Neck Swivels 270 Degrees

a brown gray sloth looks toward the camera as it climbs a tree

Biological Strategy

Brown-throated sloth

Alexandra Ralevski

Image: Nortondefeis / Wikimedia Commons / CC BY SA - Creative Commons Attribution + ShareAlike

Modify Position

Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening its prey.

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Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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The head of the three-toed sloth swivels 270 degrees thanks to extra neck vertebrae.

Most mammals have to completely turn their bodies around to find out what’s behind them – but not tree sloths. Fortunately for sloths, they have extra vertebrae (bones in their spine) that give them almost a 360-degree view of their environment, a powerful defense mechanism for one of nature’s slowest moving creatures.

Sloths can have up to three extra neck vertebrae at the base of their neck, closest to the rib cage. These extra vertebrae enable the sloth’s head to achieve a wider range of motion, giving it the ability to swivel its neck up to 270 degrees in either direction, or three-quarters of a complete rotation.

These extra vertebrae highlight a unique developmental difference between sloths and other mammals. All bones are formed through a process called ossification, and different groups of bones ossify at specific times. In most mammals, the vertebrae that are attached to the rib cage ossify before the formation of neck vertebrae. However, the sloth’s extra neck vertebrae form before the rest of the neck, similar to the rib cage vertebrae. This suggests that the sloth’s extra neck vertebrae are modified rib cage vertebrae.

It is not known exactly why these extra vertebrae evolved, but this extra range of motion makes it easier for a sloth to spot potential predators and mates, and may provide extra support for its head when hanging upside-down from trees.

Image: Stefan Laube / Wikimedia Commons /

Image: Sergiodelgado / Wikimedia Commons /

1=Three toed sloth at the Dallas World Aquarium

Image: Wikimedia Commons /

''Bradypus variegatus'' of "Museu de Zoologia" of Barcelona

Last Updated June 14, 2017

References

“Conservatism in the mammalian axial skeleton, including that exhibited by nonmammalian synapsids, is substantial. All but a few of ∼5,000 living mammal species possess exactly seven neck vertebrae; other presacral counts also show lower interspecific variation in mammals than in non-mammals. Only three genera show departures from the cervical constant: manatees (Trichechus) and tree sloths (Choloepus and Bradypus)” (Hautier et al. 2010:18903).

Journal article

Skeletal development in sloths and the evolution of mammalian vertebral patterning

PNAS. 107(44): 18903–18908. | 01/01/2010 | Hautier L, Weisbecker V, Sanchez-Villagra MR, Goswami A, Asher RJ.

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