Chapter 7 Summary

Touch is the most widely used short-range sensory modality in living organisms. Animals rely on touch (also known as tactile stimulation) to resolve conflicts, mediate courtship and mate choice, reaffirm affiliative and dominance relationships, coordinate group movements, and exchange environmental information.
Touch stimuli can vary in amplitude, temporal pattern, informational versus strategic salience, and types of sensory receptors stimulated. The latter include detection and assessment of temperature (thermoreceptors), pain (nociceptors), positions and tensions of muscles and joints (proprioceptors), and both steady (slow-adapting mechanoreceptors) and changing (fast-adapting mechanoreceptors) contacts with the receiver’s body surface.
Prokaryotes (Eubacteria and Archaea) respond to cell membrane distortion by changing ion conductances in the region of perturbation. Cell membrane sensitivity to touch is also widespread in single-celled eukaryotes but is most often limited to the membranes enclosing or just adjacent to flagella and cilia. Most invertebrates use modified cilia as their primary tactile receptors, although some nematodes and arthropods detect tactile stimuli with both ciliated and nonciliated nerve endings. Touch receptors of invertebrates are often clustered on the head and appendages, and around the genitalia.
Vertebrates have a diverse set of touch receptors. Free nerve endings monitor temperature, pain, and sexual organ stimulation. Merkel cells and Grandry corpuscles occur in dense clusters on lips, bills, digits, and around hairs and feathers. They monitor steady pressures and are slow-adapting. Encapsulated touch organs consist of bare nerve endings surrounded by two or more onionlike layers of Schwann cells. Ruffini mechanoreceptors and Golgi tendon organs act as slow-adapting proprioceptors in muscles and joints. Lanceolate, Meissner’s, Herbst’s, mucocutaneous end organs, and Pacinian corpuscles are fast-adapting touch receptors that monitor movements of external objects along a receiver’s body surface. These occur in high densities around the bases of bird feathers and mammalian vibrissae (also called sinus hairs), and in the bill tip organs of birds.
Two different sets of ion channels and associated genes are used by animal touch receptors. Those stimulated by sensory cilia admit calcium ions into the cells during stimulation, whereas nonciliary receptors favor entry of sodium ions. Both systems can be found in the same animal in nematodes, arthropods, and vertebrates.
Hydrodynamic stimuli are characterized by locally cohesive movements of fluid medium (e.g., water or air). Examples include eddies, vortices, wakes from moving animals, and short-range currents. Like near-field sounds, the amplitudes of hydrodynamic stimuli fall off quickly with distance from the source, and few animals can detect them at distances greater than 1–2 body lengths from their source. Unlike near field sounds, which persist at most for milliseconds, hydrodynamic stimuli can persist for many minutes after generation.
Biological uses of hydrodynamic stimuli include location of respiring but otherwise static prey, tracking of moving prey by following their wakes through the medium, detection of approaching predators, coordination of locomotion within schools of fish or squid, and mediation of aggressive interactions and courtship in insects.
Most invertebrates monitor hydrodynamic stimuli with cilia mounted on their body surfaces. The cilia of squid and prawns are organized into linear arrays that allow the animal to map the ambient hydrodynamic field. Fish and larval amphibians accomplish the same functions with ciliated receptors overlain with a gelatinous cupula like those in fish and amphibian ears. Superficial neuromasts, arrayed in lines along the surface of the body, monitor steady and low-frequency stimuli. Canal neuromasts are located inside small tubes with multiple openings just under the animal’s surface, and specialize in assessing higher-frequency stimuli. The combined input from the two types of neuromasts provides the animal’s brain with detailed spatial information about the amplitude, direction of flow, and spatial shape of ambient hydrodynamic fields.
The vibrissae (sinus hairs) of mammals can act as hydrodynamic sensors. Seals use the vibrissae on their snouts to track the underwater wakes of fish or conspecifics. The duck-billed platypus has hydrodynamic sensors on its bill that it can use to detect the eddies and vortices generated by the motions of aquatic invertebrate prey.
Active use of an animal’s nerves and muscles, and ionic heterogeneities between anatomical regions generate both varying and static electric fields around its body. These weak fields are detectable at most a few body-lengths away from the animal, but within that range, temporal variations in the fields are propagated with no delay. The creation of these fields is called passive electrogeneration, because it is incidental to the primary function of the relevant organs.
Most primitive fishes (lampreys, sharks, rays, skates, paddlefish, reedfish, coelacanths, and lungfish), and some amphibians use ciliated receptor cells to detect passive electric fields (passive electroreception). These electrically sensitive receptor cells are grouped into ampullary organs spread over the animal’s exterior surface but most often concentrated in the head region. Although the ancestors of advanced bony fishes lost their ampullary organs, some descendants such as catfish, knifefishes, elephantfishes, frankish, and featherback fishes, and the duck-billed platypus (a mammal) reacquired ampullary-like receptor organs. Most species use their passive electroreceptive organs to detect weak and slowly varying electrical fields produced by prey. Stingrays use passive electroreception to locate hidden conspecifics, and some catfish and sharks monitor nonbiological electrical fields to navigate at night or in murky waters.
Skates and some catfish have modified selected body muscles into electric organs that produce medium-intensity electric discharges as social signals. This is called active electrogeneration. Knifefishes, elephantfishes, and frankfish evolved similar electric organs, but use them both for electrocommunication and to probe their immediate environment for the presence of nearby objects (electrolocation). Both functions rely on modified electric receptors called tuberous organs that are sensitive to the high frequencies that make up the very-short-duration and rapidly repeated electric organ discharges. Objects of different sizes, shapes, compositions, and proximities distort the electric field around a fish in different ways. By combining the input from many tuberous receptors spread over its body, the fish’s brain can characterize the location, motion, and many properties of nearby objects. Electrolocation allows species with tuberous organs to navigate around obstacles and find prey at night or in very turbid waters.
To avoid disrupting their navigational capabilities, electrolocating fish use modulations in the rate and waveforms of their electric organ discharges for social communication. Modulations include sudden bursts of discharges at high rates, variations in discharge amplitude, subtle changes in waveforms, and brief cessations in discharging. Differences in waveform pattern created by varying the geometries and structure of their electric organs facilitate species and gender recognition in species-rich communities. In some species, waveforms are individually distinctive, allowing for recurrent social interactions, maintenance of dominance hierarchies, and sexual selection with no or little reliance on other modalities.