If there’s anything that Game of Thrones has taught us, it’s that sleeping with your sister is not a good idea. That’s certainly the case for humans. However, incest is more common than you’d think in other species.
When it comes to mating, the banded mongoose (Mungos mungo) likes to keep things in the family. The cat-like mammals conceive with their relatives at a surprisingly high rate, according to research published last week in the journal Biology Letters.
Known for their distinctive black stripes, banded mongooses live in family groups and travel the grasslands of Africa, as well as Asia and Europe. A male and female who do most of the mating dominate packs, and younger subordinates only breed occasionally. While some stray from the fold, most stay with the same pack their entire lives. Like lions, they all raise their young together.
Scientists from the U.K. and Germany observed 14 separate packs of the petite mammals over 16 years in Uganda’s Queen Elizabeth National Park. The team tracked individuals from afar to get a sense of their behavior. With hair samples from the animals’ tails, they determined how everyone was related and discerned the paternity of new mongoose pups.
About 63.6 percent of the pups came from pairings within the same family group. Meerkats and prairie dogs inbreed at similar rates, but the mongooses showed unusually high levels of mating between close relatives. In 26.7 percent of the births, parents were the genetic equivalent of half-siblings and in 7.5 percent of the births the parents were the genetic equivalent of siblings or a parent/child. Fathers paired up with daughters eight times, while no mothers mated with sons—probably because females don’t live as long as males.
It’s weird. But is it icky? That depends on how you look at it.
Our own psychological stigma against incest and inbreeding is driven by the serious consequences of picking a mate with whom you share a lot of DNA. Inbreeding produces less genetic diversity, and a child of inbreeding has a higher chance of inheriting a disease or disorder caused by a mutated recessive gene. Some attribute King Tut’s clubfoot to sibling incest between his parents. (It’s worth noting that mating with your cousin comes with significantly lower genetic risk than mating with your brother, though.)
Out in the wild inbreeding can lower the chance of survival for offspring, reduce fertility or sperm production, and leave individuals vulnerable to diseases. All this could make a population more likely to go extinct. As a result, many plants and animals have evolved innovative ways to avoid inbreeding.
So, why would a species like the banded mongoose favor breeding between relatives? Mongoose packs are apparently very West Side Story. New packs face a lot of conflict with established groups, and most interaction between individuals from other groups is pretty violent. Mating with a cousin or brother is safer than risking life and limb to mate with an outsider. “Hence, there might be a net benefit, at least to some females, of breeding within the natal group,” the researchers speculate.
Banded mongooses aren’t the only case where incest comes with clear advantages. Relatives are sometimes more invested in parental ties. In African cichlid fish, incestuous parents are less likely to fight over the logistics of caring for babies.
Incest is simply a matter of convenience in other cases. In the tropics, ambrosia beetle species invading new island ecosystems practice incest to reduce time spent finding a mate. The eggs of incestuous couples are also more likely to hatch. Weirdly, incest may preserve minute genetic adaptations to each island environment—ultimately allowing the bugs to form new species as they invade.
Longhorn crazy ants commit incest sans inbreeding. When they invade new territory, populations are low, and the queen has limited mate options. Sometimes she mates with a male relative. However, instead of producing worker ants with both parents’ genes, she produces male and female clones of the male relative and herself, respectively.
Incest is a numbers game. Prevalence depends on context, and sometimes unique advantages outweigh the genetic costs. For certain species, striking a balance between incest and outside mating is a normal, perhaps even smart, evolutionary strategy.