Mon, June 22, 2020

Battle of the sexes: is bigger better?

In the animal kingdom, bigger is often better. Larger individuals tend to be stronger than smaller ones, and their size often allows them to outcompete smaller individuals. Dominance status is also usually beneficial, and being big is often the key to obtaining dominance. Both large size and dominance status also tend to be closely related to reproductive success.

Director of the Karisoke Research Center, Felix Ndagijimana, standing with Mafunzo, one of the largest silverback gorillas we monitor.

Over the last few years, the Fossey Fund has been collaborating with scientists at the Max Planck Institute for Evolutionary Anthropology and The George Washington University to use a scientific methodology called photogrammetry that allows us to get size estimates of living gorillas. This technique enables us to answer questions about how size affects reproduction and dominance status in mountain gorillas.

So how do you measure the body size of a wild gorilla? Answer: It’s complicated. We take photographs of a gorilla with two small green parallel lasers that act as a scale on the photograph. The two lasers are separated by a known distance, and as long as they are projected perpendicular to the gorilla being photographed, the distance between the two lasers remains the same no matter the distance from which we take the photo, giving us a standardized reference point. Using this method, we can take multiple body measurements, including back breadth, body length and, in males, crest size.

“I really love the photogrammetry work we have been doing,” says Dr. Tara Stoinski, Fossey Fund CEO and one of the scientists on the study. “It has taught us so much about gorillas—how they grow, what influences their choice of mate, and what leads to reproductive success. It is also a great reminder that, like us, there is so much individual variation between gorillas, and no one size fits all.”

So what have we found? Our first paper showed that large male mountain gorillas enjoyed higher status than smaller ones, and had considerably more success in reproduction. In male mountain gorillas, bigger is indeed better.

We wanted to find out whether this also held true for female mountain gorillas. We discovered that in contrast to their male counterparts and many other mammal species, large size did not always lead to high status for female mountain gorillas: for females, bigger is not necessarily better for achieving dominance. Nevertheless, high status is still important, with more dominant females giving birth at shorter intervals than their lower status counterparts. In the long run, this is likely to result in them having more offspring than their lower status peers, boosting their reproductive success.

“It was really interesting to see that the two sexes have different strategies to maximise their reproductive success,” says Dr. Ed Wright, a scientist from the Max Planck Institute who led the study. “For males, bigger is better; but for females, rank is more important than size in determining reproductive success.”

Ubufatanye with her three babies, who were born fewer than six years apart.

This study’s findings are reflected in the recent birth patterns of one of the females we monitor. Ubufatanye is by far the highest-ranking female in her group, but she is one of the smallest of the group’s four females. However, with the birth of a baby on May 11, she achieved something we have never before seen in another female—her third birth in less than six years. This is less than half the time it would usually take a female to give birth to so many offspring, as the average time between births is four years.

“I was surprised to find that body size does not correlate with either reproductive success or dominance rank in female mountain gorillas,” says Fossey Fund’s Eric Ndayishimiye, one of the study’s coauthors. “There is much we still need to know about mountain gorillas.”

You can read the full study here.

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