Red Uakari (Cacajao calvus ucayalii) sleeping showing pale eyelids. Photo by Mark Bowler/via Getty Images
Why the red face, bald uakari monkey? Are you hot? Are you embarrassed that your Amazonian treemate isn’t wearing any clothes? Or are you sick?
A new study in Royal Society Open Science opts for choice #3. By scanning the skin architecture of bald uakari monkeys, the scientists argue that the primate’s red faces serve as an indicator of health status.
There are a couple of ways to change skin color. The first involves melanin, a natural colored pigment made by the body. This pigment sits in microscopic pouches in the skin, called granules, and helps protect against damaging UV rays. Mild sun exposure causes the skin to produce more of these melanin granules as a defensive shield. The result is a tan.
Blood flow is a second route for darkening skin. When extra blood cells course into the skin, it turns red. Such is the case when people get sunburns. Excessive exposure to the sun’s UV rays causes damage and inflammation in the skin, which widens vessels and ups the blood flow. The same happens when you blush after being embarrassed, though the trigger in that case is a hormone — adrenaline — rather than sun rays. The intensity of the redness depends on how much oxygen is carried by the blood.
To explore which option accounts for red faces in Amazonian primates, an international group of veterinarians and scientists collected skin specimens from deceased monkeys in the jungles of Peru. The primates had either died of natural of causes or been hunted for food by the local indigenous people. “No animals were killed specifically for the research, and hunters were never paid to collect samples,” the scientists write.
In the end, the team compared skin specimens from two red uakari monkeys (Cacajao calvus ucayalii) against two Poeppig’s woolly monkeys (Lagothrix poepigii), two monk sakis (Pithecia monachus), two brown capuchin monkeys (Sapajus macrocephalus) and one howler monkey (Alouatta seniculus).
Facial regions studied in the bald uakari monkey and other Peruvian neotropical primates: (a) frontal (forehead) region, (b) parietal region, (c) temporal region, (d) zygomatic (cheek) region, and (e) mandible (mouth) region. Skin samples were dissected from monkeys that either died of natural causes or were collected by subsistence hunters. Courtesy of Mayor P et al., R. Soc. open sci., 2015.
The researchers found that the red faces of uakari monkeys are caused by a higher density of blood vessels located just underneath the surface of the skin. This trait is especially true for the uakari monkey’s cheeks and forehead, which have four times as many blood vessels per square millimeter as primates without red faces. The facial skin of uakari monkeys is also 60 to 70 percent thinner than other monkeys, meaning when their blood vessels are full, the redness seems more pronounced versus regular primates.
With regards to melanin, there is none. The researchers didn’t spot any melanin granules in the facial skin of red uakari monkeys.
The team argues that if facial hue is tied to blood flow, then it might serve as a beacon for when a monkey is sick with blood parasites. As evidence, they reference a study that showed uakari faces turn white when they’re infected with the South American germ Trypanosoma cruzi. Another theory suggests that the red faces help the monkeys choose a mate, as blushing in mammals is tied to sexual hormones like estrogen and testosterone.
It will take more research to explain why these faces are red and if subtle changes in hue are cues for certain behaviors. But for now, we know the how.
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