Many have been surprised by images of primates with bright blue testicles, but it seems the pigment is present for a simple reason.
Males in the Old World Monkey family, which includes patas, mandrills, vervets, talapoins, and lesula, have bright colours in strange places. This has prompted questions about why some of them have literal blue balls.
While humans may associate blue balls with sexual frustration, this is not what is happening in the primates. Fred Bercovitch, a wildlife biologist at Kyoto University in Japan, told National Geographic that hormones are not connected to the pigment of genitalia.
Despite the lack of a hormonal connection to the colouration of the scrotum, Bercovitch says it is not completely understood. Nonetheless, in mandrills, the colouration has been linked to social status within the species.
In rain forests of equatorial Africa, male mandrills have bright faces and vibrant hinds. Typically, the brighter these areas are, the higher the primates rank. In fact, it has been found that the vibrant colours can avoid male conflicts as ranks are easily identifiable. Moreover, females prefer mandrills with brighter colours.
Additionally, Jennifer Danzy Cramer, a biological anthropologist at American Public University in Charles Town, West Virginia, explained to National Geographic that vibrantly coloured males are ‘more likely to be aggressive with and bully juvenile males’. This would correlate with colouration being connected to rank.
The advantageous colouration is caused by the Tyndall effect, which is caused by the scattering of light by the skin itself. The animal’s with seemingly blue skin have overly neat and well-ordered collagen fibres, which means even a deviation as small as a millionth of an inch would result in a different colour.
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