Definition and Distinction

Monogyny is one of several mating systems observed in nature, and it is usually referred to as a male’s tendency to mate with only a single female; the female, however, mates with multiple males. It is important to emphasize the distinctions between monogyny and polyandry

Polyandry

Polyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered...

, and monogyny and monogamy

Monogamy

Monogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...

. Polyandry is a mating system by which a female mates with more than one male; the male, in turn, can also mate with more than the one female. Monogyny, however, is a mating system by which the males mate only once. In a monogynous setting, the male mates with only the one female; the female, however, mates with numerous males. In a monogamous setting, both male and female consent to having only one mate at any one time and thus mate only with that partner for that time period. Hence, monogyny is sometimes referred to as male monogamy because the male will only mate with the one female and no one else while the female will mate with all the males. The model of monogyny arises from evolutionary circumstances in which males are restricted to mate with only one female in their lifetime.

Evolutionary Significance

Male animals, especially in species where males provide little or no parental investment (time and energy invested in current offspring at the expense of future offspring), are generally expected to maximize their fitness by mating with several females. In certain monogynous settings, however, paternal investment by the male is greater than that of males in other mating systems because the benefits of paternal protection exceed those of searching for additional mates. Paternal investment includes even dramatic examples such as the remarkable adaptation of male sacrifice via sexual cannibalism and the ability to inflict genital damage unto oneself to increase paternity success. In these circumstances, selection may favor extreme mechanisms of paternity protection that amount to a maximal investment in a single mating.

There are also circumstances in which monogyny evolves when males do not provide any paternal investment. Researchers have focused on sexual behaviour in systems where males have low paternal investment but frequently mate only once in their lifetimes, after which they are often killed by the female. Mating effort is high for these males. In particular, time and energy costs or risks incurred by males in securing a given mating could decrease the relative number of males available for mating; this type of mating is called non-promiscuous. Researchers have focused on species of web-building spiders with males that show high levels of non-promiscuous mating effort but apparently low paternal investment. The mechanism of male monogamy (monogyny) in these species is indisputably the most extreme form of non-promiscuous mating effort.

Monogyny in Ants and Honeybees

In species of ants and honeybees, there is only one female queen who mates with all the males in her colony; the males attend to the queen and mate only with her. There are circumstances, however, where a colony can become queenless, and therefore certain males must adapt to this surrounding in order to increase paternity. In ants and honeybees, there are two different types of monogynous settings. Type A are monogynous, queenright colonies where the queen is the female and everyone else is male. Type B are monogynous, worker-reproductive colonies where there is no queen but rather there are gamergates, which are mated workers who take on a queen-like role. The queen is normally the only egg producer. However, when a colony becomes queenless, some workers which have intact, undeveloped ovaries may develop them and thus become capable of laying more eggs. So in certain colonies, a singly mated worker called a gamergate reproduces as the functional queen in that colony. These workers are termed totipotent; that is, they are able to change and adapt to a different surrounding in which they no longer have a queen.

Monogyny in Spiders

Males in certain species of spiders often employ drastic methods to be paternally successful. Monogyny in spiders culminates in extreme traits, such as dramatic male self-sacrifice and emasculation of the male by the female during copulation. Since males only mate with one female in a monogynous setting, each individual male must do whatever it takes to increase his particular paternity success, even if it means sacrificing himself. Male redback spiders twist their abdomens onto the fangs of their mates during copulation and, if cannibalized (65% of matings), increase their paternity relative to males that are not cannibalized. In this way, males of the redback spiders in a monogynous setting increase their chance of paternity by actually surrendering themselves to be cannibalized by the female.

Costs and Benefits

From a male standpoint, evolutionary theory suggests that the focus of mating is to enhance paternity in order to produce viable offspring. Therefore, sexual selection theory would suggest that a male should attempt to mate with several females. This means that if a male wants to ensure that he will be paternally successful, he should mate with more than one female. When the sex ratio is male biased, however, male monogamy (monogyny) would arise as a means of increasing paternity and producing offspring; in other words, if the setting contains a sex ratio of all male to one female, then monogyny would arise as a means to produce offspring. This model predicts that a male-biased sex ratio is required for monogyny to evolve.

For Females: the advantages and benefits are significant to that of males and are clear and obvious. The monogynous female is dominant over the males and has great reproductive value.

For males: Since they can mate with only one female, they must adapt in order to increase their chance of paternity.

Male Adaptation

Males can adapt in order to increase paternity in a monogynous setting. An example of this would be the formation of gamergates in a queenless colony of honeybees and/or ants. Another example would be male-sacrifice in order to increase paternity in certain species of spiders. The costs of increasing paternity in a monogynous setting are great for the males; in certain species of spiders the male will surrender himself to be cannibalized in order to increase paternity. In this respect, the benefit for the female is that she will receive the chance to eat if she is hungry; the cost for the male is the loss of life to increase his paternity.

In certain species, male adaptation will include the process of pedipalp damage. Males in species of Nephila fenestrata, for instance, can protect their paternity by obstructing the female's genital openings with fragments of their copulatory organs. The male will actively participate in damaging his genitals by breaking off parts of his copulatory organs during mating and obstructing the female’s genital openings in order to be paternally successful.