Ripe Fruit is Better
In order to discern ripe from unripe fruit, vervet monkeys have colour vision. The most favoured and productive resources for vervet monkeys is a tree in fruit. Since fruit makes up a large proportion of their diets it is imperative to be able to tell when it is ripe.
While it is still green, fruit contains tannins and alkaloids, which often taste bitter and are sometimes even toxic. Once they ripen they lose the distastefulness and become rich in healthy sugars. Sometimes vervets do still eat the fruit while it is green.
Colour vision assists with detecting ripe fruit and in dominance displays by the males but a higher incidence of cones, the colour cells, to rods, the light-sensitive black and white cells, in the eyes means that night vision is compromised.
Before nightfall, vervets take to the trees, the larger the better, where they hide in dense foliage to escape detection by nocturnal predators. They huddle together while sleeping for assurance in the dark and for warmth during the colder months.
Generally vervets group according to rank at night with the more dominant individuals forming one sleeping group and the subordinates a separate one.
Who's the Boss?
An advantage of colour vision is that visual communication is enhanced. The scrotum of a mature vervet monkey male is bright blue while the penis is bright red. Because the dominant males always sit with their legs apart and their genitals conspicuously on display, there is no mistaking who’s the boss.
The blue colour develops at puberty due to boosted testosterone levels and resulted in the Afrikaans name for this animal “blouaap” or “blue ape”. High ranked male vervets will demonstrate their sexual maturity and social standing to other males by flashing their genitals at any given opportunity. They walk with a swagger swishing the tail aside to reveal the genitals and contrasting white fur around the anus.
The may also rise up on their hind legs in front of a sitting animal and expose the sometimes erect penis to the other’s eye-level gaze. While a dominant male always exposes himself, a subordinate male always responds by retracting its penis.
By Megan Emmett