Bigger gorillas better at attracting mates and raising young

Conservationists with WCS and the Max Planck Institute for Evolutionary Anthropology spent 12 years studying gorilla mating choices in Nouabalé-Ndoki National Park. Their findings illuminate an important factor in the evolution of great apes.

In assessing the role of size in the reproductive success of silverback gorillas, the researchers examined three physical factors for measurement: overall body length, the size of the adult male's head crest, and the size of its gluteal (backside) muscles. They then compared these physical traits with the number of female gorillas each silverback attracted, and the survival rates of their offspring.

The researchers found that the bigger the adult male, the more mates it had. However, only head-crest size and gluteal muscles strongly correlated to offspring survival and overall reproductive success. In other words, the babies of the silverbacks with bigger heads and glutes had a better chance of surviving to weaning age, and more siblings.

"Our findings of correlations between physical traits and male reproductive success could be considered evidence of a selection process in gorillas, but it is not yet proof," said WCS's Thomas Breuer, the study's lead author. "More studies would be necessary to determine the links between morphology and fitness in this and other long-lived species."

This latest study follows several others chronicling the western lowland gorillas of Nouabalé-Ndoki National Park's Mbeli Bai, a large, swampy forest clearing where the apes (and the scientists observing them) gather for long periods. The research team conducted their work from 1995 until 2007, following the lives of 19 adult males and their family groups with the help of observation platforms, telescopes, and cameras. These methods enabled the conservationists to track the number of females each silverback mated with, the number of offspring they produced, and the offsprings' survival rates.

To size up the silverbacks, the researchers used a non-invasive method called digital photogrammetry, which renders accurate measurements of individual gorillas and their characteristics from digital images by converting pixel size to actual lengths.

"By using non-invasive methods for measuring the size of individual male gorillas and their features, we are gaining insights about the factors that could be driving mate selection in our closest relatives," added Breuer.