Polyandry in Zoology
In the field of behavioural ecology polyandry is a type of breeding adaptation in which one female mates with many males. Another opposite breeding system to this is polygyny in which one male mates with many females (e.g., lions, deer, some primates, and many systems where there is an alpha male).
Polyandry is positively correlated with testicle-to-body weight across taxa (see Sperm competition). Human testicles are lighter than those of chimpanzees—including the highly promiscuous bonobo chimpanzees—but heavier than those of gorillas and orangutans.
A common example of polyandrous mating can be found in the Field Cricket Gryllus bimaculatus of the invertebrate order Orthoptera (containing crickets, grasshoppers, and groundhoppers). Females in this species will mate with any male close to them, including siblings. Widely shown in frogs (Agile frogs, Rana dalmatina), polyandry was also documented in polecat (Mustela putorius) and other mustelids.
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.
Some taxa with high social organization are eusocial, meaning that a single female (e.g., the queen bee) or caste produces offspring while the other organisms (e.g., worker bees) cooperate in caring for the young. Examples of mammalian eusociality include Damaraland mole rats and naked mole rats, among whom polyandry is the norm and polygyny has never been observed. Female mole rats compete for the status of Queen or “alpha female” (see Naked mole rat#Queen and gestation).
Commonly in canine and feline reproduction, plural ova are fertilized during the same instance of estrus by various males each, so that a single litter of puppies or kittens may have more than one father. This is called heteropaternal superfecundation; very rare in human reproduction, but is documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents were involved in paternity suits.
Related to sexual conflict, Thierry Lodé found possible evolutionary explanations for polyandry including mate competition and inbreeding avoidance.
- It is easier to ensure reproductive success (i.e., it is more likely that the female will have offspring)
- Females may be encouraging sperm competition between males post-copulation
- Multiple sperm lines may confer more variation in traits to female's offspring, this seems to be the case in the honey bee where bees from different sperm lines excel at different roles within a single hive, benefiting the health of the hive as a whole.
- Females may receive food offerings from prospective mates inciting copulation
- Offspring paternity is unknown and this can be beneficial in encouraging parental care and discouraging infanticide by males
Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, whales, around 1% of all bird species, such as jacanas, insects such as honeybees, and fish such as pipefish.
According to Gordon G. Gallup, 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.
In effect polyandry will reduce the effective population size of a given closed population.
Read more about this topic: Polyandry