The musculoskeletal system of the tail is presented in Fig. 1. The caudal vertebrae are part of the skeletal system that maintains the tail, which articulates posterior to the sacrum [28]. The morphology of the caudal vertebrae differs between the proximal and distal regions [2,3,13,22,29]. The proximal caudal vertebra, the morphology of which is similar to that of other vertebrae, provides attachments to the caudal musculature or maintains their muscular bellies [22,29]. These vertebrae articulate with one another via their zygapophyses and intervertebral discs, as do other vertebrae. In contrast with the proximal caudal vertebrae, the distal caudal vertebrae are simply shaped in a vaguely hexagonal column. They possess neither processes nor arches and articulate only with the intervertebral discs. These morphological differences between the proximal and distal caudal vertebrae might affect tail mobility. The zygapophyses involved in articulation in the proximal region of the tail permit a certain degree of sagittal flexion but resist movement in other directions [26,27,33]. Conversely, articulation only via the intervertebral discs in the distal region allows movement in almost all directions [22]. Among the taxonomic groups of species that contain tails with varying lengths, the number of caudal vertebrae tends to vary with tail length [11,22,29].

The caudal musculature contains dorsal caudal extensors and abductors and ventral caudal flexors and pelvocaudal muscles. The dorsal caudal muscles are especially considered to be elongated portions of the erector spinae muscles. These structures contribute to the movement of the tail [22,30]. The caudal musculature in tailed primates exists on both the ventral and dorsal sides, as in other mammals, and allows the tail to be flexed, extended, and abducted. The caudal muscles tend to insert more proximally in shorter-tailed primate species. The number of caudal muscles, as well as the number of vertebrae, tends to be reduced in shorter-tailed species [30].

In primates, tails play many important roles. Some New World monkeys (e.g., spider and howler monkeys) have prehensile tails and use them as a “fifth hand”; they can even suspend themselves by their tails. Most primates, however, do not have prehensile tails. Like in most other arboreal mammals, the tail in primates is generally used as a balancer, as supported by experimental studies, to increase stability in the unstable arboreal environment [17]. Indeed, when an individual walks or leaps from branch to branch, the tail whips left and right or rotates. Among the Old World monkeys, arboreal species tend to have longer tails than other species, implying that the need for balance is greater in arboreal species.

Some tails also function in social communication [1,4,14,16,21,23]. For example, tail carriage indicates the rank of a male within the troop hierarchy in some cercopithecoid species, such as the savanna monkey (Cercopithecus aethiops). The alpha male has the highest rank and carries his tail in a sigmoid manner, while the beta male has the second-highest rank and carries his tail parallel to the ground. Similar signaling has been reported in the Japanese macaque (M. fuscata). Tails may also be used for communication between male and female primates. While observing a troop of wild crab-eating macaques (M. fascicularis) in Thailand, I noted some females holding their tails high and showing their vaginas to the alpha male. Immediately after the alpha male saw a female displaying this posture, he mated with her.

The development of the tail is presented in Fig. 3. During embryonic development in amniotes (tetrapod vertebrate species that possess the amnion during development : reptiles and mammals), the body axis elongates and pairs of somites are generated along the axis. These somites are the primordia of the vertebrae and the erector spinae muscles. Somitogenesis in the caudal region proceeds in the same manner with a more anterior trunk : somites are generated sequentially from the presomitic mesoderm along the neural tube [18,24]. When the somites are added, Fgf/Wnt proteins are secreted at the tip of the tail (tail bud). The Fgf/Wnt proteins are antagonistic to retinoic acid (RA) and are present in greater quantity than RA during axial elongation. When the axial elongation terminates, however, these two opposing conditions reverse and the amount of RA at the tail bud increases. New somite formation then stops [6,25].

While the body axis elongates, Hox genes are at work, and variations in their expression characterize the body axis; they determine which embryonic portions become cervical, thoracic, lumbar, and sacrocaudal parts of the body. Studies utilizing mouse models revealed that posterior Hox genes (paralog group 10 to 13) are involved in the determination of tail length in mammals [5]. Another study demonstrated that the number of lumbar and sacral vertebrae was altered in double-knockout Hoxa10 and Hoxd11 mice [9]. Hoxb13 is also related to tail elongation in mice [36]. Another study revealed that a Hoxb13 knockout mouse had a longer tail and a greater number of caudal vertebrae than normal individuals [7].

Previous studies regarding tail development have suggested that the timing of axial elongation termination is an important developmental event for tail length determination. However, a very recent study showed that another developmental event, tail reduction, is essential in determining the final tail length [31]. It is known that the once-formed embryonic tail diminishes and is completely lost during the human embryonic period. However, until recently, the detailed developmental process behind this event has not been clarified. The authors of a recent study used human embryonic specimens to study the transition of the number of somites and clarified that the caudal somites radically decrease in number immediately after the termination of axial elongation [31]. The cellular behavior and genes involved in this tail reduction have not yet been clarified, but the transition may involve apoptosis [8].