Many vertebrates and particularly mammals have a tail, or a vestige of it like our coccyx. Why do many animals have tails? Which mammals do not have them and why?
An essential element for the survival of many animals
Mammals owe their spine and limbs to the ancestors who populated the Carboniferous seas and had to adapt to survive in living conditions that were very different from the aquatic environment. These vertebrates, tetrapods, evolved to breathe, but also to move on land where gravity required a more powerful skeleton to fight against gravity. Conquering the air environment did not happen in a day and our ancestors experienced a modification of their spine going from a column in two parts to a column in three parts:
The thoracic vertebrae which support the ribs; The sacral vertebrae, located at the level of the junction of the pelvic girdle; Caudal vertebrae form the tail of tetrapods, mammals, fish and birds. They articulate with the sacral vertebrae of all vertebrates.
In humans, there remains only a vestige of this evolution that appeared millions of years ago: our four vertebrae are fused, forming only one bone, the coccyx. Each species has a different number of caudal vertebrae: the cat has 18 to 23 coccygeal bones, the dog only 20 on average and the horse 15 to 21.
The ancestors of the first land mammals already used their tails to move as well as their forelimbs. And current mammals have become masters in the art of using this appendage that we abandoned during our evolution.
A utility that is not just for moving
Many mammals have been able to take advantage of this extension of their spinal column on land or in trees. Felines, canines, cattle, marsupials, monkeys, and even dolphins, each species has a tail and uses it to move:
The cat uses its tail to maintain its balance when walking on roofs; The dolphin propels itself in the ocean thanks to it: a real propulsion engine; The squirrel, thanks to its magnificent plumed tail, can cushion the fall or correct the landing to avoid falling next to the targeted branch.
In many mammals, it is not only a guarantee of balance, speed or elegant movements. Many animals, including dogs and cats, also use it to communicate: question marks, slow, happy or annoyed beats are valuable for understanding their intentions. In arboreal monkeys, this caudal appendage is similar to a third hand. They can walk between each branch or hang down and peel a fruit with their forelimbs to taste it. Some prefer to use their hands to hold on and use their tail as a hand. Cattle and horses will use it as a weapon to ward off unwanted flies and horseflies. The dolphin does not hesitate to use its beautiful caudal fin as a weapon when the situation requires it. Finally, for certain animals, the tail is a precious blanket with which they adorn themselves on cool nights to wrap themselves around it: foxes, wolves and Nordic dogs know how to use their caudal appendage as additional heating.
Pendulum, grappling hook, lethal weapon, we wonder why hominids finally separated themselves from a very useful limb.
Why don’t great apes and humans have tails?
While the tail of mammals serves as a counterweight to their head and allows animals to move faster, the great ape family to which we belong lacks a caudal appendage. According to zoologist and doctor David Young, author of the textbook “The discovery of evolution”, chimpanzees, gorillas and orangutans owe this absence to the vertical position. A shorter lumbar region, the loss of the tail and the shoulder blade bone located at the back allows for balance when walking, standing up, walking and running. Conversely, the presence of a tail allows the animal which moves on four legs to maintain its balance even on the ground.
A genetic mutation would have contributed to the disappearance of this caudal appendage in great apes, 25 million years ago. Passed down from generation to generation, it would have modified their means of transport. During evolution, we also inherited this mutation. The embryonic development of the human being reveals an appendix between the 31st and 35th day of the embryo. Then, these vertebrae fuse to become the coccyx, proving that at some point in our history, like all mammals, we too had a tail.
Ultimately, we now know how we lost our tail, but we still don’t know why humans move on two legs while animals do on four. Nothing to date allows us to know whether walking on our feet led to the disappearance of our tail or whether, on the contrary, its disappearance allowed vertical walking.