Extraordinary incidence of cervical ribs indicates vulnerable condition in Late Pleistocene mammoths

Jelle W.F Reumer,Frietson Galis,Clara M.A Ten Broek

doi:10.7717/peerj.318

Abstract

The number of cervical vertebrae in mammals is highly conserved at seven. We have shown that changes of this number are selected against due to a coupling with major congenital abnormalities (pleiotropic effects). Here we show that the incidence of abnormal cervical vertebral numbers in Late Pleistocene mammoths from the North Sea is high (33.3%) and approximately 10 times higher than that of extant elephants (3.6%). Abnormal numbers were due to the presence of large cervical ribs on the seventh vertebra, which we deduced from the presence of rib articulation facets on sixth (posterior side) and seventh (anterior side) cervical vertebrae. The incidence of abnormal cervical vertebral numbers in mammoths appears to be much higher than in other mammalian species, apart from exceptional sloths, manatees and dugongs and indicates a vulnerable condition. We argue that the increased incidence of cervical ribs in mammoths is probably caused by inbreeding and adverse conditions that impact early pregnancies in declining populations close to extinction in the Late Pleistocene.

Highlights

The number of cervical vertebrae in mammals is remarkably constant at seven
Articulation facets for cervical ribs on cervical vertebrae are characterized by the following combination of characteristics: (i) they have a smooth, polished-looking surface, visibly smoother than the cortical surface of the vertebrae; (ii) the surfaces have no vascular or nervous foramina; and (iii) the facets are bordered by a clear edge, distinguishing them from the surrounding cortex
The overall incidence of cervical ribs in the two species is, 3.6%. This is significantly lower than the 33.3% in mammoths, if we only consider vertebrae that can be evaluated for cervical rib articulation facets (G-test for independence, p = 0.035, Barnard’s exact test, p = 0.031)

Summary

Introduction

The number of cervical vertebrae in mammals is remarkably constant at seven. The number of cervical vertebrae varies considerably, and in mammals the number of vertebrae in more caudal vertebral regions is variable as well (Leboucq, 1898; Schultz, 1961; Starck, 1979; Narita & Kuratani, 2005). Manatees (Trichechus, Sirenia) and sloths (Bradypus and Choloepus, Xenarthra) have an exceptional number of cervical vertebrae (Bateson, 1894; Starck, 1979; Varela-Lasheras et al, 2011). Despite the extreme evolutionary conservation of the number of cervical vertebrae, intraspecific variation is not uncommon in mammals. The strong conservation of the number of cervical vertebrae implies that there must be selection against intraspecific variation of this number.

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