Ah! What is that creature? No, he didn’t dip his face in a can of paint or smear it with red fruit. This is the bald uakari (Cacajao calvus), one of the least known, but most easily identified species of primate. While its face might not be the most attractive to humans, the red face of the uakari actually serves a purpose.1 Uakaris’ bright red heads are hypothesized to be an indicator of health: the brighter the head, the healthier the monkey, and the more desirable the mate (both males and females have red heads!)1. The bright red skin of the bald uakari’s face is actually a result of very few pigmentation cells and many capillaries under the skin.2
A male bald uakari; check out his stumpy tail and bright red face! [Source]Besides their bright red domes, uakaris also stand out due to their shaggy fur and bobtails. Even though these primates are easily identifiable by sight, it’s quite hard to study them! The natural habitat of the bald uakari floods annually during heavy rains.1 Uakaris are arboreal and spend the entire wet season up in the trees. During the dry season, uakaris will occasionally come down from the canopy to forage.
Bald uakaris (Cacajaocalvus)are arboreal and spend the entire rainy season up in the trees. [Source]Bald uakaris reside in the flooded forests of Brazil and Peru, which exposes them to an unfortunate micro-“predator”. Malaria-carrying mosquitos also prosper in these flooded forests3. Uakaris are quite susceptible to malaria, just like humans. Because the uakari’s red head is a reflection of the individual’s haematologic state, it is fairly easy to identify a sick uakari by the lightening or paling of their red heads3,4.
Recent research suggests that this easy identification of sick animals might reinforce the uakaris’ preference for bright red-headed mates4. Uakaris live in multi-male, multi-female groups, and have a polygamous mating system. Members of a social group may be checking each other out as potential mates; pale-faced uakaris often get passed over for their brighter-headed comrades4. This may have driven uakari heads to be brighter and redder due to sexual selection, à la Hamilton-Zuk hypothesis4.
Bald uakari pigmentation is thought to indicate the health status of the individual, and might be a sexually-selected trait. [Source]Interestingly, the three other Cacajao species do not have bright red faces like the bald uakari, although they too live in seasonally-flooded forests and are likely also exposed to similar vector-borne diseases. More research is needed to elucidate the evolutionary and ultimate mechanism for this mysterious phenotypic trait in only one of four very closely-related species. Like many New World primates, uakaris are extremely vulnerable to habitat destruction. Luckily for the bald uakari, their red faces look quite human-like, and so they are not considered desirable as a food source5. Unfortunately, the three other uakari species are poached for food. Further, the ongoing deforestation of South American rainforests continues to threaten all four species of uakari, highlighting the need for biologists to study these elusive yet fascinating primates.
[By: Allison Lau]
References:
1Ayres, J.M.C. (1986). Uakaris and Amazonian flooded forest (Ph.D.). University of Cambridge.
2Hill CA. 1965 Maintenance of facial coloration in the red uakari Cacajao rubicundus. Int. Zoo Year B.5, 140–141
3Deane, L. M. (1992). Simian malaria in Brazil. Memorias do Instituto Oswaldo Cruz, 87, 1-20.
4Mayor, P., Mamani, J., Montes, D., González-Crespo, C., Sebastián, M. A., & Bowler, M. (2015). Proximate causes of the red face of the bald uakari monkey (Cacajao calvus). Royal Society Open Science, 2(7), 150145.
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