The less inbreeding there is in a population of animals, the healthier the future offspring will generally be. Due to this, some animals have methods to prevent inbreeding. These methods are either pre-copulatory or post-copulatory (pre-sex or post-sex) mechanisms. The ability to recognize and avoid mating with related individuals is an example of a pre-copulatory strategy. However, not all animals are able to easily recognize other members of the population that they’re related to. Therefore, some animals might rely on post-copulatory strategies to produce non-inbred and healthy offspring.
A post-copulatory strategy of avoiding inbred offspring is for a female to mate with multiple males. Doing so can essentially water down the relative’s sperm, so that it is less likely any given offspring is a product of inbreeding. This also makes it possible for the female to use cryptic female choice – a mysterious process by which females of some species can decide which male’s sperm to use to fertilize her eggs after mating with multiple males.
Speechley, Gasparinia, and Evans (2019) tested how female guppies prevent inbred offspring. Guppies (Poecilia reticulata) are a suitable species to use to test this because it is known that inbred individuals have poorer health and that polyandry (a female mating with multiple males) is common but not necessary in their normal reproductive habits.
The main questions these researchers were trying to answer were: Do female guppies avoid mating with brothers? Are they more likely to mate with more males if they’ve mated with a brother as compared to a non-relative male? What proportion of a female guppy’s offspring are from her brother if she was mated with him?
This study began by placing virgin female guppies with one (“focal”) male each. Roughly half of the females were placed with their brother while the other half were placed with a non-relative. The guppies were left together until the researchers saw them mate. After that, the focal male was removed and the researchers added three non-relative “stock” males to the female’s tank. They left these fish together for three days. The females then were separated from all males and the researchers waited for her to give birth. After the birth, the researchers collected DNA from each baby to determine how many were the children of the focal male as compared to the three stock males.
In this experiment, the female guppies mated more often with the stock males if the focal male they had mated with had been their brother. This is an important finding; this means that the females likely do recognize that they mated with a relative and so took efforts to mate with more males. As theorized, this behavior could improve the fitness and health of more of her offspring.
It is not clear how a female guppy might recognize a relative. These females spent 3 months out of their 6 month lives separated from their brothers, so the fact that there’s a difference in mating behavior indicates that there is some cue that informs the female of their relatedness. The cues could be olfactory, visual, acoustic, or some combination of these, the authors of this study theorize.
The focal males fathered a low proportion of the offspring, no matter if they were relatives of the female or not. The brothers fathered 14% of the offspring from the females they were mated with, and the non-relatives fathered 16% of the offspring from their females. This 2% difference is not very significant, but this could be explained by the fact that the focal males only mated with the females once, while the stock males had many mating opportunities. Due to this negligible difference between successful fatherhood in relatives and non-relatives, there seems to be no internal cryptic female choice mechanism at play here.
In summary, females who have mated with a brother are more likely to mate with a new male than females who mated with a non-relative. This supports the theory that polyandry is a strategy female guppies use to avoid inbreeding. This adds to a growing body of research showing that polyandry is a common and effective mechanism a variety of species use to prevent inbreeding. This is because polyandry allows the female to dilute the sperm from her relative, which increases the health of her average offspring. The researchers also found that females are somehow able to recognize kin, though how they do this is still unknown.
Citations
Speechley, E. M., Gasparini, C., and Evans, J. P. 2019. Female guppies increase their propensity for polyandry as an inbreeding avoidance strategy. Animal Behaviour 157:87-93. https://doi.org/10.1016/j.anbehav.2019.08.016
