My last blog post I introduced mating systems and gave examples of each. There are 3 main types: monogamy, polygamy (polygyny and polyandry), and promiscuity. For this blog post, I want to focus on the evolutionary advantage to males and females for some of the mating systems.
The adaptive advantage of polygyny in males and monogamy in females is easily understood. Males produce millions of sperm at a time and can copulate many times during a mating season. Females, however, only produce so many eggs at a time, and do not need to copulate multiple times in order to obtain enough sperm to successfully fertilize her egg(s).
There are examples of species where males are monogamous and other species where females are polyandrous. In these instances when and why is it advantageous for males to be monogamous or females to be polyandrous?
When males of a species are monogamous there are two reasons why this would happen. One is that the female will likely be receptive after one mating. The second is if the chances of finding another female are low. If both of these are true for that species, it is likely the male will tend toward monogamy to ensure that as many offspring as possible are his when he finds a female.
Many male spiders will either break off their genital appendages in the female’s reproductive tract or feed themselves to the female. This is done to decrease the chances that the female will copulate with another male and the offspring are more likely to be from the male who sacrificed himself.
Another example is the clown shrimp, whose females are widely dispersed in the environment, and are therefore difficult to find. Once a male finds a female, he will stay with her until she is ready to mate (known as mate guarding). This will ensure that when the female is ready to mate, the male guarding her will be the father of the majority of her offspring.
Another situation that would lead to male monogamy is explained in the mate-assistance hypothesis. This hypothesis states that when parental care and protection of offspring is especially advantageous, males will likely be monogamous.
While males guard females they not only deter other males from mating with the female, but they have also been known to protect females from predators. One example is in some cricket species, who, after sight of a predator, will allow females to enter their burrow first. This is risky for the male, but protects the female. Along with protecting the female, the male cricket will be the father of the majority of the offspring because he will have had the majority of the copulations with that female.
Another example of mate-assistance is seahorses. Male seahorses hold eggs as they gestate, and simultaneously the female will produce more eggs. In this situation the male and female match their reproductive cycles so that once the males have completed caring for the offspring, the female will have another clutch ready. Therefore, the male does not need to go in search of another female, and the female does not need to match her reproductive cycle with another male.
There are also instances where monogamy is not necessarily advantageous for the male, but male monogamy is advantageous for the female. In this case, the female will force monogamy on the male. One example of this is found in burying beetles. In this species, the male and female bury a mouse that the female will lay her eggs inside. Once they complete this process, the male will begin to release a mating pheromone to attract other females to the same spot. However, if he is successful, the new female will also lay her eggs in the same mouse, which will lead to offspring competition once the eggs from both females hatch. Therefore, it is more advantageous for the first female to stop the male from attracting other females. In this way, she is stopping him from being polygynous and forcing him to be monogamous.
That answers the first question about male monogamy. But what about female polyandry? There is increasing evidence supporting female polyandry occurring much more than previously thought. Why would females want to copulate more than once, if once is all they need to fertilize their egg(s)? There are many risks to copulating with multiple males. They would be using more energy finding and copulating with multiple males, putting themselves at risk of predation while searching for more males and during mating, and there is risk of acquiring sexually-transmitted diseases.
However, it has been found that both males and females produce more offspring when they copulate with more partners. This is not surprising for males, but why females? There are clear benefits to copulating with many males. The benefits are divided into two categories: indirect (genetic) or direct (material).
Indirect benefits are referring to benefits such as genetic benefits that would lead to more offspring with different genes that could help to ensure that some of the female’s offspring will survive. One example of a genetic benefit is copulating with multiple males reduces the risk of the female having an infertile partner. This is termed the fertility insurance hypothesis. This is supported by the observation that eggs of polyandrous female red-winged blackbirds are somewhat more likely to hatch than monogamous ones. Also, in Gunnison’s prairie dogs females become pregnant 100% of the time if polyandrous, but only 92% of the time if monogamous.
Direct benefits are referring to benefits directly to the female. One example is through resource acquisition. In some species, if a female mates with a male, he will allow her in his territory, and she will be able to forage in that area.
There are many more examples than what I have shared with you. There are also many more hypotheses when thinking about mating systems and why some species exhibit one over the other. There is more being found out every day that adds to our understanding of mating systems and what about certain species life history contributes to the mating system they exhibit.
Alcock, John. 2013. Animal Behavior: Tenth edition.