The Masters of Signalling

Some like it... blue?

Not necessarily. It seems that if a vervet monkey (Chlorocebus pygerythrus) shows you its blue balls a bit too often, it is not happy to see you... it's a signal of dominance.

Richard Dawkins, a popular evolutionary biologist, suggested that the monkeys show their genitals off for a reason which can be explained by the handicap principle. The handicap principle states that giving 'reliable signals' must cost the signaler something - something that a weaker, less capable animal - the receiver - cannot afford. The receiver then knows that the signal represents quality, and since it is unable to compete, the receiver knows it has lost the competition.

The vervet monkey's scrotum is neon blue and its penis is bright red, so one is simply 'forced' to look at it. It's un-unseeable. We, humans, know very well how fragile testicles are... and how important they are. So, when a vervet monkey proudly displays its genitals, it means to signal that it's tough enough to display them without having to worry about anything happening to them.

It's possibly also an important sex signal. Just like the peacock's mesmerizing tail, the vervet monkey's hypnotizingly blue scrotum serves as a signal of quality to females. Many other primates are bright colored to signal quality.

There is just one thing about the color of the vervet monkeys' scrota that perplexes scientists to this day: their intra- and inter-indivudal variation.

A 2013 study showed that the color of the scrotum grew paler with age, heavier males had a bluer scrotum and those with long canines had a paler one.

The monkeys display their scrotum in so many contexts that it's difficult to give a simple answer as to what the true signal is supposed to mean. A 2020 study aimed to investigate this. The researchers found that the frequency of signalling increased with dominance, but none of the variables they used could explain why there is such variation in the coloration of the monkeys' scrota, so we still don't have an answer!

You see, the matter of blue balls isn't complicated only in humans.

And the females?

Well, as you might have guessed, they don't have a scrotum. The skin around their mammary glands, though, is blue, just like the scrotum of the male. It is, however, covered with fur, so it's not as visible as the strikingly blue balls.

This tweeted picture shows a mother. Her mammary glands are swollen and the blue skin color is somewhat visible.

The vervet monkeys are also good at acoustic signalling. They have distinct alarm calls they make in particular contexts. The sound they make depends on what predator is approaching them, and what hunting technique this predator has. This allows them to choose the ideal method of escape. Robert Seyfarth, who studied them, thinks that these calls might be precursors to language.

Moreover, Linne A. Isbel and Laura R. Bidner from the University of California discovered that the alarm calls also serve as predator deterrents. Their calls successfully deterred leopards, which were more likely to attack when the monkeys were more quiet and turned away once they heard the call.

Photo credit (GNU): Yoky

Common name:Vervet monkey
Scientific name:Chlorocebus pygerythrus
IUCN status:Least concern
Population trend:Decreasing
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