A bachelor herd of Impala ceases to forage to stand warily at our approach. Impala (Aepyceros melampus) are among the most abundant mammalian fauna here, and like the adorable dik-diks (who I will eventually post about), they are often passed up without much attention. Color-wise, the most eye-catching features of impala are the black stripes on the rear, black-tipped ears, and lone black spot on the sides of the abdomen. Herds of impala are most often seen running away, white tails flapping back and forth all the while. But it took me several weeks to notice they are actually bicolored animals, split into a darker tan coat on the top half and a pale coat on the lower half. This color pattern is common among herbivorous middle-sized mammals and is thought to be an antipredator adaptation called countershading. The marked dorsoventral contrast breaks up the outline of the animal, making it more difficult for predators to detect amid surrounding vegetation. Downwelling light also illuminates the top part of the body more brightly than the bottom half (termed counter-illumination), which reduces the effect of shadows and allows the entire body of the animal to be more uniformly colored against the background. I shot this photo in low light in the evening just after the sun had set (my favorite type of light), and without strong sunlight the dark dorsal part of the coat becomes even more distinguishable.
The impala mating system is a resource-defense polygyny, in which a single male will defend a harem of females and juveniles to retain exclusivity in resource use which is attractive to receptive females. For those males who are unable to usurp an alpha male from his harem, they congregate into groups of bachelors to prevent themselves from becoming easy pickings to predators. While in bachelor herds, impala will graze together and mock fight to up their skills in hopes of eventually attaining a harem themselves. Most impala in bachelor herds are young males with smaller horns, either short and straight or scimitar-shaped, though large horned males that are ousted from their harems due to lower body condition or fighting ability are also present. Bachelor herds will sometimes encompass a male and his harem, aiming to separate and drive him out to allow for a successor. We were fortunate to witness this strange arrangement of male and female impala together last week. In a normal setting, female impalas are seen grazing and moving in tandem, followed closely behind by a vigilant male.
Photographed in situ [1]