Exceptions to the 7 cervical vertebrae constraint
Except for infrequent individual variations, only a few mammalian species frequently have other than 7 cervical vertebrae. This includes 3 species of sloth in 2 genera in the superorder Xenarthra, 2 species in the manatee genus and 1 specie in the dugong genus in the superorder Afrothertheria. There are no known examples among the third (more numerous) placental superorder, the Boreotheria.
Varela-Lasheras et al. (2011) examined skeletal specimens of these species and classified vertebrae anterior to the clearly thoracic vertebrae as having either cervical or a mixture of cervical and thoracic (transitional) features and whether there were small ribs on any of these. They also examined skeletons of other families in the two superorders.
The 3 specimens of the two-toed sloth, Choloepus hoffmanni, all had 5 cervical vertebrae and 1 transitional vertebra with small ribs. Of 13 examples of the two-toed sloth, Choloepus didactylus, 7 had 6 cervical and 1 transitional, which had small ribs, 5 had 5 cervical and 2 transitional with small ribs on both, and 1 has 6 cervical and 2 transitional with small ribs on both
Of the 11 specimens of the three-toed sloth, Bradypus tridactylus, 9 had 8 cervical and 1 transitional, of which 7 had small ribs, and 2 had 7 cervical and 2 transitional, both with small ribs.
They also examined specimens from other Xenarthran species. All 27 specimens from 6 species of armadillos had 7 cervical vertebrae. For one species of anteater, 7 specimens had 7 cervical vertebrae one had 6 cervical and 1 transitional.
Nine specimens of the Caribbean manatee had 5 cervical vertebrae and 1 transitional with a small rib, 1 had 6 cervical, 2 had 4 cervical and 2 transitional, both with ribs, and 1 each had 6 cervical with no ribs or 5 cervical and 2 transitional with ribs on both. Two examples of the African manatee had 6 cervical and 3 had 5 cervical and 1 transitional with ribs.
Specimens were examined from 3 species of hyraxes (Afrotherians), and 13 had 7 cervical vertebrae, 1 had 6 cervical vertebrae, and 1 had 6 cervical vertebrae and 1 transitional. None of these had small ribs.
It can be seen that species that depart from the canonical 7 vertebrae show considerable individual variability.
Is there any particular feature that might be destabilizing the constraint? Sanchez-Villagra (2013) suggested that the 7 cervical vertebrae constraint was related to an increase in the metabolic rate of mammaliaforms, and subsequent exceptions are associated with low metabolic rate. Sloths and sirenians (manatees and dugongs) have low metabolic rates compared to most eutherian mammals of similar sizes, are confined to tropical or sub-tropical habitats, and have vascular bundles in their extremities that are thought to act as counter-current heat exchangers to preserve warmth(Rommel & Caplan 2003, Gilmore et al. 2000).
Xenarthrans and Afro therians are thought to have diverged from the linage from which all other eutherians derive about 105 mya ( ). It might (initially) seem irrelevant, but in no Xenarthrans or Afro therians do the testes and/or epididymus (the organ in which sperm mature and are stored) leave their embryonic location near the kidneys. In many Boreotherians and marsupials these organs migrate through the abdominal wall and lodge either subcutaneously (inguinal) or exterior to the torso in a specialized organ, the scrotum. Various theories have been proposed to explain the complex pattern of location of these organs in mammals, and temperature plays a role in most of these theories.
The Cool Spermatogenesis Hypothesis (Moore 1926) states that higher testicular temperatures impair sperm formation. The Mutation Hypothesis (Short 1997) higher temperatures result in mutations, particularly on the Y chromosome, during the high rate of oxidative metabolism for sperm maturation in the epididymis. The latter hypothesis provides a straightforward explanation for why no birds (where the male sex chromosomes are homozygous) have scrotums.
Lovegrove (2014, 2019) elaborated on these hypotheses with the Endothermic Pulses Hypothesis. He used maximum likelihood methods to analyze when major changes occurred based on data on position of the testes, mean body temperatures, and body size for 83 families of placental mammals. He concluded that the pattern of testes position could be best explained by the scrotum originating in an ancestor of all Boreotherians, and that the examples without scrotums were evolutionary reversals, rather than scrotums arising more than once. He placed this event as shortly after the wide-spread extinction event 66 mya that wiped out all dinosaurs except the bird lineage.
Prior to this event, all mammals had been small, slow, nocturnal, burrowing, and frequently insectivores. These characteristics are thought to have contributed to there survival. Based on his survey of body temperatures of extant mammals, Lovegrove argues that prior to this event for more than 100 million years all mammals had maximum body temperatures of no more than 36 degrees C. The extinction event allowed mammals to occupy many new ecological niches. In the Boreotherian lineage this frequently required successive increases in size and speed, and requisite increases in body temperatures.
The Xenarthra consists of anteaters, armadillos, and sloths. They are second only to the Monotremata in having low basal metabolic rates (Lovegrove 1996). Lovegrove (2000) suggest two reasons large non-carnivorous mammals would require a high metabolic rate, both connected to mobility. One is to escape predators and the other is to be able to move large distances to seek out sufficient food. Many Xenarthra are covered in spines or armor, which removes the necessity of speed to escape predators. Xenarthra also have the longest history of using specialized diets among mammalian lineages (Eisenberg 1981). If sufficient food would always be available in a small area, only a slow rate of metabolism would be required for any required movements.
Morrison (1945) reported that a pregnant-3-toed sloth lost body temperature when exposed to the cold only about a third as fast as previously observed in non-pregnant sloths. Thus, pregnant sloths may have some alternative, although not very effective, means of generating or conserving heat.
Although in the case of sloths, the necessity of gestation at a low body temperature during gestation may play a role in their 7-cervical vertebrae exception, other evidence suggests that it may a requirement for homeothermy. Monotremes have low metabolic rates but are either homeothermic at all times or are part of gestation, as do many other mammals, even those that enter torpor during other parts of gestation (see Exploration 1)
Despite a low metabolic rate, manatees have been reported to have a fairly high and stable body temperature (35.6-36.4 degrees C). In Florida, they congregate during winter around warm springs and near power plant cooling water outlets and move to the cooler rivers in the spring (Irvine 1993). Both male and female manatees, as do cetaceans and true seals, have vascular bundles around their reproductive organs that receive cooler superficial blood (Rommel et al. 2001). This is apparently necessary because of dissipation of heat developed by the axial propulsion muscles if hindered by the large body mass and a thick layer of fat. Gestation in mammals lasts about 12 months and most births occur in the late spring or summer. Thus, mating and early development occur in a warmer environment summer in subtropical rivers) than in the case of cetaceans and seals. In sirenians, this cooling organ may be necessary to maintain some degree of homeothermy for the embryo. The exception of the 7-cervical vertebrae constraint may be related to avoiding hyperthermia.
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Species with cervical ribs.
Galis et al. (2022) predicted that adult lorisids would have more cervical ribs that other primate species, including the closely related galagids (bushbabies) based on two factors. Lorisids have a low metabolic rate, which they expected would reduce selection against such ribs. They also utilize their forelimbs less vigorously than other primates, reducing selection associated with thoracic outlet syndrome.
39% of 82 skeletal specimens from 3 lorisid species had cervical ribs. For 981 specimens from 50 species belonging to 9 primate families, only 8 (0.8%) had cervical ribs, including none in 88 galagid specimens from 3 species.
The last common ancestors of lorisids and galagids was about 40 mya. Its activity was likely moderate, resembling that of modern lemurs. Thus, the lower metabolism, slower movements, and geographical separation of lorisids from the galagids likely occurred after the divergence.
Cervical ribs of isolated or stressed populations
There have been reports of high incidence of cervical ribs in several subpopulations of mammals, including humans.
A higher incidence of cervical ribs was found in fossil specimens of 3 megaherbivore species facing extinction in northwestern Europe in the late Pleistocene. Reumer et al. (2014) examined sixth and seventh cervical vertebra of 9 fossil wooly mammoths (Mammuthus primigenius) and found articulation facets indicating they had had cervical ribs in 3 specimens. They found only one such out of 28 specimens from 2 extant elephant species.
Van der Geer and Galis (2017) examined 32 C7 vertebrae from fossil wooly rhinoceroses recovered in the Netherlands and North Sea. 16% showed evidence of an anterior shift in the cervical/thoracic boundary. No such shift was observed in the C7 vertebrae of 56 skeletal museum specimens from 5 rhinoceros species.
Cuxurt-Erruz et al. (2024) found that 4 out of 14 specimens from fossils of the Irish giant deer, Megaloceros giganteus, had cervical ribs. This compared to less than 2% in 179 samples from 20 extant species of deer. In this and the previous examples the authors suggested two possible explanations for this high incidence. During the Late Pleistocene there was a decrease in the vegetation consumed by these megaherbivores, thus contributing to their extinction. Poor nutrition could have resulted in abnormal development. Alternatively, the high incidence could be due to more genetic instability due to inbreeding in a reduced population size.
They looked at the incidence in an extant species, the Pere David’s deer (Elaphurus davidianus) which disappeared in the wild but was propagated by captive breeding of a very small population. 3 out of 11 specimens from this population had cervical ribs. This supports the idea that cervical ribs can result from inbreeding. This species has now been successively reintroduced into the wild in China. The hypothesis predicts that the incidence should decrease as the population expands and stabilizing selection resumes.
This hypothesis is also supported in studies of breeds produced by artificial selection.
Brocal et al. (2018) examined CT scans of 19 breeds of dogs with 4-20 examples of each breed. In this initial study the incidence of cervical ribs in Pugs stood out, and this breed was studied in greater depth. For the non-Pug breeds, 16 out of 333 examples (4.8%) had cervical ribs, compared to 45 out of 97 (46%) Pugs.