Control of scrotal colour in the vervet monkey

The vervet monkey has a vivid blue scrotum which pales when the animal falls in social rank. Histology and electron microscopy showed dopa-positive melanocytes in the dermis each packed with fully melanised melanosomes. By transmitted light the scrotal skin was brown on a red background: by reflected light the colour was blue. Thus the blue colour is due to Tyndall scattering over a layer of melanin. Variation of scrotal colour was not due to changes in melanocyte number or dispersion of melanosomes. Pallor was induced by injecting fluid, and blueness could be restored by removing fluid. It is concluded that the blue-to-white colour variation is modulated by the degree of dermal hydration.