Incest in humans is, and almost always has been, one of the most common of all cultural taboos. Most modern societies have laws regarding incest, not least because children born of close incestuous union have a greatly increased risk of disorders, death and disability because they are more likely to be affected by recessive or deleterious traits.
Within the rest of the natural world, of course, the social taboos associated with incest don’t exist, but many animal species will still avoid mating with their close relatives if there are alternative partners available. However, if there are no other options, some animals, especially insects and other invertebrates, regularly mate with their own siblings. Mites are particularly prone to this. As soon as straw itch mites hatch from their eggs, they begin to attack their mother and suck out her juices. They hatch sexually mature, so just as soon as they have finished eating their mother, the newborn males grab their sisters and mate with them.
The female Adactylidium mite, meanwhile, lives her entire life as a parasite on the eggs of a tiny insect called a thrip. From the moment she is born, she already has several females and a single male developing within her body. The baby females inside her take it in turns to copulate with their brother even before they are born. As they moult and grow, they start devouring their mother from the inside. Eventually, the mother mite is little more than a swollen bag of discarded shells and faeces, and the tiny impregnated females within her now cut holes in their mother’s body so they can emerge and find their own thrip eggs to live on. Unequipped to move or even feed on his own, the lone male will die still within his mother’s dead remains. In essence, all male Adactylidium mites die before they have even been born. But his sister’s don’t fare much better, though – within a few short days, they too will have been eaten alive from the inside by their own offspring.
And it’s not just mites that have overly-incestuous relationships. The date stone beetle feeds on and spends at least part of its life within dates, which are the fruit of the date palm. As soon as a clutch of beetle eggs hatch, brother and sister will quickly mate with one another, but all is not lost if a young female misses out on this opportunity. She will go off in search of another date stone, boring into it and excavating a chamber within. She will then lay a clutch of unfertilised eggs, all of which will hatch into males. The female mates with her first son that reaches maturity and then she lays a larger batch of fertilised eggs that will hatch into both males and females.
As for the beetle’s son/lover… well, she eats him just as soon as mating is complete, and then she goes on to devour his poor brothers, who ended up serving no purpose whatsoever. This certainly puts Oedipus to shame.
Of course, it’s probably worth noting that many kinds of animals – including aphids, crayfish, snails, and even reptiles as big as Komodo dragons – can reproduce asexually without a mate. And you surely can’t get more incestuous than breeding with yourself…