated with not only achieving the highest dominance rank in a group, but also how long a male was able to maintain dominance, and the number of females who join the group. Interestingly, body length did not seem to matter.

“From an evolutionary perspective, reproductive success is everything for male gorillas,” says the study’s lead author Edward Wright of the Max Planck Institute. “But until now, we’ve not understood why some gorillas sire many offspring, and others none at all. While crest and back size are not the only indicators of mating success, this study provides our first proof of a physical trait that influences female attraction and mating.”

Over more than 50 years, the Fossey Fund has tracked and studied a number of legendary silverbacks that validate the “size matters” theory. Cantsbee, Mafunzo, Musilikale and Isabukuru collectively sired dozens of children, and their many descendants are one of the reasons the mountain gorilla population is rebounding from the brink of extinction.

“There are certainly other factors that contribute to gorilla rank and dominance among males,” Ndayishimiye says. “But gorillas with a higher crest-back score had more reproductive success. They were more aggressive, and the large size helped them attain a higher rank in the group.”