Monogamy--a bond between two partners of opposite sex--is a relatively rare phenomenon in mammals (3-5%, from a total of 4000 mammalian species). The duration of the bond may vary from one breeding period to life-long. Monogamy does not exclude 'genetic promiscuity', i.e., extra-pair mating. In fact, this is rather common. Monogamy is an intrinsically unstable mating strategy. Benefits include the (relative) certainty of access to the partner's reproductive potential, but the chief disadvantage is that access to other potential partners is strongly diminished, particularly in those cases where males exhibit strong mate-guarding behavior. Mate-guarding, in particular the guarding by males of females with a good territory or of females that accept the territory of a male, has been shown to be the most important selective pressure leading to the evolution of monogamy. A monogamic bond strongly favors the evolution of male investment in the raising of offspring, as is the case in most birds (90% of bird species are monogamic and most exhibit biparental care of young). Mammals exhibit this type of behavior to a far lesser extent (female mammals monopolize the feeding of newly born young). Most male mammals do not look after their offspring; humans are an exception in this respect. Like most mammals, humans are not strictly monogamic. A tendency to social monogamy has evolved, however, and is subject to strong reinforcement by cultural factors, particularly religion. As a result, in a number of cultures monogamy is the predominant mating system; however, most cultures (about 85%) are polygamic. For humans, the optimal evolutionary strategy is monogamy when necessary, polygamy when possible.