Variation of the number of proximal caudal vertebrae with tail reduction in Old World monkeys

Abstract

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.

Skeletal determinants of tail length are different between macaque species groups

“…Proximal vertebrae have transverse processes and a spinous process, on which tail muscles insert, and consist of the main part of tail movement (swing). For these reasons previous studies focused on the number or length of the proximal caudal vertebrae and/or the morphology of the sacrum to find parameters that help estimate (reconstruct) the tail length 16,20,33 . However, our results showed the number and length of caudal vertebrae in the distal region varied more than the proximal region, and correlate more strongly with RTL.…”

“…Primates’ vertebrae are categorized into three types: proximal, transition, and distal 8,18–20 (Fig. 1).…”

“…The tail is composed of caudal vertebrae that are linearly arranged, and the number and length of caudal vertebrae are the skeletal determinants of tail length. Primates’ caudal vertebrae are categorized into three types based on their morphology: proximal, transition, and distal 8,18–20 . Among these three morphology types the muscular and tendon attachments are different 21,22 .…”

“…Among these three morphology types the muscular and tendon attachments are different 21,22 . The proximal vertebrae are the main vertebrae that enables large joint motion at the base of tail and contribute on tail swinging, thus the region have been studied for the estimation of tail length 16,20 . Tojima 20 suggested that the number of proximal vertebrae represents tail length; and Russo 16 showed that the length of proximal vertebrae, especially the transition vertebra, is a good estimator of the tail length.…”

Comparative Anatomy of Caudal Musculature Attachments in Catarrhines with Different Tail Length

“…Tojima (2014) revealed that the number of proximal caudal vertebrae, which provide insertion places to caudal muscles, correlated with relative tail length (the percentage of tail length against head and body length) in Papionini. Together with this insight, the result of this study suggested a possibility that the number of proximal caudal vertebrae and the insertion places of caudal muscles are highly related.…”