Receptors Detect Tiny Disturbances

photograph of alligator with mouth open in front of log

Biological Strategy

American alligator

AskNature Team

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Sense Touch and Mechanical Forces in a Living System

Perceiving touch enables living systems to detect other living systems around them and environmental conditions, such as air movement, water currents, and temperature. This ability can help them sense danger as well as opportunity, as when a Venus flytrap’s hairs sense the presence of an insect to eat. Sometimes, a living system senses touch or mechanical forces at a coarse scale; other times, at a sensitive scale that detects very subtle differences. For example, a human elbow is not nearly as sensitive to textures as human fingertips. Fingertips have dermal ridges and many nerve endings that increase sensitivity, enabling them to explore the environment in detailed ways. Elbows don’t need to sense at that level of detail.

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Sense Shape and Pattern in a Living System

Living systems must identify other living systems and objects to navigate, feed, escape predators, find resources, and more. The ability of living systems to “see” varies widely, with “seeing” including not only sight, but other means of sensing shapes or patterns, such as smell or echolocation. For example, a hawk can see much more detail than humans while other organisms can see far less detail. Nevertheless, each living system is capable of detecting shapes and patterns to the extent it needs to for survival. In addition to perceiving physical shapes or patterns, living systems can also sense landscape patterns at various scales. The purple sea urchin, for example, does not have eyes, yet can pick out fine details in its environment by using its entire surface as a compound eye. The urchin’s spines shield light coming from wide angles, further refining its focus.

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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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Faces of alligators detect tiny disturbances in the water and discriminate objects using touch-sensitive receptor cells in their scales.

The American alligator is a large reptile recognizable by its heavily armored body covered in thick scales. These alligators live in swamps, rivers, and lakes of the southeastern United States. Even with a scaly and tough-looking exterior, parts of the alligator’s body are highly sensitive to touch—even more sensitive than a human’s fingertips. This is achieved with an array of sensory organs in the alligator’s scales.

The touch-sensitive organs are called integumentary sensory organs. On the surface of a scale, the sensory organs appear as small, dark dome-shaped spots typically ~0.5 mm in diameter. Thousands of these sensory organs cover the alligator’s face. They are especially dense around the teeth, inside the mouth, and at the tip of the snout.

Under each dome lies a dense network of touch-sensitive receptor cells (known as mechanoreceptors) and nerves. When part of the dome is pushed down by an object the alligator encounters or even moving water, this stimulates the touch-sensitive receptor cells. The cells are sensitive to deflections as small as 4 μm, just a fraction of the width of a human hair. The stimulated receptor cells then send signals along connected nerve cells to the central nervous system, which includes the alligator’s brain. There are a variety of receptor cells and nerves in each dome-shaped organ: some respond to continuous stimulation (like a constant push) while others are more sensitive to changing stimuli (like vibrations).

Researchers hypothesize that these extremely touch-sensitive areas around the mouth help the alligator locate, capture, and examine prey even when visual and sound cues are absent. The alligator will open its mouth slightly while searching for food in the water, using its integumentary sensory organs to detect disturbances in the water from a moving object. Once the alligator finds and bites the object, it can examine the object with its mouth to see if it’s edible.

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Last Updated October 20, 2017

References

“Integumentary sensory organs (ISOs) are densely distributed on the jaws of crocodilians…[Each ISO] was innervated by networks of afferents [nerve fibers] supplying multiple different mechanoreceptors…Rapidly adapting, slowly adapting type I and slowly adapting type II responses were identified based on neuronal responses. Several rapidly adapting units responded maximally to vibrations at 20–35Hz, consistent with reports of the ISOsʼ role in detecting prey-generated water surface ripples. Despite crocodiliansʼ armored bodies, the ISOs imparted a mechanical sensitivity exceeding that of primate fingertips. We
conclude that crocodilian ISOs have diverse functions, including detection of water movements, indicating when to bite based on direct contact of pursued prey, and fine tactile discrimination of items held in the jaws.” (Leitch and Catania 2012:4217)

Journal article