Compared to the rest of our primate family, we have strikingly little hair on our faces, which is how humans came to be called "hairless apes". But there's another species that – while not hairless exactly ­– still manages to stay fairly clean-shaven: the aptly named bald uakari monkey. And there’s something else that makes the uakari stand out: its face is very, very red. Tomato red. Fire engine red. Or, to be most accurate, blood red.

Several years ago, scientist Mark Changizi suspected that skin could act as a sort of mood ring, a signal to indicate something useful about what's going on underneath.

Some primates, like baboons and chimps, use skin colour to display sexual arousal (but they display that on their genitals, not their faces). For mandrills and some macaques, facial colouration can provide useful intel about fertility, and female rhesus macaques also appear more interested in males with redder faces.

But none of its primate cousins comes close to the persistent redness of the uakari monkey's face. Relatively small (weighing in at around two to four kilograms), the uakaris are found only in a few small strips of the Amazon rainforest, and for as much as 30 years researchers have been trying to work out exactly why the monkeys have so much facial skin on show – and why it's so, so red.

Could the redness be a good way of indicating health? When bald uakaris get sick, their skin becomes dull and pale, so a bright red face might be a sign of a healthy, parasite-free monkey – one that's worth mating with. This hypothesis, however, has never been confirmed, and until now nobody has worked out what causes the redness in the first place.

But recently, a group of researchers led by Pedro Mayor of the Autonomous University of Barcelona set out to solve the long-standing colour mystery. To do so, they collected skin samples from a variety of dead uakaris and some other Amazonian monkeys.* They published their results this week in the journal Royal Society Open Science

What did they discover?

First, the uakaris' faces look so red because of a huge number of small blood-carrying capillaries just beneath the skin's surface. That skin is also quite thin: it's thinner than the facial samples from other monkeys, and also thinner than skin samples from the uakaris' torsos and legs.

Second, we can see all the blood flowing beneath an uakari's skin because there isn't any pigment in it at all: the skin is, in a sense, transparent. In comparison, the skin samples from every other monkey species the researchers looked at – woolly monkeys, saki monkeys, howler monkeys, capuchins – contained lots and lots of melanin (the pigment that gives skin its colour).

Together, these features allow the uakari monkeys to show off how much blood they have just underneath their faces at any point in time. And that means any significant blood loss due to injury or illness will soon show up on their faces: their facial skin is an "honest indicator" of their health. They can't fake it.

For example, researchers and zookeepers have noticed that uakaris become paler after they become infected with the malaria parasite, or when they contract Chagas disease, both of which affect blood. It could be that uakaris evolved alongside these parasites, with the monkeys' red faces eventually emerging as a way of distinguishing healthy from sick individuals.

Then again, when an uakari monkey becomes sexually aroused, its blood pressure increases, and its face becomes redder as well. So the red faces could also have evolved as a means of signalling sexual interest.

As is so often the case, we'll need further research to tell us more, the scientists conclude.

* None of the monkeys were killed especially for this study; some died naturally and others were hunted as part of the routine subsistence hunting of Amazonian indigenous groups. Hunters gave up skin samples voluntarily; they were never paid.