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Mating Systems

Mating systems may be conceptualized as a compilation of behavioral strategies that individuals in a population utilize to obtain mates. Discussions of mating systems usually include reference to: 1) number of mates acquired by individuals of each sex, 2) manner(s) of mate acquisition, 3) presence of "pair bonds," and 4) patterns of parental care provided by each sex. These four factors are associated in consistent ways referred to as polygyny, monogamy, and polyandry.

The ability of a portion of the population to control the access of others to potential mates is a critical factor determining the mating system. Control can be direct, as in the physical herding of mates and exclusion of others competing for mates. Control of access to potential mates can also be indirect, as in the control of resources critical for either mate attraction or successful reproduction. The greater the degree of control or monopolization, the greater the resulting variance in mating success, and the greater the level of intrasexual competition.

What, then, causes one population (or species) to have one type of mating system, and another population to have a different type?

Environment -----> Mating system

Ecological constraints impose limits on the degree to which sexual selection operates (within phylogenetic constraints). Two variables seem critical: 1) distribution of resources, and 2) distribution of mates.

If resources are distributed uniformly in the environment, then there is little opportunity for resource monopolization, and polygamy based on resource control will be difficult. On the other hand, if resources are unevenly distributed, the potential for obtaining additional mates increases because some individuals may be able to control a larger quantity or better quality of resources than others. If mates are clumped (perhaps because of a localized resource), then defense of multiple mates may be possible. The assumption that increased male PI results in lowered reproductive variance as a absolute rule is false.

The temporal distribution of mates can also affect the mating system. If females become sexually receptive in unison (a short mating season), there is little potential for individual males to monopolize multiple females. If the time involved in acquiring a single mate is a significant portion of the total time that mates are available, trends toward polygamy will be minimal.

Emlen and Oring (1977) derive the following "ecological classification" from the above principles:

MONOGAMY Neither sex has the opportunity of monopolizing additional members of the opposite sex. Fitness often maximized through shared parental care. Eg. Doves, Gibbons.

POLYGYNY Individual males control or gain access to multiple females

Resource defense polygyny Males control resource important for female reproduction and some males control more resources than others. Eg. Marmots, Honeyguides.

Female defense polygny 'Herding' of females by males. Eg. Elephant seals, Langurs.

Male dominance polygyny Resources not monopolizable, males aggregate and females select mates. Two types:

Explosive breeding assemblages Females are sexually receptive in unison, reducing the degree of polygyny. Eg. Wood frogs.

Leks Females not synchronized, and the 'dominant' males receive a high proportion of the matings.

RAPID MULTIPLE CLUTCH POLYGAMY Both sexes reproduce via several mates, and no individuals are able to monopolize multiple mates at the same time.

POLYANDRY Individual females control access to multiple males.

Resource defense polyandry Some females are able to control territories with several males, who provide parental care for the female's offspring. Eg. American Jacana, Wilson's Phalarope, Saddle-backed Tamarins.

Male defense polyandry Females are able to keep other females away from their mates.

Read Chap. 7 !!!

Bonobo