Sunday, January 29, 2012

Palaeophysiology of sloths: Why some giant sloths probably evolved from tree-dwelling sloths

The venous return of sloths shares some peculiarities with a few other mammals, many of which are tree dwelling (clinging to branches) or diving (seals and whales). The special circulatory features include a double vena cava and an large internal vertebral vein. In the sloth (Choloepus), the two venae cavae connect with this epidural vertebral vein. The vein is as large in diameter as the spinal cord, which is in fact displaced laterally. All of this is described in the classic work by M. Goffart, "Function and Form in the Sloth" (1971).

In a review paper I published in 1990 (Comp. Biochem. Physiol. Vol. 95A, No. 1, pp. 23-25), I pointed out that a range of mammals which experience a shortage of blood to the limbs share certain unusual vascular features (not only those already mentioned but also features such as vascular bundles - a highly developed form of counter-current heat exchanging arrangement of arteries and veins, of the kind sometimes known as a rete mirabile). The shortage of blood to the limbs can be caused by diving, and diversion of blood to the trunk and away from the limbs, or by prolonged clinging postures, which will also tend to reduce blood supply to the limbs and divert it to trunk.

We cannot observe the circulation in the limbs of extinct giant ground sloths, but we can determine that they shared the internal vertebral veins found in living sloths (Goffart writes that they also occurred in the "gigantic ground-living fossil sloths"). It is reasonable to deduce that ground sloths either shared the clinging habit of modern sloths, or more likely, they evolved from tree sloths with this habit and retained the venous arrangement in the spine.

Here is something relevant from the Wikipedia article on sloths:

"The two-toed sloths of today are far more closely related to one particular group of ground sloths than to the living three-toed sloths. Whether these ground-dwelling Megalonychidae were descended from tree-climbing ancestors or whether the two-toed sloths are really miniature ground sloths converted (or reverted) to arboreal life cannot presently be determined to satisfaction. The latter possibility seems slightly more likely, given the fact that the small ground sloths Acratocnus and Neocnus which were also able to climb are among the closer relatives of the two-toed sloths, and that these together were related to the huge ground sloths Megalonyx and Megalocnus."



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