And a Tale of the Tail
By Eric M. Jones
YEARS AGO, IT WAS DISCOVERED that almost every museum display presenting a dinosaur with a tail was wrong. This is the kind of thing a six-year-old might have innocently discovered. “Daddy, if dinosaurs dragged their tails on the ground, why aren’t there any dinosaur-tail marks?” Err? ... Oops ...!
This observation sent all the paleontologist scurrying down to their dusty basement fossil collections. Indeed, there were no fossilized dinosaur tail marks discovered among the many thousands of fossilized dinosaur footprints. Not one. So apparently dinosaurs never dragged their tails. This makes sense, because the tails would wear out constantly being scratched over rocks. Apparently they used them for signaling, defense, balance, but they very rarely touched the ground.
Animals use their tails for many purposes. Tails can counterbalance the changes in center of gravity with corresponding movements of its tail. So this helps them while running at high speeds, changes in movement, climbing trees. Some animals have prehensile tails, some have no tails. Some, like snakes and worms are all tail. Sea creatures use tails for propulsion. But none of these animals is as interesting or as successful as human beings ... who have no tails.
So what causes animals to have tails or no tails? People who misunderstand evolution might think that an animal who doesn’t use some part of itself eventually loses it. This isn’t necessarily true. Male mammals have nipples, and appendixes. There are dozens of extra parts on animals that don’t seem to have any current function, yet they stick around just because they aren’t much of a penalty either. And some parts that seem a real burden to a creature still remain because Mother Nature says, “If you are fit enough to carry this ridiculous cumbersome feature around with you, it will demonstrate to potential mates that your DNA is probably damned good.”
Likewise, to say that humans don’t have tails because they didn’t need them, is simply a shorthand way of saying, that in the environment humans found themselves in, not having a tail was not a breeding or survival disadvantage. This double-negative is not the same as saying that not having a tail was a breeding or survival advantage. Far from it.
The easiest answer for why Humans don’t have tails is to understand that they evolved in a family of apes that had no use for tails. They did not live in trees, they used complex motions of their upper bodies for balance when walking bipedal, and were too big to get much use from them. Tails merely slowed them down. To see the truth in this, watch any professional football game. Humans get along better without tails. Of course if football were played in trees, it might be a different result.
The taxonomic superfamily Hominidae comprises orangutans, gorillas, chimpanzee, bonobo, and humans in the Great Ape family. They are all tailless, as too is the gibbon of the Lesser Ape family. None of these “apes” ever seem to have had tales, even if they evolved from earlier creatures who did. They all have a so-called vestigial tail, the coccyx, which is Greek for “cuckoo bird” possibly because of its resemblance to one. But it is a mistake to think this assemblage of a few fused terminal vertebrae has no function, or that it is some prehistoric shrunken appendage. Many important muscles, tendons and ligaments attach to it. Occasionally humans are born with a tail, but this is simply a “soft tail,” with no vertebrae, and is regarded as an abnormality rather than a true vestigial tail, even when it is located where the tail is expected to be.
Our closest sister group is the monkeys, most of which have tails. We diverged from them around twenty-five million years ago and evolved mostly on the African plains. Humans stood upright and developed a superior cooling system of bare skin covered with sweat glands for perspiration which enabled them to chase down virtually any game they wanted. They could hunt in packs, had stereo vision, enormous intelligences, and could communicate with each other. So far, Humans have been successful.
Then there’s the physics: Let’s assume an animal needs a tail for whatever reason. The strength of the tail is proportional to the square of the diameter of the tail (the cross-sectional area of the tail). So if the Great Ape is five times the height of the smaller primate, its mass is 125 times as much, while its tail is only twenty-five times stronger—or only one-fifth as strong in proportion to the increase in mass. So you can see that the proportional strength of a tail (assuming it maintains its proportions) falls away as the animal increases in mass, reducing its usefulness. This can be seen in larger land animals in general where the tail either gets enormously thick, like that of a kangaroo or crocodile, or very small like an elephant or a horse (a horse has long hairs but only a small tail). Most large animals have tiny tails to keep flies off you-know-where. And that’s about it.
Tails seem to be used for propulsion in water and balance on land, especially in trees. It is noteworthy that while most predators have tails, prey don’t have much unless they are tree dwellers. Some tails also assist in cooling. Furry animals are constantly up against their thermal-performance limits. Blood flow to hairless skin such as the rat’s tail accounts for seventeen percent of the heat dissipation from the rat. Small lizards can shed their tails when attacked. The attacker is distracted by the wriggling tail and the animal escapes. Then the animal grows another.
So why did dinosaurs have tails? Dinosaurs were reptiles and tails seemed to be necessary for balance. This is especially true of big bipedal predators like the
Many dinosaur tails evolved into weapons. Ankylosaurus had a club on its tail roughly the size of a sack of cement. Stegosaurus had a spiked tail.
There is speculation that some dinosaurs cracked their tails like a bullwhip, but the jury is still out on this. The biomechanics of this seem farfetched. Dr. Nathan P. Myhrvold (formerly Chief Technology Officer at Microsoft, and co-founder of Intellectual Ventures) and Dr. Philip J. Currie (of the Royal Tyrrell Museum of Paleontology) and others have been studying the question of whether or not sauropods could have cracked their long tails like a bullwhip, either for communication or defense. Many dismiss the idea because they think it would cause too much damage to the tail. But some tails are damaged and have fused vertebrae in just the way one would suspect, if this tail cracking had taken place.
Most anthropologists believe that not having a tail became an advantage in the ape family when they began to experience the benefits of upright posture and locomotion. Then the musculature used by the tail started to take on the role of supporting the pelvic floor. Problems with the pelvic floor in older humans is common and debilitating. This is never a problem with monkeys which tend to be quadrupedal.
Apes and bears (or most species that walk erect on two legs) can balance themselves without a tail. These creatures evolved other systems (inner ear, vision, etc.) to be able to detect changes in their center of gravity to correct their balance.
So humans did not lose their tail through evolution, they evolved from a family of primates that didn’t have one.
But what about the future? Humans are now in charge of their own evolution, and the matter of tails might be revisited. They might be quite chic in the nightclub scene. Might there be any advantages to genetically engineering tails? Could they catch on as sexual enhancements? There are already adult sex-toy tails and we see designers making brainwave controlled ears. Are brainwave-controlled prehensile tails far ... ah ... behind?
I kind of like the idea. 
Further Reading
When Did Apes Lose Their Tales?
Coccyx.
Six Million Years of Savanna.
Did Dinosaurs Break the Sound Barrier?
Eric M. Jones is the Associate Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur, working in the experimental aircraft community, NASA, space transportation companies, and the International Space Station.
