Polyandry in animals
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).[1] 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). Polyandrous behavior is also prominent in many other insect species, including the red flour beetle, the adzuki bean weevil, and the species of spider Stegodyphus lineatus. Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, around 1% of all bird species, such as jacanas and dunnocks, insects such as honeybees, and fish such as pipefish.
This article is about polyandry in non-human species. For polyandry in humans, see Polyandry.Predictors of polyandry[edit]
It is theorized that polyandry is more prevalent in organisms where incompatibility is more costly, and where this incompatibility is more likely.[2] The former is especially true in viviparous organisms.[3] Where the cost of having a low-quality father is significant, however, an organism is less likely to be polyandrous.[2]
Paternity sharing[edit]
Males that compete for more mating opportunities are the ones that have shown less cooperation in breeding. However, there are other males that in order to gain access to females and mating opportunities practice parental care. Such parental care is mostly seen in polyandrous mating systems; but they must go through the cost of sharing paternity and parental care. This has been observed in avians like acorn woodpeckers (Melanerpes formicivorus),[12] and fishes as the cichlid fish (Julidochromis transcriptus).[23] Observations that show male relatedness suggest the importance for male social relationships in taking care of offspring. These social relationships in males are seen in primates, Geoffroy's tamarins, Saguinus geoffroyi.[24] High proportions of related males at the half sibling or higher were found.[24]
As for frog species, rhacaphorid frog (Kurixalus eiffingeri), is one of the few that is polyandrous and exhibits parental care of eggs. The male frogs sit on the egg clutches, maintaining contact with the eggs.[25] Parental care and males genetic contribution were found significantly correlated meaning that male frogs with a higher paternity attended their eggs far more significantly than those of partial paternity for the clutch.[25] Females that mate with more than one male obtain the aid of both and as a result of all three parents providing food and care to the offspring there is an increase in the survival of their young. The amount of help is determined based on mating share and paternity share. (Mating Systems, p 275)
In polyandrous mating it has been suggested that male cooperation may outweigh the costs of sharing paternity in situations of scarcity, of foods and of high competition levels for land or females. Female size and a large breeding territory defended by more males may force male cooperation. For males, such factors can promote an increase in reproductive success, leaving behind the cost of paternity share (Mating Systems, p277). Comb-crested jacana's (Irediparra gallinacea) has shown genetic evidence for paternity assurance.[26] The comb-crested jacana females had one to three more mates available for copulation. In order to guarantee paternity to their male mates, the females would spend time in the territory of the male, laying her eggs.[26]
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., non-reproductive female worker bees) cooperate in caring for the young. Examples of mammalian eusociality include Damaraland mole-rats and naked mole-rats,[27][28] among whom polyandry is the norm and polygyny has never been observed.[29]
Polyandry and infanticide avoidance[edit]
Infanticide avoidance is one of the major reasons animals, like mammals, are inclined to choose polyandry.[30] Infanticide is observed to be the major cause of mortality in various mammals. Polyandry is thought to evolve from their offspring's vulnerability to infanticide.[31] There is evidence that polyandry is an effective strategy that is adapted by many to increase their offspring survival through the avoidance of male infanticide. Bank voles (Myodes glareolus) has supported the infanticide avoidance hypothesis where polyandrous females' offspring have a higher survival than monandrous females' offspring.[32]
Polyandry benefit exception: green turtles[edit]
High levels of multiple paternity have widely been seen and reported in snakes, marine turtles,[33] and lizards.[18] However, Lee and Hayes (2004) have evidence that suggests that there is no set data in green turtles that significantly shows that there are potential fitness gains through polyandry. Instead, it is suggested that the multiple mating is a result of male aggression. Both Wright[4] and Lee and Hayes studies exhibit similar results that show a lack of support of the indirect benefits of polyandry. In addition, they pose that there is no relationship between multiple paternity and reproductive success. Multiple male paternity is present in green turtles, but no significant fitness benefits have been found. Observations done on clutches showed that there was no correlation of reproductive success when mating with multiple males.[19]
Callitrichidae[edit]
Current research suggests that polyandry is the dominant social structure in the New World monkey family Callitrichidae. The Callitrichidae includes marmosets and tamarins, two groups of small New World monkeys found in South America. Wild groups usually consist of three to ten individuals, with one reproductively active female, one or more reproductive males, and several nonreproductive helpers that can be either male or female. Cooperative polyandry is not the only mating system found in these primates. Polyandrous, monogamous, and polygynous groups can be found within the same population, and a group can even change mating systems, making it the most flexible mating system of any non-human primate.[34] Unlike most primates who typically give birth to single young, twins are the average litter size for tamarins and marmosets. The entire group participates in raising the offspring, sharing the responsibilities of infant carrying, feeding, and grooming. The presence of nonreproductive helpers appears to be the most important factor in determining which mating system is used, as ecological and environmental variability have not been found to have a significant impact. Goldizen (1987) proposed the hypothesis that monogamy in Callitrichidae should develop only in groups with nonreproductive helpers to help raise the young, and in the absence of these helpers, both polyandrous males and females would have higher reproductive success than those in lone monogamous pairs. Indeed, in studies of Saguinus fuscicollis, common name saddle-back tamarin, no monogamous lone pairs have ever been seen to attempt a breeding cycle.[35]