Chapter 10

Mating Systems

1. Mating systems can be defined in terms of the number of sexual partners an individual acquires during a breeding season. Monogamy occurs when males and females both have only a single mate. Polygyny occurs when males have multiple partners, but females have just one, whereas polyandry occurs when females have multiple partners and males have just one. When both sexes mate multiply, it is termed polygynandry if there is a pair-bond between males and females, and promiscuity if there is no association between mates beyond sperm transfer.

2. Monogamy is likely to evolve when the costs to mating multiply are prohibitively high for both sexes and neither sex can monopolize access to mates or resources. This can occur because potential mates do not form groups and roam widely, making them potentially costly to locate, when individuals have the ability to restrict mating behavior in their partner, or when resources are so critical to successful reproduction that both parents are necessary to rear young.

3. Polyandry is likely to evolve when the benefits of mating multiply are higher for females than remaining monogamous. These potential benefits include indirect benefits in the form of genetic advantages for offspring or direct benefits for the female. Potential genetic benefits include higher individual offspring quality or viability, greater genetic compatibility between the female and her mate, and greater genetic diversity among a female’s offspring. Females also may mate with many partners to avoid the risk of inbreeding. Potential direct benefits include access to additional resources, access to additional parental care, better protection from sexually harassing males, and reduced risk of infanticide.

4. The diversity of polygynous mating systems has evolved in response to different patterns of female distribution, which are often driven by the distribution of resources or the risk of predation. These female decisions in turn affect the profitability of different kinds of mating tactics that males use to gain access to females. When females or the resources they need are clumped in space, female defense or resource defense polygyny becomes more likely. If, however, females are widely dispersed, or male density is high, males may engage in nonterritorial scramble competition for mates, or they may acquire mates by displaying at a lek where females come specifically to seek out mates and nothing else.

5. Multiple mating by both sexes is likely to evolve when neither sex is able to gain an advantage over the other. Species living in social groups, often because of resource or offspring-care needs, are likely to form long-term pair-bonds and to be polygynandrous when multiple caregivers are required to successfully raise young. In contrast, when there is no pair-bond and often uniparental or no form of parental care, both sexes may mate promiscuously, or indiscriminately. Promiscuous mating systems are often the result of sexual conflict between males and females.