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Mating systems in mammals

About mammals > Reproductive processes
Burchell’s zebras

Depending on the environment, mammals will adopt various mating strategies. In some species such as beaver (genus Castor), the maintenance of a pond, lodge, and dam to maintain water level and insure security of the offspring requires the efforts of both parents, and thus both sexes must combine efforts to raise young to adulthood successfully. In such species, the sexes not only combine gametes, but also efforts and they remain together throughout the mating season, or for life. The term monogamy describes such systems, where animals remain with one mate either annually or permanently.

Only 3% of mammals are monogamous but monogamy is found within nearly all mammalian orders and predominates in some families, for example in foxes, wild dogs and gibbons. Some examples of seasonal monogamy would be in red foxes (Vulpes vulpes), in which males provide parental care, but where couples break-up after rearing of young. Permanent monogamy would be best exemplified in North American beavers that mate for life (Castor canadensis). Generally speaking, monogamy occurs in species where the support from the males is instrumental to the rearing of young, and males gain more from limiting the number of offspring (by staying with one female) and investing instead in the growth of their offspring. However, many so-called monogamous males will opportunistically attempt breeding with other females, paired or not, to increase their genetic fitness, and true monogamy occurs rarely when lack of potential mates occurs, or under pressures from the environment.

Polygynous mating systems occur when males do not provide paternal care, and hence pursue matings with numerous females. Such are probably best exemplified in ungulates and pinnipeds, where males maintain access to several females simultaneously. In these harems, males try to control female breeding by asserting their dominance over other males, and if successful, winning males sire offspring from several females, whereas females only breed with a single male.

Not all females accept a single male, and in some species, both males and females will mate with numerous individuals. Promiscuity describes such a system, and is probably best exemplified in wide-ranging carnivores such as mink or wolverines. In these species, females cannot compare mates because they are spatially widely scattered, so females may mate with numerous individuals, and rely on other mechanisms to choose mates such as sperm competition. Promiscuous females also occur in certain social structures where uncertainty of paternity in males prevents them from killing the offspring of the female (infanticide). Such a social structure occurs in prides of lions (Panthera leo).

Finally, a mating system exists where male alliances may form to care for the offspring and allow females to spend most of their energy producing, and not caring for, offspring. This system, called polyandry, may occur in mammals under extremely biased sex-ratio in adulthood where males are extremely abundant, and females extremely rare, as for example with the African wild dog or the naked mole rat, with one “queen” and all else workers. This scenario is much less common but occurs in some human cultures.

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