Nine out of ten Earthlings agree: Nothing beats a hot pair of twins. If you are already attracted to someone, the only thing that can possibly improve their overall hotness is discovering that there is two of them. In fact, that’s the best theory I’ve heard yet to describe why identical twins, as a phenomenon, are so popular in everything from DoubleMint Gum commercials to Playboy spreads: we singletons tend to objectify them as the same person with the advantage of having two bodies. But biologically speaking, does having a clone confer any advantage to you as an individual, or even to you as a species?
First, a primer on twinning. Dizygotic twins, otherwise known as fraternal — or, in the case of two females, which is more common, sororal — twins are the product of two separate eggs, and form in two separate placentas. In humans, having any kind of twin is a gamble — even a fraternal twin is five to seven times more likely to die in the womb than a singleton fetus, and at much higher risk of mental retardation, learning disabilities, respiratory problems, cerebral palsy, and a host of other health problems. But in the animal world, di- or polyzygotic young are the norm; we call them litters. In a cruel world, a species usually cannot count on only children to further itself, and so hedges its bets with siblings.
More rare in humans and other animals are monozygotic twins; that is, identical twins developed from a single egg and placenta. You might be a twin or know a twin who looks very different from his or her womb-mate, as environmental factors such as lifestyle choices and childhood illnesses cause certain genes to express themselves in one twin and not another. Identical twins may share the same DNA, but don’t bear the same fingerprints. I’m a singleton myself, but sometimes I imagine a hypothetical twin brother I might have had who works out, is a vegetarian, and hasn’t been drinking coffee daily since age 14. He is 6’1″, physically fit, has a normal haircut, and I secretly hate his guts.
Twins in human reproduction seem to be a happy accident; after all, twins make up a mere 2% of the world’s population, with identical twins or triplets constituting only 8 percent of those, or 0.2% of all people. But what about species in which twinnage is fairly common? Can producing two or more genetically identical offspring be a successful reproductive strategy? At first glance, the animals that frequently have twins have little in common: ferrets, cats, sheep and deer all frequently bear twins, and polar bears almost exclusively do. But for popping out passels of identical bundles of joy, one mammal has the rest beat: the nine-banded armadillo, which as a rule produces litters of identical quadruplets.