Understanding mating systems starts with the underlying assymetry between females, which invest heavily in a relatively small number of gametes, and males, which invest little in each of a large number of gametes. This fundamental difference between the sexes means that males and females enter into mating with differing, and sometimes conflicting points of view. We start with descriptions and examples of the different types of mating systems--monogamous, polygynous and polyandrous--and then delve deeply into the intriguing issue of how animals choose their mates.

How many males, how many females?

Mating systems describe patterns of male/female pairings. Mating systems tend to be species-specific, although alternative strategies may be expressed by individuals within a species.

  • monogamy--one male, one female, often pair bonded
    • mate guarding
    • copulatory plugs
    • prolonged copulation
    • aggressive behavior mate assistance
    • serial monogamy
  • polygyny--one male, many females
    • scramble competition polygyny (term coined by Thornhill and Alcock (1983)
    • leks
    • female defense polygyny
    • resource defense polygyny (male territoriality centered on a resource needed for breeding success)
  • polyandry--one female, many males because female choice of mates seems to drive many mating systems, polyandry, at least superficially seems to be rare. In other words, in a population males vary in their quality and therefore in their attractiveness to females, so while all females have access to mating, only a small proportion of the males do (polygyny). There are a few cases of resource defense polyandry, in which the female is territorial and males coexist on her territory.
  • promiscuity
  • forced copulations as a reproductive strategy

"In light of sexual selection, the extreme rarity of polyandry (particularly in pair bonding animals) and the moderate rarity of monogamy becomes more explicable. The relative frequency of the three basic mating systems reflects the action of sexual selection on the range of possible ecological conditions; it is usually possible for some males to monopolize several females, or for females to gain by choosing those few males that will have the greatest positive effect on their fitness. Polygyny is the common outcome"p. 89, Thornhill, R. and Alcock, J. 1983. The evolution of insect mating systems. Harvard University Press.

Orians, G. H. 1969. On the evolution of mating systems in birds and mammals. Am. Nat. 103:589

Emlen, S. T. and Oring, L. W. 1977. Ecology, sexual selection and the evolution of mating systems. Science197:215-233.

Female mate choice

In general females exert more choice concerning their mates than do males of the same species. The theoretical argument explaining female choosiness is simple; females invest more in their gametes (eggs), make fewer gametes, and are more likely to be present when the young emerge than are males. The difference between female and male gametic investment, and the relative rarity of female gametes, combined with the likelihood that the female is stuck with parental care, drive females to carefully select their mates.

What effect does female mate choice have on males? Any trait that is preferred by females will be strongly favored by natural selection. This type of natural selection, called sexual selection, explains the elaborate ornamentation of males of many species, such as the peacock's tail.

Females should make their choices among males based on the likelihood that mating with the chosen male will enhance the survival of the female's offspring. This could be because the male possesses resources, such as food within a territory, or because the male has "good genes", which will then be passed on to their offspring.

"Good genes" models are complicated because any trait chosen by females will be under strong selection and the assessment value of the trait will be lost. Also, separating honest signals representing good genes from deceitful signals may be difficult for females.

How might honest signals of good genes evolve?

Male mate choice

Male mate choice is much rarer than female mate choice, but plays an important role in some mating systems. Male mate choice is likely to appear when:

  • the male has substantial involvement in parental care
  • females vary greatly in quality as mates or possess territories of varying quality
  • the mating system is monogamous

page 11-1
copyright ©2001 Michael D. Breed, all rights reserved