Consider the Armadillo
Nature-Nurture-Noise (2)
Armadillos are mammal but not at all prototypical. They are very odd-looking creature. Among their other idiosyncrasies armadillos are virtually hairless, their nakedness covered by a hard carapace lending them a somewhat reptilian exterior. But of armadillo quirks, one above all will concern us here, because it aptly illustrates the theme of this book, which is the inexorable and pervasive role of chance in making us who we, individually, are.
That seems a lot for an armadillo to answer for, and they, alone, can’t. But armadillos have one singular trait that will help shape our intuitions for what’s to come. First, though, some background on this remarkable creature.
The story begins on the Mexican Border with Texas. The border is a river, the Rio Grande. On the Mexican side a particularly intrepid species of armadillo has flourished for thousands of years. It is called the nine-banded armadillo, Daspyus novemcinctus. Daspyus means rough foot in Latin; novemcinctus means nine banded, in reference to the nine elastic rings of tissue around the torso, between the plates of the carapace.
Like all twenty species of armadillo, the nine-banded species originated in South America, where it remains common. Unlike all but one other armadillo species, the nine-bandeds expanded their range outside of South America. The other species to exit South America stopped in Central America; the nine-bandeds though, kept moving northward all the way to northernmost Mexico. So, the Mexican population is already far from is area of origin.
The Rio Grande long remained a barrier to further northward expansion, until the 1850s, when a few Mexican nine-banded armadillos somehow managed to cross it. The ones who managed this feat may have done so as a last resort while being pursued by one of their many predators. A jaguar perhaps, or a cougar, or a wolf, or a coyote. In any case it was probably an act of desperation, when all the other anti-predator strategies failed.
An armadillo’s primary defense is, of course that external armor, which is made of very tough stuff. Many erroneously believe that nine-banded armadillos can curl up into an impervious armored ball. Only three-banded armadillos can actually manage to do so. Even when nine-banded armadillos curl up to the max, much valuable flesh remains unprotected. Hence, in response to a predatory attack, they usually dig for their lives with their long, thick claws and powerful forearms. Once partly buried or wedged in rocks, its head and belly protected, an armadillo is generally safe, even if its rear and tail are exposed. It is very difficult for most predators to extract a half-buried armadillo. Their armored backsides and semi-armored tails limit damage from even the most persistent predator. Most of the time.
Those hapless armadillos that don’t make it sufficiently far down the hole can only ball up, which doesn’t much deter many predators, including humans. Unfortunately for armadillos, we humans can also harvest the ones with their butts in the air. Long before armadillos crossed the river they were valued for their meat throughout their range. But Americans (Texans) didn’t take to it at first, disdaining it as poor man’s pork, and during the Great Depression, “Hoover hogs”, in honor of the man most responsible for overcoming their reticence.
Armadillos also have a unique behavioral defense, their first line of defense actually: they can jump from a four-point stance straight up in the air, up to four feet it is claimed. This sudden leap seems to disconcert predators, including human hunters, buying precious time for the dash and dig. If there is nowhere suitable to dig, but a body of water is at hand, they employ a dash and dive strategy. For surprisingly, armadillos can hold their breath for a long time, up to six minutes it is alleged. They then trot across a river or lake bottom.
Even six minutes can only get an armadillo so far, after which they deploy the last weapon in their escape arsenal. Their oxygen-supply exhausted, armadillos bolt for the surface and inhale large volumes of air to fill not only their lungs, but their intestines and some other cavities as well. They then float like a cork and dog paddle the rest of the way. The rest of the way can’t be too far though. The breadth of the Mississippi river for example, is much too far.
The much smaller but still considerable Rio Grande River would have deterred all but the most motivated individuals. No matter how motivated most would have died in the process or turned back. Only a very few individuals succeeded, judging by their remarkably low genetic variation in the U.S.
Such low levels of genetic variation are generally not conducive to evolutionary success. But succeed these strange creatures did. In fact, they massively proliferated, first in Texas, then increasingly northward and eastward. Today armadillos can be found throughout Oklahoma, Arkansas and Missouri to the north, and to the east, from North Carolina to Florida. Recently armadillos have colonized southern Illinois and they are not done yet. Armadillos are projected to eventually reach Pennsylvania, New Jersey and Connecticut, aided in part by global warming. For the limiting factor in armadillo expansions is climate. They originated in a warm and wet environment; that legacy remains a constraint. No long periods of sub-freezing temperatures and no arid environments for this intrepid species.
Into the Lab
While expanding their range, armadillos particularly abounded in eastern Texas. Locals first came to notice them as road kill. Their defensive strategies are ill suited for speeding automobiles, so their corpses soon littered the roadways, a nuisance called “hillbilly speed bumps” by local wags. By 1900 living armadillos were a common sight in East Texas despite their nocturnal habits. One enterprising biologist at the University of Texas, Oliver Peterson, decided to bring them into the lab for a closer look. He was particularly interested in their embryonic development. Peterson couldn’t get them to breed in his lab, but he had no problem capturing pregnant females. Though squeamish about indoor coitus, these female armadillos had no qualms about raising their litters in captivity.
Peterson began his observations with the neonates. He quickly recognized that they always came in packages of four and that each package consisted solely of one sex; some litters were all female, others all male. Same sex quadruplets. Very strange. His further investigations revealed that the quads were genetically identical, having derived from one fertilized egg. In human twins this condition is termed “monozygotic”. Armadillo quadruplets are monozygotic twins squared.
And Peterson recognized that his armadillos could be a perfect animal model for human twin studies, a burgeoning new field of genetic research. The 19th century geneticist, Francis Galton, was among the first to study twins. It was Galton who bequeathed us the categories Nature and Nurture, so seductively alliterative, so attractively simple, so deeply simplistic.
Peterson was a careful observer. In carefully observing armadillo quad clones he noticed individual differences in the arrangement of the bony elements on their heads, called scutes. It’s natural to assume that these differences must be caused by differences in the environments to which they were exposed. But the scute patterns are established in utero, an environment common to all littermates. So the differences in scute arrangements were not caused by genetic differences, nor, apparently, differences in their environments. Puzzling.
Further investigation revealed a host of other traits, present at birth, that varied within quad clones. Body size, for example, also the size of major organs including hearts, brains, livers and kidneys. Moreover, hormone levels vary noticeably within quads, as do those of key neurotransmitters in the brain. In essence, each individual armadillo, in each and every set of quad clones, is quite anatomically and physiologically distinct from its genetically identical siblings from the same environment.
Galton’s nature-nurture dichotomy doesn’t provide an adequate framework for understanding these individual differences. In explaining individual differences in armadillo quads there’s a lot left over once we’ve accounted for genes and environment. Sometimes, what’s left over accounts for more of the individual differences than Nature and Nurture combined.