April 8 (UPI) -- To calculate the size of a live gorilla, scientists must rely on an advanced imaging technique called photogrammetry. But when gorillas size each other up, they just listen.
New research, published Thursday in the journal Scientific Reports, suggests chest beating provides accurate information about a male gorilla's size.
To communicate their dominance to potential female mates and rival males, gorillas use cupped hands to rapidly drum their chests. The beating sound can be heard from more than half-a-mile away.
"We think that it is likely that both males and females attend to this information. Body size is key for male gorillas," first study author Edward Wright told UPI in an email.
"Our previous study showed that larger males are more dominant and have a higher reproductive success than smaller males," said Wright, a researcher at the Max Planck Institute for Evolutionary Anthropology in Germany.
In theory, a bit of deception could offer an advantage to males in their attempts to woo mates or defend territory. However, males needs accurate information when picking their battles.
"Rival males are likely to use body size information conveyed in chest beats to assess the competitive ability of chest beating males," Wright said. "This will help them decide whether to initiate, escalate or retreat in contests with them."
Not all vocal signals in the animal kingdom are honest, but there are significant risks to deceit -- like ending up in a fight with a rival twice your size.
To better understand the accuracy of the information communicated by a gorilla's chest beating, scientists recorded male gorillas beating their chests.
They used photogrammetry to estimate each male gorilla's size, with the data revealing that bigger males produced chest beats with lower peak frequencies than their more diminutive peers.
Though scientists can now be certain that gorillas are providing accurate information about their body size, researchers say more work needs to be done to understand exactly how males and females use the information provided by chest beating.
Chest beats, the researchers note, vary in ways other than frequency -- and may carry information beyond body size.
The scientists said they suspect chest beating not only helps males attract mates and size up rivals, but also to identify themselves, maintain social hierarchies and defend territory.
"We were surprised to find that temporal characteristics of the chest beats such as the duration of the chest beat or the number of beats comprised in chest beats were not related to body size," Wright said. "We know that body size is conveyed in chest beats but what other information might be transmitted as well? Age? Dominance rank? Individual identity?"