While this male mandrill may look unfriendly, mandrills are social animals that live in large groups in Africa's rainforests. Each pack is led by a dominant, alpha male.

These brightly colored, or "fatted" alpha males--as seen in this picture--are the only ones to sire offspring, and have much higher levels of testosterone than the paler, "non-fatted" males. The red color on the male's face and genitalia also indicate its dominance within the group.

This remarkable coloration becomes more pronounced as the male approaches sexual maturity. By comparison, the female's colors are a drab brown and olive.