Polyandry is defined as “the mating of one female with more than one male while each male mates with only one female.” Exclusive polyandry (as opposed to polyandry in concert with polygyny) is very rare, occurring in only about 1% of animal populations, most being shorebirds like the sandpiper. The basis of polyandry is a sex role reversal. The females compete for the males and are larger and more colorful, while the males take on the parental role. With the sex role reversal, a natural selection against older males evolves. This is accomplished by the females tending to select the males with the best sperm in order to give the female the most offspring possible. Younger males will more likely have fertile sperm; therefore impregnating the female on more instances than an older male with less fertile sperm.
There are two major types of polyandry: simultaneous and sequential. Simultaneous polyandry is when the female controls a very large territory. Inside this territory, the female has multiple smaller nesting territories with different males. The female mates with all males simultaneously, keeping control of the smaller territories. Another form of simultaneous polyandry, cooperative simultaneous polyandry, is when the female only has one nesting area where she mates with multiple males producing a clutch of eggs of mixed parentage with all males contributing to the eggs. Sequential polyandry, the most common form, is where the female mates and produces a clutch of eggs with one male, then leaves the male to incubate and rear the eggs while moving on to another male in a different nesting territory. Here, the female moves from one male to another, leaving the male in full responsibility of the eggs instead of sharing the responsibility.
There are many genetic benefits of polyandry. The first being fertility insurance. This hypothesis suggests that by mating with multiple males, the female is guaranteed to fertilize all of her eggs. The multiple partners potentially make up for one male that may not be able to fertilize the eggs. The good genes hypothesis states that the females have multiple mates because she is in search of the male that will pass along the best genes to her offspring. By finding this male, the female is increasing the survival rate of her offspring. The genetic compatibility hypothesis is one that suggests that the female finds multiple mates in order to find the most compatible genetic match for her eggs. While looking for a good match, she is also eliminating the males that are least compatible with her eggs.
The material benefits of polyandry can be seen through three hypotheses. The more resource hypothesis suggests that the more mates the female has, the more males she has to care for her clutch. The better protection hypothesis states that by having multiple partners, the female is better protected from predators. The infanticide reduction hypothesis is one that claims that since the female has multiple males, she has a lower infanticide rate because the males do not know which progeny belong to them. This prevents the males from killing other male’s young. Although some infanticide occurs between females and other female’s eggs, it is minimal among males.