Monogamy polygamy and promiscuity

There are five basic types of animal mating systems (Table 6.1). Monogamy involves a pair-bond between one male and one female, whereas in polygamy, which includes polygyny, polyandry and polygynandry, social bonds involve multiple males and/or females. Promiscuity refers to the practice of mating in the absence of any social ties. Note that many species will adopt two or more different mating systems, and the examples used throughout this text are not meant to imply that a particular species engages only in the mating system under discussion.

Social monogamy is actually very rare in most taxonomic groups, one notable exception being an estimated 90 per cent of bird species. Because it is generally so uncommon, behavioural ecologists have long been interested in why any species should choose social monogamy. In a number of species, including the California mouse (Peromyscus californicus) (Gubernick and Teferi, 2000), black-winged stilts (Himantopus himantopus) (Cuervo, 2003) and largemouth bass (Micropterus salmoides) (DeWoody et al., 2000), offspring survival is substantially higher when both parents are looking after their young. This is known as biparental care and is generally more common in birds than in mammals because both male and female birds can incubate eggs and bring food to nestlings, whereas gestation and lactation in mammals mean that much of the parental care is performed by females. Biparental care, therefore, may at least partially explain why social monogamy is so common in birds.

Table 6.1 The five basic types of animal mating systems

Mating system males females Examples3

Mating system males females Examples3

Table 6.1 The five basic types of animal mating systems

Monogamy

1

1

Prairie vole (Microtus ochrogaster) Hammerhead shark (Sphyrna tiburo) Polynesian megapodes (Megapodius pritchardii)

Polygyny

1

Multiple

Red-winged blackbirds (Agelaius phoeniceus) Fanged frog (Limnonectes kuhlii) Spotted-winged fruit bat (Balionycteris maculata)

Polyandry

Multiple

1

Galapagos hawk (Buteo galapagoensis) Gulf pipefish (Syngnathus scovelli)

Polygynandry

Multiple

Multiple

Variegated pupfish (Cyprinodon variegatus) Smith's longspur (Calcarius pictus) Water strider (Aquarius remigis) Jamaican fruit-eating bat (Artibeus jamaicensis)

Promiscuity

Multiple

Multiple

Soay sheep (Ovis aries) Long-tailed manakins (Chiroxiphia linearis)

"Examples refer to social mating systems, which in some cases may differ from genetic mating systems.

"Examples refer to social mating systems, which in some cases may differ from genetic mating systems.

If offspring can survive without paternal care, and if a male can make himself attractive to multiple mates, then polygyny may result. In many species this occurs when resources such as food are distributed patchily, because males can then defend high quality territories that will each attract multiple females. In Gunnison's prairie dogs (Cynomys gunnisoni) for example, monogamy prevails when resources are uniformly distributed whereas polygyny or polygynandry is often found when resources are distributed in patches that are guarded by one or several males (Travis, Slobodchikoff and Keim, 1995).

Very occasionally, the sexual roles of males and females are reversed and females, which in these cases tend to be larger and more colourful than males, will compete for and defend territories to which they attract multiple males. The males will then perform most of the parental care. This mating system is known as polyandry, of which the American jacana (Jacana spinosa) is a well-studied example. In this species the female defends large territories on a pond or lake, and in each territory several males will each defend their own floating nest and incubate the eggs that the female lays there. The most likely explanation for this unusual mating system is the habitat in which it occurs (Emlen, Wrege and Webster, 1998). Suitable nest sites are scarce and predation is high. If female jacanas laid only one clutch at a time, then very few of her offspring would survive and the fitness of both males and females would be low. If, however, females simultaneously lay multiple clutches and a proportion of these survive, the female will increase her fitness. Although males appear to be disadvantaged by this mating system, they may have little choice in the matter when there is such strong competition for suitable nest sites.

Polygynandry refers to the situation in which two or more males within a group are bonded socially with two or more females. This differs from promiscuity, a system in which any female can mate with any male without any social ties being formed. Differentiating between polygynandry and promiscuity may require a detailed study of a particular social group, and in fact the two terms are sometimes used interchangeably. Promiscuity is very common in mammals, occurring in at least 133 mammalian species (Wolff and Macdonald, 2004). It has also been documented in birds such as sage grouse (Centrocercus urophasianus) (Wiley, 1973) and in a number of fish species including guppies (Poecilia reticulata) (Endler, 1983). Promiscuity can have high fitness benefits to males if they can fertilize multiple females. Females may also benefit from promiscuous mating, as illustrated by field experiments on a number of species, including adders (Vipera berus) (Madsen et al., 1992) and crickets (Gryllus bimaculatus) (Tregenza and Wedell, 1998), that have shown increased offspring survival when females mated with multiple males. This may result from one or more of a number of factors, including genetically variable offspring, increased parental investment and a reduced risk of male infanticide.

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