In a monogamous mating system, consistent pair bonds are formed between two individuals with sexual exclusivity. Polygamy is a general term used to describe a system in which an individual mates with 2 or more partners of the opposite sex. Two subsets exist within this definition. In a polygynous system, one male mates with two or more females. In a polyandrous system, one female mates with two or more males.
Polygynandry is a mating system defined by the sexual sharing among two or more females and two or more males. This term is often used interchangeably with promiscuity, but it differs because pair bonds form among specific individuals. In a promiscuous regime, no pair bonds are formed.
Across the board, mating systems develop to optimize the reproductive success of individuals. Monogamy will prevail when it is most fitness enhancing for those involved. For example, if females are dispersed, which is the case in some mammal and many bird species, monogamy will likely be the dominant strategy observed. If females are forced into a small area, such as female elephant seals cramped along a shoreline, males will use the opportunity to sequester females and maximize their reproductive output.
Monogamy will also prevail if the fitness of the offspring is dependent upon biparental care. In many bird species, the male and female are both fully capable of incubating the eggs and feeding hatchlings. If only one parent is involved, the probability of the chicks’ survival is compromised. Female-enforced monogamy is when a female eliminates the risk of other competing females from copulating with her chosen mate. A deep-sea angler fish male is born as a small, helpless sperm packet who lacks a digestive system and is entirely dependent upon his destine host female. He swims though the deep ocean in search of her, and has specialized mouth parts that assist in him in his permanent attachment to her.
Polygyny is by far the most commonly observed mating system in Animalia, and its presence is based on the extent to which males can monopolize females. This model applies to species in which the territory of a male contains useful resources for the female and her offspring. The environment plays a significant role in this mating system because females are easier to sequester when resources are spatially clumped. For example, female elephant seals in the
Polyandry has been considered an evolutionary problem because it was thought to challenge one of the major underpinnings of sexual selection. It is the assumption that the disparity between male and female gamete investments (anisogamy) has favored the selection for diverging mating habits between males and females. Females are thought to be the “choosier” sex because they invest more energy into generating eggs, while males tend to favor quantity over quality and their fitness will not be lowered if they mate with a female of low genetic quality.
This line of thinking has gone under extreme scrutiny recently, as there has been accumulating molecular evidence revealing multiple paternity (polyandry) to be a common practice in animals. Multiple hypotheses have developed to explain this. The material benefits hypothesis predicts females who mate with more than one male are privileged to access more resources, parental care, and a limitless sperm supply. The genetic benefits hypothesis broadly predicts females who are mated with multiple males will produce offspring who are the result of superior ejaculates. If the offspring are males, they potentially have the same high quality sperm and will be able to heighten the fitness of the mother.
There are unique cases that do not fit in the categories described above. For example, we see an investment role reversal in Red Phalaropes of Alaska. Females exhibit traditionally masculine qualities. They are bigger and more colorful than males and play no role in incubating or caring for the young. The female spends about a week courting a single male and laying her eggs in his nest. Females of this species are able to generate eggs quickly, and after a week the female begins searching for another mate. Males incubate the eggs and nurture the chicks alone.
Polygynandry and promiscuity are similar yet distinctly different mating systems. Polygynandrous females regularly form pair bonds and copulate with several males at the same time as males form pair bonds and copulate with several females. The polygynandrous European badger lives in social groups with as many as five mothers and five fathers that produce multiple-paternity litters. It is thought the primary reason for this is described by the resource dispersion hypothesis: polygynandrous groups may arise because limited optimal resources may force more than one breeding pair within a territory. There is usually a dominant pair and a number of subordinates within a social group. In some species the subordinates do not mate, but in many cases subordinate females produce litters of mixed paternity.
Promiscuous species are those in which pair bonds do not form and males and females are likely to copulate with more than one individual of the opposite sex. It is a unique system because subordinate males have the opportunity to access females and male reproductive success is a function of post-copulatory strategies such as sperm competition and cryptic female choice. For example, when brown headed cowbirds live in areas with abundant resources, territorial lines are not well defined and promiscuity is often reported. The brown headed cowbird is a nest parasite that lays its eggs in other birds’ nests and provides no parental care for its young. Promiscuity may be the obvious choice given the birds’ behaviors and their lack of parental care.