Waves of Shortening Used to Move

Biological Strategy

Boa constrictor

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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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Bodies of boas and pythons can move in a worm-like extended fashion by passing waves of shortening down their bodies.

“A few snakes, such as boas and pythons, can move forward when extended lengthwise in a wormlike fashion rather than with their bodies bent into curves. They’re not fast, and the behavior seems to be used for a stealthy approach to prey. With backbones fixing their lengths, they can’t work exactly like earthworms–they instead use waves of shortening that pass rearward along a powerful lengthwise band of ventral muscle. So the top of the snake moves steadily. Meanwhile, its bottom has a stop and start motion as successive segments are fixed to the ground while the ones behind the fixed ones are shortened and those in front of the fixed ones are lengthened. Moving segments are raised slightly above the fixed ones–it looks as if the snake is walking on its ribs, although it is not. Gray (1968) and Gans (1974) give good descriptions.” (Vogel 2003:488-489)

Last Updated September 14, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

17/06/2013 | Steven Vogel

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