Sticking their necks out for evolution: Why sloths and manatees have unusually long (or short) necks

May 6, 2011

As a rule all mammals have the same number of vertebrae in their necks regardless of whether they are a giraffe, a mouse, or a human. But both sloths and manatees are exceptions to this rule having abnormal numbers of cervical vertebrae. New research published in BioMed Central's open access journal EvoDevo shows how such different species have evolved their unusual necks.

Birds, and amphibians have varying number of vertebrae in their necks, swans have 22-25, but mammals, regardless of size of animal or the animal's neck, only have seven. Aberrant neck vertebrae are usually correlated with an increase in risk of stillbirth, and neuronal problems in mammals. These pleiotropic events are often associated with physical problems, such as thoracic outlet syndrome, due to misplaced or crushed nerves, muscles and blood vessels.

The only mammals which have evolved different numbers of neck vertebrae without any apparent problems are sloths and . Two-toed sloths (Choloepus) have 5-7 neck vertebrae while three-toed sloths (Bradypus) have 8 or 9. There is some controversy over whether these changes are due to homeotic alteration, where mutation of a gene, such as Hox, causes incorrect skeletal patterning, or are due to an alteration in primaxial/abaxial patterning, where the thoracic structure overwrites that of the cervical vertebrae. Homeotic alteration would affect systems throughout the body and explains the associated effects seen in other mammals. Alteration in primaxial/abaxial patterning would only affect the vertebrae but may explain how sloths escape adverse effects.

After looking at evidence from sloths and related species such as anteaters and , none of which showed unusual neck structures, it became apparent that the conversion of vertebrae from cervical to thoracic for Choleopus suggested a complete foreshortening of the neck and there were ribs associated with the 7th vertebra. For C. hoffmanni these were complete ribs fused to the sternum, but for C. didactylus they were rudimentary ribs suggesting a transitional structure. In contrast, the 8th vertebra for Bradypus was still cervical and most resembled a normal 6th vertebra but the 9th vertebra had rudimentary ribs. None of these vertebrae patterns can be explained by alteration in primaxial/abaxial patterning. Furthermore other skeletal abnormalities were found in sloths including fusion of vertebrae, defective production of cartilage, ossification of sternum and pelvic girdle, abnormal fibrous bands connected to rudimentary ribs, and asymmetric ribs, which can only be explained as side effects of homeotic alteration.

Similarly the skeletons of manatees (Trichechus) were compared to dugongs and hyraxes. Manatee vertebrae and skeletons also showed similar alteration to the sloths. Surprisingly about half of the dugongs and a couple of the hyraxes studied also had reduced number of neck vertebrae and all of these had other skeletal abnormalities.

Dr Galis from the Netherlands Centre for Biodiversity Naturalis said, "Our research casts doubts on the validity of the aberrant primaxial/abaxial patterning theory and instead lends weight to Bateson's theory of homeotic transformation where the identity of the has been changed from cervical to thoracic (or visa-versa depending on whether the neck if shorter as for Choloepus or longer as for Bradypus). The defects seen are most similar to mice with Hox mutations."

Dr Galis continued, "We think that it is the slow lifestyle and low metabolic rate which has allowed evolution to alter the neck length of without any of the side effects seen for other . Their low metabolic rates protect them from cancer and their low activity rates protect them from thoracic outlet syndrome." So, for a sloth, being slow allowed them to evolve unusual necks.

Explore further: When vertebrae cross dress: How sloths got their long neck (w/ Video)

More information: Breaking evolutionary and pleiotropic constraints in mammals. On sloths, manatees and homeotic mutations.
Irma Varela-Lasheras, Alexander J Bakker, Steven D van der Mije, Johan AJ Metz, Joris van Alphen and Frietson Galis
EvoDevo (in press)

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1 / 5 (5) May 07, 2011
I am ashamed that money is being spent on this type of meaningless research. The theory of evolution is such a fraud and an utter waste of funding. Evolution is irrelevant to modern medicine. No doctor ever improved diagnostics because they knew about Darwins speculation. We need observational science, not more historical speculation.
5 / 5 (2) May 11, 2011
Wrong, Johannes:
"Researchers in the field of evolutionary medicine have suggested that evolutionary biology should not simply be an optional topic in medical school, but instead should be taught as one of the basic medical sciences.[1]"

Presumably, the real reason for your protest is that you have an invisible-friend-in-the-sky, who objects to evolutionary science.
1 / 5 (3) May 12, 2011
So evolutionists recommend evolution? Thats a surprise...Evolution is not science, because it cannot be observed or tested in a controlled environment. Darwinian evolution is at best so called historical science, a speculation about what might have happened in the distant past, where no one was there to observe how some slime grew legs over millions of years. It should be classified in the faith section along with UFO's, big bang and other pseudo-science.
5 / 5 (2) May 13, 2011
Johannes, bacterial evolution by natural selection has been demonstrated in the lab. Consider nylon-eating bacteria becoming dominant when nylon is deliberately made available.
5 / 5 (1) May 13, 2011
I am ashamed that money is being spent on this type of meaningless research.
I am apalled that you could be ashamed about people trying to learn the truth the Universe.

The theory of evolution is such a fraud and an utter waste of funding.
The theory of Evolution fits the physical evidence. The Bible fails to the fit the evidence. The world is not young, there was no Great Flood and the first two chapters of Genesis can't even agree on when Man was created much less actually fit the physical evidence.

Evolution is irrelevant to modern medicine.
Yeah that is why we have to deal with genetic diseases in humans created by a perfect all knowing god. Or perhaps it is because the diseases protected human from other diseases as can be seen in sickle cell anemia and cystic fibrosis. Evolution or an incompetent god. I go for the one the fits the evidence. Evolution.
5 / 5 (1) May 13, 2011
No doctor ever improved diagnostics because they knew about Darwins speculation.
Would you care to look at the two diseases above. Of course not that would be dealing truth instead of fantasy.

We need observational science, not more historical speculation.
We need less lies from Creationists that won't even discuss this.

When was the Flood? I bet won't answer because you KNOW that it completely unsupported by evidence.

5 / 5 (1) May 13, 2011
IMO, the Bible "Flood" story is a classic piece of misunderstood folklore, passed on through many generations by word of mouth and probably including more than one major flood in the Middle East, such as the Mediterranean tsunami, caused by Mount Etna about 8,000 years ago


and the well-known flooding of the former below-sea-level lake where the modern-day Black Sea is, about 7600 years ago


The point is that, as far as the local people were concerned, this area WAS the whole world - they didn't know any other, so that's how it was described in the pre-historic

stories! Of course, Biblical literalists wouldn't be literalists without naively assuming they meant the whole of planet Earth! But then you can't expect a literalist to get to the truth of anything, can you?
not rated yet May 13, 2011
We seem to be ever-so-slightly off topic with this entire thread! Maybe not too many people are that interested in a sloth's neck!

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