Mating systems in the animal kingdom are the complex patterns by which individuals within a species associate for reproduction. These patterns are shaped by evolutionary pressures and the specific ecology of a species, falling within the field of behavioral ecology. Mating systems determine how reproductive opportunities, parental care, and resources are distributed among males and females. They ultimately influence the social structure of a group and the genetic diversity passed on to the next generation.
Defining the Primary Forms of Mating Systems
The most common way to classify mating systems is by counting the number of partners an individual has during a breeding cycle, leading to four primary forms. Monogamy describes a pair bond formed between one male and one female, lasting for at least one breeding season. This system is common in many bird species, such as albatrosses, where both parents are required to successfully raise the young.
Polygamy describes any system where an individual mates with multiple partners. The most frequent form is polygyny, involving one male mating with multiple females, common in mammals like red deer. The reverse is polyandry, a relatively rare system where one female mates with multiple males, often occurring when males perform the majority of parental care, such as in the jacana bird. Finally, promiscuity (polygynandry) involves multiple males and multiple females mating without forming stable pair bonds, typically seen in species like chimpanzees where parental care is minimal.
Ecological and Evolutionary Drivers
The evolution of a species’ mating system is profoundly influenced by the relative investment each sex makes in their offspring. Parental investment is the time, energy, or resource a parent expends to increase the survival of current offspring, often at the cost of producing future offspring. Because females typically invest more initially (producing larger gametes and often bearing the young), they become the more choosy sex.
When offspring survival requires the effort of two parents, such as in environments where resources are widely scattered, selection favors the male remaining to provide care, promoting monogamy. This is known as the male-assistance hypothesis: a male maximizes success by ensuring the survival of his offspring rather than seeking additional matings. Conversely, if a female can successfully raise young alone, a male’s strategy is to maximize his number of partners, leading to polygyny.
The distribution of essential resources also dictates mating system structure. If resources like food or nesting sites are clumped and can be defended, a male may monopolize them, attracting multiple females to his territory. This scenario, called resource defense polygyny, means that ecological factors determine a male’s ability to control access to mates. The need for a specific nesting territory, rather than the act of mating itself, can be the limiting factor that males compete to control.
The Distinction Between Social and Genetic Mating
Modern genetic analysis has revealed a complexity in animal relationships, distinguishing between social mating and genetic mating. Social mating describes the observed behavioral association, such as a male and female sharing a nest or territory and cooperatively raising young. Historically, scientists assumed this social bond implied exclusive sexual fidelity, known as genetic monogamy.
However, genetic studies, particularly in birds, have shown that Extra-Pair Copulations (EPCs) are common, even in socially monogamous species. EPCs are matings between an individual in a pair bond and a partner outside that bond, resulting in Extra-Pair Paternity (EPP). For instance, in eastern bluebirds, up to 35% of the young in a nest may be sired by a male other than the social father.
This phenomenon shows that social and genetic strategies can be mixed. Individuals gain the benefits of a stable partnership, such as biparental care, while also pursuing genetic diversity. Females may seek EPCs to acquire “good genes” from a genetically superior male, even if he will not provide parental care. This insight highlights that the biological reality of mating systems is often more fluid than initial behavioral observations suggested.