The mammalian class is generally characterized by the presence of a caudal appendage, commonly known as the tail, which extends from the vertebral column. However, certain lineages within the Mammalia have diverged significantly from this norm, exhibiting a complete lack of a visible external tail. This absence represents a profound anatomical shift that occurred at different times and for various reasons across the evolutionary timeline. Understanding these exceptions requires exploring the specific environmental and physiological pressures that led to the disappearance of this limb.
What Does a Tail Do?
The tail is a highly versatile and muscular appendage that performs multiple functions for the mammals that possess one. For arboreal species, it is utilized as a dynamic counterbalance, crucial for maintaining stability while navigating narrow branches or leaping. Many New World monkeys possess prehensile tails, which function as a fifth limb for grasping and supporting the animal’s entire body weight while climbing. In terrestrial animals, the tail provides a rudder-like function for sudden changes in direction during high-speed pursuits, such as seen in large cats. Beyond mechanical uses, the tail serves as a tool for communication and defense, conveying social signals or swatting away biting insects.
Mammals That Lack External Tails
The most prominent group of mammals recognized for their tailless state is the Hominoidea, or the great apes, which includes humans, chimpanzees, gorillas, orangutans, and gibbons. This entire group shares the trait of having no projecting external tail, setting them apart from Old World and New World monkeys. Beyond primates, the absence of an external tail appears in several other mammalian groups due to convergent evolutionary pathways. Marine mammals, such as dolphins, whales, and manatees, lack the bony, flexible caudal vertebrae of a typical tail. Terrestrial species also exhibit significant reduction, including certain rodents like guinea pigs and capybaras, as well as koalas and various species of bears.
Why Evolution Removed the Tail
Apes and Upright Locomotion
The loss of the tail in the hominoid lineage, which occurred approximately 25 million years ago, is linked to changes in locomotion and posture. As ancestral apes began to spend more time on the ground and adopted a more upright stance, the tail lost its utility as a balancing appendage. The flexible tail structure became an impediment to the stability required for walking upright and sitting. Furthermore, the loss of a tail simplified the musculature around the pelvic region, making it stronger and more stable to support the torso during upright movement.
Genetic Mechanism
This anatomical change was driven by a small genetic alteration involving the TBXT gene, a major regulator of tail development. Researchers discovered that a mobile genetic element, an Alu element, inserted into an intron of the TBXT gene in the ancestral ape genome. This insertion caused an alternative splicing event, disrupting the gene’s instructions for building a full tail. Selective pressure then favored individuals with this mutation, as a tailless structure was better suited for new modes of movement like knuckle-walking and bipedalism.
Hydrodynamics and Liability
For marine mammals, the evolutionary pressure was entirely related to hydrodynamics and efficiency in water. A long, bony tail would create significant drag, slowing the animal and requiring more energy for swimming. The specialized flukes evolved to be powerful, rigid, and horizontally oriented, maximizing thrust and streamlining the body shape. In other cases, like burrowing rodents or bears, a protruding tail could simply be a liability, getting snagged or becoming a hindrance in tight, confined spaces.
The Vestige: A Tiny Reminder
Despite the external absence of a tail, the skeletal structure of all tailless mammals retains a remnant of their tailed ancestors. In humans and other apes, this structure is the coccyx, or tailbone, which is composed of three to five small, fused vertebrae at the very base of the spine. The presence of these caudal vertebrae serves as clear evidence that the ancestors of these species did, in fact, possess a functional tail. The coccyx is not entirely functionless in modern humans, even if it does not perform the dynamic roles of a tail. It acts as a crucial anchor point for several muscles and ligaments of the pelvic floor, including the gluteus maximus and the levator ani muscle. Furthermore, it is a component of the tripod structure that supports body weight when a person is sitting.