CT Visualization of Vasculature Channels in Modern and Fossil Sloth Ungual Claws

Abstract

One of the interesting features of modern sloths is the vasculature patterns of their upper and lower limbs. Even though sloths are endothermic like other mammals, their mean body temperature tends to be lower, more variable, and more linked to the ambient temperature than other placental mammals. Like some other mammals from environments with temperature extremes, they possess rete arteriovenous bundles, which work as an effective heat exchange mechanism, where heat from distally flowing arterial blood is transferred to proximally flowing venous blood. This minimizes the heat loss from the extremities while still maintaining blood flow. The temperature gradient increases in the rete system when the animal’s core temperature decreases, which cools the limbs and preserves core body heat. When the body temperature rises the flow through the superficial venous system increases, which promotes heat loss and body cooling. This begs the question of how this complex vasculature system may or may not extend to the distal phalanges and claws of the sloth, perhaps their most distinctive feature. One possibility could be that to work with the limb cooling function of the rete system, the distal phalanx and claw may be poorly vascularized so as not to increase chances for heat loss. However, considering the extensive and important use of their claws, they may require extensive vascularization to withstand the large amount of stress they are often under. This study uses Micro CT scans of sloth ungual phalanges to visualize their vascular channels. In both modern and extinct giant ground sloths, vascular networks through the bone of the ungual phalanx are quite extensive, and are very clearly linked to externally visible channels and impressions left by the proper digital arteries. In ground sloths these internal vascular channels quickly branch on either side and continue distally, letting off smaller branches as they go. These channels are rather large, even appearing to be artificial drill holes on one specimen that has the distal end broken off. Modern sloths show a similar pattern, but their vascular channels take up proportionally less space in the bone than in the giant ground sloths. This difference may be due extinct giant ground sloths’ more varied use of their claws. These complex patterns show that sloth claws require a significant blood supply, and may indicate that the rete system is effective enough that by the time the arterial blood gets to the distal phalanx it has been sufficiently cooled to not be a significant source of heat loss.