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Sloths and the mammalian neck

Sloths and the mammalian neck. Skeletal development in sloths and the evolution of mammalian vertebral patterning. Hautiera et al., PNAS 2010. Almost all mammals have 7 neck vertebrae (before first rib). Three-toed sloths have 9. Why?.

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Sloths and the mammalian neck

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  1. Sloths and the mammalian neck Skeletal development in sloths and the evolution of mammalian vertebral patterning. Hautiera et al., PNAS 2010. Department of Kinesiology and Applied Physiology

  2. Almost all mammals have 7 neck vertebrae (before first rib). Three-toed sloths have 9. Why? Giraffe http://commons.wikimedia.org/wiki/File:Giraffe_skeleton.jpg, 2010-10-19 Echidna http://commons.wikimedia.org/wiki/, www.skullcleaning.com , 2010-10-19. Bat www.ucmp.berkeley.edu, 2010-10-19. Department of Kinesiology and Applied Physiology

  3. Three toed sloth. Nine cervical vertebrae. www.flickr.com/photos/artour_a/482203590/, www.skullcleaning.com , 2010-10-19. Department of Kinesiology and Applied Physiology

  4. Hypothesis 1: An evolutionary mistake or generated extra cervical vertebrae in sloths. (“Une erreur de la Nature”, Le Comte de Buffon,1765.) Hypothesis 2: The rib cage and shoulder girdles “moved down” the trunk in sloths. Authors propose that “neck vertebrae” ≠ “cervical vertebrae”. Neck vertebrae=vertebrae without ribs. Cervical vertebrae defined by timing of ossification in utero. Consistent with expression patterns of developmental genes (Hox). Department of Kinesiology and Applied Physiology

  5. Unborn three-toed sloth skeletons at progressive developmental stages (A,B,C,D). Blue=neural arches, red=vertebral centra, green=shoulder girdle, orange=pelvic girdle. Hautiera et al., PNAS 2010. Department of Kinesiology and Applied Physiology

  6. Dasypus=armadillo. Bradypus=3-toed sloth. Both are placental mammals of superorder Xenarthra (meaning strange joints). Slide shows: In marsupial and placental mammals, including armadillos (closely related to sloths), first 7 vertebrae osssify after first thoracic, and Hox5 and Hox6 genes are expressed for C6-7 and T1-2. Authors use this to define cervical vertebrae. Three-toed sloth (Bradypus) also has 7 cervical vertebrae (defined by developmental timing & gene expression) but 9 neck (ribless) vertebrae. Hautiera et al., PNAS 2010. Department of Kinesiology and Applied Physiology

  7. Authors conclude hypothesis 2 is correct: Sloths have 7 cervical vertebrae like other mammals, but ribs & shoulder girdles have “moved down”. Department of Kinesiology and Applied Physiology

  8. Good scientific theories make predictions Manatees, two-toed sloths are only other mammals without seven neck vertebrae - both have six. Authors predict that when ossification patterns of vertebrae in these animals are examined, they will have seven “cervical” (late-osssifying) vertebrae like other mammals. Department of Kinesiology and Applied Physiology

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