Polygynandry is a reproductive strategy that occurs when two or more males have an exclusive sexual relationship with two or more females. The numbers of males and females need not be equal. In vertebrate species studied so far, the number of males usually is lower than the number of females in their breeding groups. Some animals follow this behavior regularly, others resort to it in uncharacteristic circumstances. Polygynandry also may occur in mating systems that typically are polygynous, when a population becomes too large for males to maintain exclusive harems successfully.