Spare Parts Page 7
FASCINATING FAR END FACTOID
Baby rattlesnakes are born without rattles. They don’t form the first segment of their rattle until one to two weeks of age, when they shed their skin for the first time. Each time a rattlesnake sheds its skin, which can happen two to three times per year, a new section is added to the rattle.4
For several unusual animals, the tail is a distinct feature of difference. Consider the axolotl, a salamander native to Mexico. His tail is decorated with fins, which means he’s happily at home both on land and in the water. The satanic leaf-tailed gecko, named for what looks like horns on top of its head, can not only change colors but also has a tail that looks like a leaf, two perfect covers that enable him (or her) to hide from predators in Madagascar, the only place on earth where you can find him (or her).5 But none of the unusual modern tails come close to that of a creature called Microraptor gui, a possible ancestor for both dinosaurs and birds. This little fellow, whose first name comes from the Greek and Latin words meaning small thief, was a member of the Sauropsida, a group that includes all living birds and reptiles as well as their ancestors. He might grow as large as 2.5 feet from nose to tail, had a feathered body, four wings, and tail feathers, but did not fly. Fossils of these animals, on display at various museums in China, show a fascinating addition to the world of plain old tails, one so striking that the raptor was given a role as a guide named Guido—a subtle play on both guide and the name Microraptor gui guaranteed to make paleontologists smile—in Universal Studio’s animated film The Land Before Time XII: The Great Day of the Flyers (2007).6, 7
Tails have also figured prominently in our mythology, often imparting to one fantastical character a trait from another life form. Taking the best known in alphabetical order, we can start with Cerberus, the very large dog with three heads who guards the Gates of Hell. He has a snake’s tail, as does Chimera, the mother of the Sphinx. The Gryphon, a.k.a. Griffin or Griffon, a character known for stalking, has the tail of a lion. Manticore, the Persian winged nightmare, has a lion’s body, a human head, three rows of sharp shark’s teeth, and a tail that shoots missiles. Satyrs have a male body and a horse’s tail. Mermaids have a female body and a fish’s tail. The sea god Triton has two fish tails instead of legs.8
The Greek & Roman Sea God Triton, Son of Poseidon/Neptune
Back here in Real Life, for the many animals with tails, the three most interesting ways to use one may be for talking, walking, and attracting the opposite sex.
Talking with your tail. We humans communicate primarily with spoken words, but we also use physical signals. We smile to say “yes,” frown to say “no,” and, depending on the angle of the wrist and the motion of the hand, wave to say “hello,” “goodbye,” or “forget about it.” Clearly, animals sometimes vocalize with intent. Who could mistake the warning in a canine growl or a feline hiss? But, like us, animals also “speak” with other body parts, such as a dog with his expressive tail. Dogs learn about one an other by sniffing scent glands in the anal area. Thus Monique Udell, founder of the Canine Cognition and Behavior Lab at the University of Florida in Gainesville, explains that a dog will cover his genital area with his tail if he’s not ready to trust another dog. Tucking his tail over his rear and under his body is “a way to say, ‘I’m nervous here … I don’t really want to have this interaction’—like a human a crossing their arms and shifting away.”9 As for that wagging tail we humans generally see as a sign of happiness or friendship, that can be a complex communication among animals. In 2013, Marcello Siniscalchi and his team at the University of Bari Aldo Moro in Italy noticed that a dog is more likely to wag his tail to the right rather than the left and that while this has no immediate meaning for two-legged animals, it seemed to say something important to the four-legged ones (other dogs). Fitting forty-three dogs with vests equipped to monitor heartbeats, the researchers videotaped the animals and found that when a dog watched another dog wag his tail to the right, he (the watcher) remained relaxed and mellow. But when he saw another dog wag to the left, his reaction was quite the opposite. Dogs that saw that left-leaning wag were likely to whine, tuck their own tails under their bodies, salivate, and, in extreme cases, run away. The working theory is that the wag-to-the-right ties into left brain approach behavior and the wag-to-the-left is a right brain signal linked to withdrawal behavior. Until we know for sure, although you’ve always considered a wagging tail a sign of happiness, you may want to check which way the tail is wagging before reaching out to great a canine stranger.10
Walking with your tail. The most obvious propeller tail in the animal kingdom is the one at the back end of a fish. No surprise there. But here’s something you may not have known: kangaroos also use their tails to move. Researchers at the University of Colorado (Boulder), Simon Fraser University in Burnaby (Canada), and the University of New South Wales in Sydney (Australia) have discovered that, yes, the kangaroo’s broad tail is useful in balancing the body simply when standing or when two roos raise their fists when battling to establish who’s first in the kangaroo kingdom. The surprise is that it definitely makes the kangaroo’s classic hopping walk more efficient. “We found that when a kangaroo is walking, it uses its tail just like a leg. They use it to support, propel and power their motion. In fact, they perform as much mechanical work with their tails as we do with one of our legs,” said Maxwell Donelan of Simon Fraser University, corresponding author for the study. “I’m envious,” added University of Colorado coauthor Rodger Kram, a competitive master runner. “When they hop faster, they don’t use energy at a faster rate. They have the ability to move faster and not get tired, the ultimate goal of a runner.”11
Sexting with your tail. In The Descent of Man, Darwin makes the sensible observation that individuals adapted to their environment will thrive and survive to produce thriving and surviving offspring. For example, a monkey with a long tail who can swing easily from branch to branch, collecting food as he goes, is more likely to survive and reproduce than is one with a stubby tail or no tail at all. But it takes two to procreate, so the question was, how do male and female survivors get together? Darwin’s answer was a really adventurous one, thoroughly justifying the full title of his second book on the majesty of evolution, which isn’t the commonly quoted The Descent of Man. It’s The Descent of Man and Selection in Regards to Sex.
In the patriarchal nineteenth-century world, where dominant males were raised to compete for submissive females, Darwin proposed that what brought the sexes together were strong male secondary sexual characteristics such as broad shoulders, which promised the female partners better chance of, yes, survival. It was a thoroughly rational suggestion—until it ran into the peacock’s tail.12 Anyone with eyes to see could tell that peahens preferred males with elaborate feather fans at the rear end even though the tails were not strictly survivor equipment. In fact, these decorative tails might actually grow too heavy to lug around, get in the way in a fight between males, and even attract predators, all situations contrary to the survival of the super tail males.13 But Darwin was a realist. He decided, first in On the Origin of Species and then in The Descent of Man, that love is love, just as mysterious among peacocks and peahens as among men and women. If the girl birds liked boy birds with big tails, then another Darwinian concept, sexual selection, would lead to the coupling that passed on to the next generation those apparently useless but certainly beautiful tails. He was right: love is love. But modern science says Darwin was also onto something else from the start. Experts now propose that the number of “eyes” in a peacock’s tail is not simply decorative. It’s a clue to the strength of the bird’s immune system, so it may be that peacocks with really great tails may be less likely to be infested with parasites and thus turn out to be survivors after all. The idea remains to be proven conclusively, but Darwin wasn’t going to be the one to do that. By April 1860, five months after publishing the first edition of On the Origin of Species and a long time spent considering the characteristics of the peacoc
k’s tail, Darwin had had enough. In a letter to his friend, Asa Gray, the American physician-turned-botanist Harvard professor of natural history who coined the phrase “survival of the fittest,” he wrote: “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”14, 15
GETTING BY WITH NO TAIL AT ALL
“In the old days,” begins a classic Native American folk tale, “Bear had a long, black, thick, shining tail, a beautiful tail that Fox envied. One very cold winter, plotting to deprive Bear of his proudest possession, Fox set a trap, cutting a hole in the ice covering the lake and surrounding it with fat, tasty fish. As Bear walked by, Fox dipped his own tail into the icy water and pulled it out with a fish attached to the end. See, he said to Bear. If you stick your tail in the water, you can catch as many fish as you can eat. Just sit there with you tail through the hole in the ice and pull it out each time you feels a fish hook on. Bear sat there so long he fell asleep, and the next day when Fox came by, shouting to wake him up, the poor creature sat up with a start—leaving his frozen tail broken off behind, under the ice.” And that, the tale tells us, is why Bear has no tail today.16
The Manx cat doesn’t have one, either, although his lack of a rear-end appendage is due not to a dip in icy water but to a mutation in the genes that form a tail. First bred on the Isle of Man off the west coast of England, some Manx cats have an indentation at the base of the spine where the tail should be. Others may have a tiny nub, some a longer stump, and some may still have a full tail. There are also tailless chickens and tail-less fancy show rats, yes, show rats. Among the canines, there is a relatively long list of dogs born with a stub of a tail or none at all, among them English Sheep Dogs, Boston Terriers, Brittany Spaniels, English Bulldogs, Jack Russell Terriers, King Charles Spaniels, and Queen Elizabeth’s favorite, the Pembroke Welsh Corgi.17
Less pleasant and certainly not genetic is the loss of a tail due to docking, the amputation of a perfectly healthy body part to conform to the human—not animal—standards of what looks good. Docking dates all the way back to antiquity, when people believed that cutting off the tail reduced the dog’s risk of rabies. Centuries later, when Jean-Baptiste Lamarck’s theory that acquired characteristics such as a docked tail could be passed on from parent to offspring was in vogue, and dog owner knew that long-tailed working, hunting, or fighting dogs were at greater risk of injury, docking continued apace. Today, in America, some show dog breed standards still support the practice.18 Happily, veterinarians and their organizations have begun to fight back. In 1999, the American Veterinary Medical Association urged their members to explain to dog owners the risks and potential complications of unnecessary surgery. The World Small Animal Veterinary Association and the Association of Veterinarians for Animal Rights released the following position statement: “The Association of Veterinarians for Animal Rights, the Canadian Veterinary Medical Association, the Council of the Royal College of Veterinary Surgeons (UK), the Australian Veterinary Association have all joined to protest the operations. Belgium, Denmark, Finland, Germany, Great Britain, Greece, Italy, Luxembourg, The Netherlands, Norway, Portugal, Sweden, and Switzerland have outright banned cosmetic tail docking (and the even more unnecessary cropping of a dog’s ears). Dogs with cropped ears have been ineligible to compete in shows under United Kingdom kennel club rules since 1898. Unfortunately the American Kennel Club still supports and sometimes requires cosmetic surgery for show dogs.”19 (As an aside, perhaps the most interesting fact is that dogs that have lost—or never had—a tail are still able to communicate with other dogs. Like people who cannot hear, they seem to have established a sign language to take the place of that left- or right-wagging tail with which other dogs speak.)
But contrary to common belief, it was not the human tail that Darwin labeled vestigial. It was the coccyx, the structure we call the human tailbone.
THE MAN WHO INVENTED THE HUMAN TAIL
The coccyx is a group of small bones at the end of your spine. The ancient Greek physician Galen thought it looked like a bird’s beak, so the Romans christened it coccyx and the Greeks, kokkyx, both words meaning cuckoo, presumably an echo of the bird’s distinctive cry.
In other animals, the coccyx serves as the anchor for a tail, so in 1871, when Darwin labeled the human coccyx vestigial, he was implying that that we had once had a tail. And why not? Tails are so universal, so useful, and sometimes so magical that one might reasonably wonder what happened to ours.
Nothing.
We never had one, nor did our immediate evolutionary ancestors, members of the Hominidae family that includes the great apes: orangutans, gorillas, chimpanzees, gibbons, and pongos, as well as the Old World tailless monkeys native to Africa and Asia.20 But for some reason, the German zoologist Ernst Heinrich Philipp August Haeckel (1834–1919), an ardent proponent of Darwin’s theories, decided to make it his mission to prove that we had had a tail that we lost while swimming in the evolutionary stream.
Haeckel was something of a zoological linguist, the man who coined the biological terms anthropogony (the study of human origins), ecology, phylum (species), phylogeny (the development of a species), stem cell, and Protista, the name of a group of one-celled organisms such as yeasts that have some characteristics of plants and some of animals.21 In his magnum opus, The Riddle of the Universe at the Close of the Nineteenth Century (1899), he explained and endorsed monism, the idea that everything comes from one thing, a different way of saying we are all descended from a common ancestor. “The history of the embryo (ontogeny) must be completed by a second, equally valuable, and closely connected branch of thought—the history of race (phylogeny),” he wrote. “Both of these branches of evolutionary science, are, in my opinion, in the closest causal connection; this arises from the reciprocal action of the laws of heredity and adaptation … ‘ontogenesis is a brief and rapid recapitulation of phylogenesis, determined by the physiological functions of heredity (generation) and adaptation (maintenance).’ ”22
Haeckel has more or less faded from history, although reproduction copies of The Riddle are still available on Amazon. Haeckel has faded from history, but his legacy remains with us in the phrase every biology student learns at his high school science teacher’s knee: Ontogeny recapitulates phylogeny. This is the idea that as embryos every single one of us travels through the multiple stages of evolution that brought us to where we are now. Haeckel’s proof was this famous chart, redrawn in 1892 by George John Romanes (1848–1894), a Canadian-born English biologist and strong supporter of Darwin, whose primary interest lay in what he called comparative psychology, that is, the possibility of similar thinking processes among all vertebrates (animals with a backbone).23
The Similarity among Embryos
Illustration from Ernst Heackel’s Anthropogenie (1874)
Depicting the various stages in the development of fish, salamander, turtle, chick, pig, cow, rabbit, and human embryos.
The first time you see these pictures, you are likely to think, “Wow. We’re all the same.”
Well, think again, because, no, we’re not.
It is true that we are all chordates, animals with a notochord (the hollow tube that holds our spinal cord) and that for a while in our fetal development we, like all vertebrates—fish, amphibians such as frogs, all reptiles, birds, and other mammals24—appear to have a tail. It is also true that modern embryologists and morphologists (people who study the structure of living things) recognize a relationship between ontogeny and phylogeny, but it is not an inseparable link.25
Those of us who accept evolution as truth know that all vertebrates arose from some common ancestor who once lived in water and then, in some cases, migrated to dry land. We also know that at some stages in our embryonic development, we share common features but not common futures. For example, early on, all vertebrate embryos have “arm buds,” but humans do not become puppies with paws or birds with wings. Our fingers and toes are webbed for a while, but we do not become frogs, either. Inste
ad, our human genes and chromosomes send us on to become humans. Our arm buds become arms, and, although about one in every two thousand babies is born with some digit webbing, most commonly the second and third toes, for most of us enzymes dissolve the webbing by two months into fetal life.26 More or less the same thing happens with what looks like a tail but is actually an extension of the nascent spine that shows up around the fourth week of pregnancy and then, a few weeks later, as our vertebrae and spinal cord develop, shrinks back into the tailbone, that is, the coccyx.27
Ah, you may say, but what about babies born with tails? In biological terms, an atavism is a sort of evolutionary backslide, the appearance of a trait that had long ago disappeared as an animal made its way up the species ladder. For example, all birds currently living on earth are toothless. But recent studies suggest that long ago—in the Mesozoic Era, the time of “middle animals” that includes the Triassic (251–199.6 million years ago), the Jurassic (199.6–145.5 million years ago), and the Cretaceous (145.5–65.5 million years ago)—birds did have the genes for enameled teeth that were eventually replaced by the specialized hardened material that covers the upper and lower jaws of all modern birds. In 2006, a group of German scientists was able to induce the atavistic formation of teeth in a mutant chicken embryo they christened Talpid 2 (ta-2).28 Both the Germans and a similar team at the University of Lyon explain that atavistic incidents may occur when an individual’s DNA retains the genes for a previous trait and a mutation enables that gene or genes to override the one or ones for later structures or traits. Nonetheless, both also note that the modern chickens with teeth would be as rare as, well, hens’ teeth.29