This study aims to describe the histology of the facial skin in the bald uakari monkey to better understand the proximate causes for changes in intensity (luminance or redness) in the red face of this species. Although several authors have observed that the facial colour in dead and sick uakaris turns pale, the hypothesis still has not been verified and little is known about the developmental and mechanistic processes that result in the red colour of the uakari face. Ayres suggested that the red face is a signal by which individuals can assess the health status of potential mates, enabling them to select mates with low parasite burdens and, therefore, good resistance to parasites. The bald uakari monkey ( Cacajao calvus), is a vulnerable ‘red-faced’ monkey emblematic of Amazonian flooded forests. In that sense, the Hamilton–Zuk hypothesis suggests that secondary sexual ornaments may reliably reflect ability to resist parasites by revealing current health status. Handicap theories of mate choice propose that only individuals of ‘superior quality’ will be able to express exaggerated secondary sexual ornaments, such as bright or intense coloration.
The reasons underlying female preference for ornamented males also remain a matter of debate. Although changes in skin redness are apparently linked to variation in blood flow and oxygenation, the mechanism for this is unknown. Oestrogen regulates variation in blood flow reducing systemic vascular resistance and increasing the cardiac output, resulting in changes of the red colour of the skin. Whereas in females, facial redness is directly linked to oestrogen in males, facial coloration is indirectly linked to testosterone, which is aromatized by aromatase to oestrogen. These oestrogen receptors are developed after puberty and are only present in sexual skin. The facial redness is influenced by the degree of epidermal blood flow through action on oestrogen-dependent receptors in the hairless face. Additionally, rhesus macaque females are more attracted by male reddish faces. In these species, facial redness varies throughout the ovarian cycle and contains information about the timing of the fertile phase. Sometimes, as in baboons and chimpanzees, these are brightly coloured external genital organs, and in other species, such as Japanese macaques, mandrills and rhesus macaques, facial coloration acts as a visual signal. Primates exhibit a vivid and colourful array of visual signals.
Thus, social functions, such as female mate preference, may be the primary designers of the evolution of primate facial colour patterns across species. While the facial colour pattern is used primarily for species recognition, secondary colour variations serve to assess individual identity, providing rich sources of information about behaviour and condition that are essential for social interactions. Primates process information signalled by faces more rapidly than other stimuli, and within the order, a wide range of phenotypes for function, location, colour and shape create a high diversity of facial colour patterns.
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