17.1 Origin and Diversity

Evolutionary origin

Most vertebrates have tongues. Cartilaginous and bony fishes have a “tongue” attached to the floor of the mouth but their structure does not contain muscle. Jawless fish like lampreys do have a muscular tongue. After they attach to the body of a host using their mouth as a suction cup with keratinous “teeth”, they use their tongue muscles to rasp their abrasive tongue against the scales and skin of the host to cause the oozing of the body fluids on which they feed. The tongue structure and associated muscles in the lampreys are not homologous (did not have the same origin) to those of the tongues of tetrapods. There is no evidence, therefore, that the tetrapod tongue would have appeared among fishes.

The tongues of tetrapods seem to have originated among amphibians. Frogs and salamanders have muscular tongues that they use to ingest food and the structure is generally homologous to those of other classes of vertebrates.

Diversity

The tongue has assumed an extensive variety of sizes and shapes in tetrapods, commonly reflecting the diet and foraging modes of animals. Its association with the hyoid allows it to have a cartilaginous or even bony structure in many species.

Many mammals have a tongue that is much longer, relative to body size, and versatile than that of humans. This is particularly true in quadruped herbivores. Having their forelimbs on the ground, many of these animals use their tongues to collect and manipulate food before ingesting it.

Some predators have tongues that are especially adapted for catching prey. Examples such as woodpeckers, anteaters, frogs and chameleons are present in all classes of tetrapods. The tips of the tongues can be pointy, sticky and have barbs to stick to or impale their prey. The tongue is usually long for increased range of action and it can be projectible.

Anteaters

The giant anteater has no teeth and is capable of only very limited jaw movement. It relies on limited rotation of the lower jaw to open and close its mouth. This is accomplished by its relatively underdeveloped masticatory muscles. Jaw depression creates an oral opening large enough for the slender tongue to flick out.

The tongue is typically 60 cm long with a small rounded tip. Along its length it is covered in backward-curving papillae and coated in thick, sticky saliva secreted from large salivary glands. This allows the giant anteater to collect insects by extending the tongue into ant or termite mounds. During feeding, the tongue moves in and out around 160 times per minute and the anteater swallows at a higher rate than most other mammals.

Woodpeckers

These birds feed mainly on insects and their larvae taken from living and dead trees, and other arthropods, along with fruit, nuts and sap from live trees. The family is noted for its ability to acquire wood-boring grubs using the bill for hammering. The insect prey most commonly taken are those found inside tree trunks and in crevices in the bark. These include beetles and their grubs, ants, termites, spiders, and caterpillars. They may be obtained either by gleaning or, more famously, by excavating wood. Having hammered a hole into the wood, the prey is impaled by a long tongue that is pointy and barbed near the tip. The long tongue is then retracted into the mouth. The muscle slides over the hyoid cartilage which has posterior horns that can extend around the brain to attach to the superior base of the beak. This allows room to fit the entire retracted tongue into the mouth.

Chameleons

The chameleon has a catapult system that allows it to launch the tongue up to 1.5 body lengths away to catch prey. Tongue projection occurs extremely quickly, reaching the prey in as little as 0.07 seconds with an accelerations that exceed 40 gravities. The power with which the tongue is launched (~3000 W kg−1) exceeds the physiological limit of muscles, indicating the presence of an amplification system. The muscle therefore loads an elastic system that launches the tongue when triggered.

This system is formed by a highly modified hyoid bone, tongue muscles, and a collagenous sheath. The hyoid bone has an elongated projection, called the entoglossal process. A cylindric sheath of collagen covers the entoglossal process and connects it to the surrounding accelerator muscle. This muscle is also cylindric and its fibers are arranged radially. When the accelerator muscle contracts around the entoglossal process, it becomes thinner and has to expand longitudinally along the length of the bone because muscle tissue is not compressible. This longitudinal expansion stretches the collagenous sheath longitudinaly, storing a large amount of energy that will produce the work when the tongue is launched. Continued contraction of the accelerator muscle triggers the system. The entoglossal process end abruptly at its anterior end. When the accelerator muscle expands longitudinally it slides over the entoglossal process until some of it slides off the tip. This produces and anterior anchorage that allows the collagen sheath to release the stored energy like a stretched rubber band, launching itselft and the accelerator muscle anteriorly.

If the target is hit, the prey is usually captured in the sticky pad that covers the anterior end of the tongue. A retractor muscle, the hyoglossus, connects the accelerator muscle to the hyoid and is responsible for drawing the tongue back into the mouth.

Other groups of animals also exhibit specializations of elongated structures that facilitate ingestion. These organs may be analogous to tongues, such as the proboscis of a butterfly or the radula on a mollusc. These structures are not homologous with the tongues found in vertebrates, however, and important differences in structure and function can be observed. For example, the proboscis of a butterfly is formed by two jaws held together to form a tube and it is used to suck instead of licking.

Summary

The tetrapod tongue with mucosa and muscles originated in amphibians. It diversified greatly with key roles in food ingestion, gustation, swallowing and communication. Specializations of tongue structure and function are found in all classes of tetrapods, particularly among predator species.

Key terms

Proboscis, entoglossal process, accelerator muscle, radula, hyoglosus muscle,

Figure credits

Figure 1 by Sweeting, Roger [Photographer] (2016) Mouth of a river lamprey. Freshwater Biological Association, UK - Mouth of a river lamprey. http://www.environmentdata.org/archive/fbaia:2676, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=63207440

Figure 2 by Postdlf at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8856815

Figure 3 by Ellen from Ann Arbor, MI, USA - Giant anteater sticking his tongue outUploaded by Snowmanradio, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=10291152

Figure 4 by Soldier of Wasteland - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=52460032

Figure 5 Public Domain, https://commons.wikimedia.org/w/index.php?curid=785566