In primates, tail length is subject to wide variation, and the tail may even be absent. Tail length varies greatly between each species group of the genus Macaca, which is explained by climatic factors and/or phylogeographic history. Here, tail length variability was studied in hybrids of the Japanese (M. fuscata) and Taiwanese (Macaca cyclopis) macaque, with various degrees of hybridization being evaluated through autosomal allele typing. Relative tail length (percent of crown-rump length) correlated well with the number of caudal vertebrae. Length profiles of caudal vertebrae of hybrids and parent species revealed a common pattern: the length of several proximal-most vertebrae do not differ greatly; then from the third or fourth vertebra, the length rapidly increases and peaks at around the fifth to seventh vertebra; then the length plateaus for several vertebrae and finally shows a gentle decrease. As the number of caudal vertebrae and relative tail length increase, peak vertebral length and lengths of proximal vertebrae also increase, except that of the first vertebra, which only shows a slight increase. Peak vertebral length and the number of caudal vertebrae explained 92 % of the variance in the relative tail length of hybrids. Relative tail length correlated considerably well with the degree of hybridization, with no significant deviation from the regression line being observed. Thus, neither significant heterosis nor hybrid depression occurred.

Tail morphology in primates is important for interpreting functional adaptation and phylogeny. Tail length is probably the most remarkable trait. Establishing usable methods to predict the tail length of extinct primates is part of the basis for the reconstruction of primate evolution, particularly of hominoids. Previous studies revealed that sacral morphology often predicts tail length. However, most previous studies have only attempted this by using categories (short, long, etc.). A problem with those studies is that the range of short tail length is wide. Accordingly, this study aimed to quantitatively estimate tail length in catarrhines with intermediate tail lengths. Sacra and proximal caudal vertebrae (first to third) of 89 hybrid individuals between Japanese macaque (Macaca fuscata) and Formosan rock macaque (Macaca cyclopis) were measured. These hybrid macaques were phylogenetically controlled but varied greatly in their relative tail length (tail length/head and body length = 18.8-88.8%), and thus were an excellent sample to obtain general regression formulae to estimate catarrhine tail length. A total of 15 predicting models were devised and five formulae performed well. The utility of these formulae were confirmed by application to 15 species/subspecies of catarrhine taxa.

Introduction: Numerous case reports have indicated that the "human tail" is not always a harmless protrusion but can be associated with anomalies such as occult dysraphic malformations. However, the definition and classification of this anomaly have not been discussed. A prevailing hypothesis is that the "human tail" is a residual embryonic tail. Herein, we attempted to classify and define the human tail and investigate the frequency of this anomaly.Materials and Methods: We first defined the human tail as a protrusion on the dorsal side of the lumbar, sacrococcygeal, and para-anal regions identified after birth. We collected case reports written in English, Japanese, French, German, and Italian that were published from the 1880s to the present.Results: We discovered two important findings: (1) the cause of this anomaly may differ even though the "tails" resemble each other closely in appearance, and (2) its position tends to be correlated with the type of anomaly and its associated cause. We propose a new classification of the human tail based on these findings. Conclusions: Our classification may facilitate more accurate treatment and precise case descriptions of the human tail.

Tail length in primates can vary greatly between species or even between local conspecific populations, and the tail is markedly reduced in several lineages. In Old World monkeys, tail length is considered as an important feature reflecting their phylogeny and adaptations. The number of caudal vertebrae is one of the important factors which determine tail length, and it is known that this number varies with tail length. Caudal vertebrae can be divided into two types (proximal and distal), and tail mobility and function are considered to be different in these two regions. Though the number of vertebrae in each region is important for understanding tail length evolution in Old World monkeys, there have been few attempts to investigate this matter. This study focused only on the proximal caudal vertebrae, which are more easily preserved than the distal ones, and tested if there is variation in their number with tail length or phylogenic differences. As a result, two important findings were obtained: (1) the variation of the number of proximal caudal vertebrae was different among the phylogenic groups, and (2) especially in Papionini, there was a great variation in the number of proximal caudal vertebrae, and it correlated strongly with relative tail length [RTL = (tail length/head and body length (sitting height)) × 100%]. I speculate that these variations in the number of proximal caudal vertebrae were possibly caused by a change of the embryonic developmental mechanism of tail morphogenesis, a common mechanism of morphological evolution. To clarify the mechanisms and evolutionary trends of the variation in the proximal caudal vertebrae, not only morphological approaches but also developmental biological approaches will be necessary in the future.

Although the human tail is completely absent at birth, the embryonic tail is formed just as in other tailed amniotes. Since all morphological variations are created from variations in developmental processes, elucidation of the tail reduction process during embryonic development may be necessary to clarify the human evolutionary process. The tail has also been of great interest to the medical community. The congenital anomaly referred to as 'human tail', i.e. the occurrence of a tail-like structure, has been reported and was thought to represent a vestige of the embryonic tail; however, this hypothesis has not been verified. Accordingly, in this study, we aimed to establish a new method to visualize all somites in an embryo. We used sagittal-sectioned embryos from Carnegie Stage (CS) 13 to CS23. All samples were obtained from the Congenital Anomaly Research Center, Kyoto University, Japan. Combining photomicroscopy and three-dimensional reconstruction, we clearly visualized and labeled all somites. We found that the number of somites peaked at CS16 and dramatically decreased by approximately five somites. Tail reduction with a decrease in somites has also been observed in other short-tailed amniotes; thus, this result suggested the possibility that there is a common mechanism for morphogenesis of short tails in amniote species. Additionally, our findings provided important insights into the cause of the congenital anomaly known as 'human tail'.