Gorillas beating their chests may prevent fighting

To the animal kingdom now and some interesting insights into why mountain gorillas do that thing they’re most famous for. Katie Haylor reports...

Katie - In the forests of Uganda, Rwanda and the Democratic Republic of Congo mountain gorillas beat their chests. But why? This could all be about saying, "Hey, look how big I am." According to a paper out this week in Nature Scientific Reports, by measuring the size of male mountain gorillas and recording the sounds of their chest beats, scientist Edward Wright and colleagues found ...

Edward - Larger males produce chest beats with lower frequencies than smaller males.

Katie - In other words, the larger gorillas emitted lower sounding chest beat and the smaller gorillas made a higher sounding chest beat. The team first had to measure how big the gorillas were, which can't be easy as for obvious reasons you can't really get that close.

Edward - So we use what we call the parallel laser technique. So we project two small green parallel lasers onto the gorilla, and they're separated by a known distance. And then we take a photograph with these two lasers in the photograph, which is then used as a scale to measure several different body parts of interest. And we also take care not to shine the lasers in their eyes. So we're usually taking photographs from behind the gorilla or from the side, but never from the front.

Katie - Using the parallel laser technique, the team managed to figure out the distance across the shoulders of 25 males. Then it's a question of recording the chest beats - which Edward says the gorillas don't do all that often, and you don't necessarily know when they're going to do it. So they ended up with recordings from six of the 25 gorillas.

Edward - Sometimes you can predict when they're going to do chest beats. Sometimes they hoot before they chest beat.

[gorilla hooting and then chest beating sfx]

Edward - We're not really sure what the function of the hooting is, but perhaps it inflates their lungs with air. And so when a gorilla starts to hoot, okay, maybe in the next few seconds or few minutes they may chest beat. Other times they just chest beat out of the blue. And so it's quite difficult to be at the right time at the right place.

So we put this together in a statistical model, controlling for several other variables like age. And then we found that larger individuals chest beated at lower frequencies than smaller individuals. And this was a robust, significant relationship.

Katie - Curiously, Edward isn't sure exactly why larger males have a lower chest beat.

Edward - It could be the volume of the lungs, hand size, and those two probably correlate with each other anyway. Gorillas also have laryngeal air sacs - big spaces inside the chest cavity. So we think that larger individuals may have larger laryngeal air sacs, which could also be influencing the frequency of the chest beat. But we're not exactly sure if it's a combination of those features or if it's just one.

Katie - Now it's not only males that do this chest beating, but it is more common in male adults than in females or younger ones.

Edward - The main goal I think is to advertise how strong they are. If you're another male and you hear a chest beat and you're able to determine or assess their competitive ability, that's really useful because then you might decide, "He's much bigger than me. I'm not going to pick a fight with him because I'm likely to lose." If you're a female and you hear a big strong male chest beat, "Ooh, that might be a good male for me to mate with." So I think it's definitely involved in male-male competition and female choice.

Katie - But what if you're a smaller male gorilla? Is it dangerous to lay your cards on the table and let others around you know how big or small you are?

Edward - One of the roles of chest beating is to maybe resolve conflicts before you get physical contact and aggression. Physical contact can be extremely fierce. And so it's very risky even for the larger male. The chest beat sort of acts as a, "Okay, you're bigger, I'm smaller. I'm going to let you have this one." But if you're bigger, you're going to say, "No, I'm going to take what I want. Because if we really did come to a fight, I'm very likely to win." So perhaps individuals only really fight when the size difference, so the fighting ability, is quite similar.