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.
If you live in Texas or anywhere in the American South, you might recognize the nine-banded armadillo as a bowl full of roadkill or, more colorfully, the “hillbilly speed bump.” That’s because the nine-banded armadillo — whose bands of ossified armor can number between seven and ten depending on region — cannot roll into a ball like its three-banded cousin, but instead reacts to an immediate threat by jumping three or four feet straight into the air, a strategy which sufficiently surprises a coyote but which does not impress the radiator grill of an oncoming 18-wheeler.
The nine-banded armadillo is native to South America, where it is as comfortable on the Argentine pampas as in the Brazilian rainforest — it is part of the Edentata family with the sloths and anteaters, a family of mammals that lack incisors. But the armadillo also thrives on the hills, if not the highways, of Texas. It is the last straggler of the Great American Interchange, a Yankee Swap of animals between the Northern and Southern continents that has been going on since the Cenozoic. Unlike the hummingbird and the opossum, South American conquistadors who have been on our side of the Isthmus for over 3 million years, the armadillo only started colonizing the United States in the past hundred years or so. Human introduction — particularly during the Great Depression, where they were hunted and eaten as “Hoover Hogs” — is largely to blame, but you can also chalk it up to the fact that they’re surprisingly good swimmers for something with its own suit of armor. Because armadillos are burrowers by nature, they’ve evolved the ability to hold their breath for up to six minutes while they’re face-deep in dirt, an adaptation that also suits them when they need to cross bodies of water like the Rio Grande. If the river’s too wide for the heavy-bodied armadillo to ford by simply walking along the bottom, it can inflate its intestines full of air and become its own flotation device.
Armadillos have thrived in the U.S. for many reasons. They are adaptable insectivores, for one. Being recent arrivals, they have no predators specialized enough to hunt them. (Coyotes with can openers?) If chased, they can dig a burrow fast enough to wedge themselves into, presenting pursuers with only a bony posterior. But of greatest importance is their fecundity: armadillos can produce litters of four every year for fourteen years, and reach sexual maturity at age 1. But here we reach the difficult question: if genetic diversity drives evolution within a species, why on Earth would an armadillo evolve to produce litters of genetically identical offspring? Wouldn’t twins be counterproductive to a healthy gene pool?
Let me see if I can explain my scientific inquiry here without indulging in any incest jokes about the inhabitants of rural Texas. Let’s say Armadillo A and Armadillo B, two female armadillos with adjacent territories, each bear four pups who are identical quadruplets: the female Aa’s and the male Bb’s. To simplify things, we’ll say for the sake of argument that two of the Aa’s are hit by pick-up trucks, one Bb is shot by a bored redneck, and one Bb dies at the hands of the Mexican drug cartel during a deep undercover mission for the DEA. But the other two Aa sisters and two Bb brothers, being close neighbors, fall in armadillo love and have their own armadillo babies. The resulting two sets of cousins are now closely related enough to be siblings in any other species, and the aunts and uncles of those armadillo babies are no different, genetically, from their parents. You can imagine how, after several generations, the armadillo population of this county in Texas is in something of a genetic bottleneck, and there’s more than one reason the local armadillos don’t have any front teeth. (Dang it.)
Lord help them if the quads are born with a real genetic defect, for then all four are likely doomed. Or if the quads are genetically superior to other litters and all survive to breed, it’s a mixed blessing: it’s fantastic news for the parents but ultimately leads to local incest down the line. In armadillo reproduction, it’s all or nothing; no singular armadillo has any genetic advantage over its brothers or sisters, and shares the exact same probability of survival. Yet it must be to the armadillo’s advantage as a species to bear identical quadruplets, despite the enormous risk, or else they’d have singletons or fraternal twins. So why does the armadillo go monozygotic at all? It’s partially the fault of its weird physiology: armadillos can delay the development of a fertilized egg for up to four months until conditions are favorable, which is to their advantage, and that egg lives in a uniquely shaped uterus that can only accommodate one egg. In most species, that would necessarily mean one offspring. But by splitting into four, the armadillo mother quadruples the chances of what is, genetically speaking, still only one baby with four bodies.
As far as my incest & inbreeding scenario, it’s conjecture that’s never been proven to happen. Perhaps the armadillo only has a survival rate of 1 out of 4 in its first year. Perhaps it travels too far in its lifetime over its enormous range to put its genes in the same pool as its identical siblings. Or perhaps there is some mechanism that prevents incest in armadillo culture; a pheromonal “Do Not Enter” sign. In any case, it’s clear that the armadillo’s best armor isn’t its bony plates. It has conquered the nighttime lands from Nebraska to Uruguay by counting on being a constant loser; expecting to be prey, it quadruples its chances by quadrupling its genetic self. There is safety in numbers, and the armadillo’s best protection is multiplicity. You can afford to die once or twice when you are many.