In behavioral ecology, polyandry is a class of mating system where one female mates with several males in a breeding season. Polyandry is often compared to the polygyny system based on the cost and benefits incurred by members of each sex. Polygyny is where one male mates with several females in a breeding season (e.g., lions, deer, some primates, and many systems where there is an alpha male). A common example of polyandrous mating can be found in the field cricket (Gryllus bimaculatus) of the insect order Orthoptera (containing crickets, grasshoppers, and groundhoppers). Polyandrous behavior is also prominent in many other insect species, including honeybees, the red flour beetle, the adzuki bean weevil, and the species of spider Stegodyphus lineatus. Polyandry also occurs in some mammals including primates such as marmosets and the marsupial genera Antechinus and bandicoots, and in around 1% of all bird species,^{[citation needed]} such as jacanas and dunnocks, and in fish such as pipefish.

It is theorized that polyandry is more prevalent in organisms where incompatibility is more costly, and where this incompatibility is more likely. The former is especially true in viviparous organisms. Where the cost of having a low-quality father is significant, however, an organism is less likely to be polyandrous.

The adaptive significance of polyandry in animals is controversial. Polyandry has direct benefits for females allowing fertilization assurance, provision of resources, and parental care for their offspring. House mice (Mus musculus musculus) have shown indirect, genetic benefits, where females have increased offspring survival through multiple mating, showing that practicing polyandry mating results in an increase in offspring viability. In a meta analysis, including 10 different orders of insects, polyandry increased the production of eggs by females specifically in Lepidopterans and Orthopterans. Indirect benefits of mating for females can be gained through sperm competition to attain "good genes" (Good-sperm hypothesis), cryptic female choice, increased genetic quality, and genetic diversity. Females spiders (Pisaura mirabilis) store more sperm from gift-giving males suggesting that sperm storage is under female control through cryptic sperm choice. The increase in sperm storage from the gift giving males might allow females to produce "sexy sons" that also give gifts and increase the fitness of offspring. Sperm storage and fertilization success increased with copulation duration, suggesting an advantage in sperm competition. Polyandry also works as evolutionary bet-hedging to avoid extinction of female lineage due to male infertility or mating failure (bet-hedging polyandry hypothesis).

Many reptile species also demonstrate polyandry, especially among members of the tortoise family (Testudinidae). Through polyandry and long-term sperm storage, recent studies have found evidence for the ability of female tortoises to produce clutches of eggs that demonstrate multiple paternity. Predictably, these hatchlings showed an increase in genetic variability compared to those sired by a single male. Potential for multiple paternity within a clutch is primarily a result of sperm storage across reproductive cycles, since studies have confirmed the presence of multiple males' sperm in the female tortoise reproductive tract simultaneously. As a result of clutches with greater variation in paternal genes and increased sperm competition, females can maximize both the genetic quality and number of offspring. Multiple paternities within a single clutch is therefore considered an effective strategy to increase the reproductive success and fitness of female tortoises.

Acorn woodpeckers provide substantial information on breeding systems because they practice monogamy, polyandry, and polygyny. In polyandry the presence of more male breeders in acorn woodpeckers has shown that females reproduce the optimal clutch size and that with paternity sharing between males it is behaviorally more stable increasing male and female fitness. As opposed to polygyny, where it has been observed that polygynous females that lay a large number of eggs exceeding the optimal clutch size reduces the fitness of the group. Polygynous trios compared to polyandrous trios showed that having more eggs lowered the group fitness. Chao (1997) studies using acorn woodpeckers suggest that other mating systems are practiced at a lower rate when compared to polyandry because it is polyandry that can maximize the fitness of males and females when obtaining optimal clutch size. In polyandrous mating, optimal clutch size is obtained because there is only one female and it becomes more stable when all members remain together.

Multiple mating is also seen in wattled jacanas where females have a group of males close by for mating. Copulations are easily seen in jacanas, facilitating the observation of females copulating continuously with various males. The continuous copulations and the close proximity to mates have allowed the females to fly in within minutes leading to the expected sperm mixing and allowing the most viable sperm to fertilize most of the clutch or to increase genetic diversity for benefits in disease resistance. Snow and Andrade (2005) concluded that the redback spider (Latrodectus hasselti) increases their ability to manipulate the paternity of their offspring by using the spermathecae to store multiple sperms. Additionally, a study on leaf-cutting ants (Acromyrmex echinatior) supported the hypothesis that sperm mixing indeed occurs in polyandrous social insects. It was further found that the eggs in the queen ants showed to be completely mixed and used randomly during egg laying.

According to Gordon G. Gallup, as a form of adapting to multiple mating in females, human penile shape is indicative of an evolutionary history of polyandry. Male humans evolved to have a wedge- or spoon-shaped glans and to perform repeated thrusting motions during copulation in order to draw foreign semen back away from the cervix and thus to compete with sperm of other males.

Polyandry may also impose costs on females, exposing them to diseases, increased predation risk, time and energy costs, and even physical harm due to sexual harassment. Polyandrous females encounter sexual harassment when courted by males resulting to be costly to females. In order to reduce the costs to the females, females will take part in polyandry. For example, in the bee species Anthidium maculosum, like in many other species, the high cost of resisting mating may exceed the cost of accepting numerous males in a breeding if copulating takes a short time, and therefore females in some species are inclined to being polyandrous. In the polyandrous system, sexual coercion has been observed to be one of the major reasons for why females begin to mate with multiple males to outweigh the cost under different circumstances. The sexual conflict hypothesis suggest that polyandry can occur due to sexual coercion preventing females from obtaining any benefits. Sexual coercion has been seen in three different forms: forced copulation, sexual harassment, and intimidation. Forced copulation is seen in those males that cannot gain access to females for mating. As a result of the lack of access to mates, males are more likely to engage in sexually aggressive behaviors that could result in physical harm and death for females, as has been observed in elephant seals and ducks. Intimidation was seen in social primates, who coerce females into mating, therefore, pushing them to mate with their own aggressor. Males use intimidation as a method of punishment in females that do not mate with them.