A mating system describes the structure of sexual relationships within a species, encompassing the number of partners individuals have and the duration of their association over time. These patterns represent a species’ reproductive strategy, which has evolved to maximize the passage of genes to the next generation. The specific system a species adopts influences the genetic structure and diversity within its population.
Primary Categories of Mating Systems
The classification of mating systems is based on the number of partners each sex has during a breeding cycle, leading to four fundamental categories. Monogamy involves one male and one female forming an exclusive pair bond that lasts for at least one breeding season or longer. This pattern is common among many bird species, such as albatrosses, where both parents are needed to successfully raise the highly dependent offspring. However, while some species like the prairie vole exhibit strict sexual monogamy, many socially monogamous species engage in extra-pair copulations, meaning the pair raises young together, but not all offspring are genetically related to the social father.
Polygyny is the most common form of polygamy, where a single male mates with multiple females. This system often results in highly skewed reproductive success, with a few dominant males siring most of the offspring, as seen in species like the elephant seal. Polygyny can take different forms, such as resource-defense polygyny, where males defend a territory containing resources attractive to females, or female-defense polygyny, where males directly defend a group of females, sometimes called a harem, as observed in some gorilla populations.
Polyandry is the reverse, involving one female mating with multiple males, which is a comparatively rare system in nature. In species like the spotted sandpiper or the Jacana bird, the female will lay clutches of eggs for different males, and the males take on the primary role of incubating the eggs and caring for the young. A related system is promiscuity, or polygynandry, occurs where both males and females mate with multiple partners in a group setting, and no long-term pair bonds are formed, exemplified by species such as chimpanzees.
The Driving Force of Parental Investment
The relative amount of energy and resources each parent contributes to the offspring, known as parental investment, is a powerful determinant of a species’ mating system. In most animal species, females initially invest more due to the inherent costs of producing large, nutrient-rich eggs or carrying a fetus through gestation. This investment asymmetry often dictates that the sex investing less (typically the male) will compete intensely for access to the female to maximize their reproductive output.
For species whose young are helpless and require extensive feeding and protection, the need for biparental care favors the evolution of monogamy. A male’s fitness is highest when he stays with a single female to help rear the young, ensuring higher offspring survival and quality. Many bird species fall into this category, as two parents are often required to bring enough food to the nest to support a clutch of rapidly growing chicks.
Conversely, in species where offspring are precocial and require minimal post-birth care, males can increase their reproductive success by seeking out additional mates rather than staying with one. This lower requirement for male parental investment makes polygyny an advantageous strategy for males, as they can inseminate multiple females. Species with male-only care, such as pipefish, where the male carries the eggs in a pouch, are an exception. Because the female is freed from the burden of parental care, she can produce more eggs and compete to mate with multiple males, leading to a polyandrous system.
Ecological and Resource Influences on System Choice
Environmental factors influence the spatial distribution of mates and resources, thereby shaping the feasibility of different mating strategies. The distribution of resources, such as food or suitable nesting sites, directly affects how females are dispersed across the environment. If essential resources are scattered and difficult to defend, females tend to be widely dispersed, making it impractical for a male to monopolize multiple partners. In such environments, a male’s best option may be to stay with one female to ensure the survival of their shared offspring, which often promotes monogamy.
When resources are highly clumped, it becomes possible for a male to defend a territory that attracts multiple females. This scenario favors the evolution of resource-defense polygyny, where the male controls access to the resource, thereby controlling access to the females who need it. For example, the yellow-rumped honeyguide male may defend a beehive, which serves as a food source for females, allowing him to mate with all the females that aggregate there.
Population density also plays a role; in very low-density environments, finding a second mate may be so time-consuming or difficult that monogamy is favored simply because mates are scarce. The ability of one sex to monopolize the other—mate defendability—is thus a central concept. If females gather naturally, males will evolve strategies to defend those groups, but if females are highly mobile and widely spread, males must adopt a different reproductive tactic, which determines the species’ predominant mating system.