6.1. Adaptations for Ingestion

Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking.

Diversity

Animals have evolved a great variety of mouth shapes and sizes in response to the selective pressures associated with feeding. The first fishes had mouths, but bony jaws evolved later and allowed for an impressive diversification in the group. Fishes, amphibians, reptiles and mammals have bony jaws and hard teeth for powerful biting, piercing and grinding. Birds, on the other hand, evolved much lighter keratinized beaks that facilitate flight. Outstandingly long mouths and tongues can be found in nectar feeders and anteaters. Strong beaks and skulls are present in woodpeckers that hit tree trunks with the beak to find insect larvae for food. Some examples of structural adaptations for feeding are presented below.

Trunk

The trunk, or proboscis, is a fusion of the nose and upper lip, although in early fetal life, the upper lip and trunk are separated. The trunk is elongated and specialised to become the elephant's most important and versatile appendage. It contains up to 150,000 separate muscle fascicles, with no bone and little fat. These paired muscles consist of two major types: superficial (surface) and internal. The former are divided into dorsals, ventrals, and laterals while the latter are divided into transverse and radiating muscles. The muscles of the trunk connect to a bony opening in the skull. The trunk encloses two air passages that run side by side and are separated by a nasal septum composed of tiny muscle units that stretch horizontally between the nostrils. A unique proboscis nerve, formed by the maxillary and facial nerves, runs along both sides of the trunk. It controls the muscles to produce lateral flexion, ventral flexion and extension of the trunk, the dilation of its walls for sucking in water, the compression of its walls, to expel the water into the mouth and a pinching movement at the tip of the trunk, which allows the animal to manipulate small objects precisely.

Elephant trunks have multiple functions in addition to bringing food to the mouth. These include breathing, olfaction, touching, grasping, and sound production. The trunk's ability to make powerful twisting and coiling movements allows it to collect food, wrestle with other elephants, and lift up to 350 kg. The trunk can be used for delicate tasks, such as wiping an eye and checking an orifice, and it is capable of cracking a peanut shell without breaking the seed. An elephant can reach items at heights of up to 7 m with its trunk and dig for water under mud or sand. Elephants can suck up water both to drink and to spray on their bodies. An adult Asian elephant is capable of holding 8.5 liters of water in its trunk. When swimming, the elephant uses its trunk as a snorkel. Trunks are most developed in elephants, but they are also found in several other mammals, such as the tapir and elephant seal.

Beak

The beak, bill, or rostrum is an external anatomical structure of birds that is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young. The terms beak and rostrum are also used to refer to a similar mouth part in some dicynodonts, Ornithischians, cephalopods, cetaceans, billfishes, pufferfishes, turtles, Anuran tadpoles and sirens.

Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections—the upper and lower mandibles—are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes known as nares lead to the respiratory system. In contrast with mammal jaws, the beaks of some birds (like seagulls) can articulate at multiple points instead of two. This allows them to widen the beak as they open it, and swallow a wider food item than would fit the width of the closed beak. This is specially useful for birds as they that lack teeth and most of them have to swallow food items whole.

As the jaws and teeth of mammals, bird beaks have been shown to evolve in close association with the physical properties of the most common food items in the diet. A classical example is the radiation of Darwin's finches in the Galapagos Islands (Fig. 8). A single species arrived to the islands from the continent and colonized them. It then gradually diversified into several species that specialized in the consumption of different seeds. The morphology of the beak evolved quickly to better exploit the food resource of each specialized new species.

Tentacles

A tentacle is a flexible, mobile, elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. The tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the "eye stalks" of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.

A variety of shapes and sizes of tentacles is found around the mouths of Cnidarians, Molluscs and Bryozoans. Trypanorhynch cestodes are parasitic tapeworms found in fishes. Their scolex shows four tentacles covered by spines (Fig. 2). These tentacles help the adult cestode to attach to the intestine of the shark or ray host. The same tentacles are also present in the larvae. Among vertebrates, two short tentacles are found in the legless amphibians called caecilians. They are located on each side of the head, between their eyes and nostrils. Their function, however is most likely olfactory or tactile, but it is not used to direct food into the mouth. Similarly, the star-nosed mole, Condylura cristata, of North America, has 22 short but conspicuous tentacles around its nose. They are mobile and extremely sensitive, helping the animal to find its way about the burrow and detect prey. They are about 1–4 mm long and hold about 25,000 touch receptors, giving this mole a very delicate sense of touch. These tentacles have not been seen to direct food toward the mouth either.

Barbels and Whiskers

Many fishes have barbels around the mouth. Birds and especially mammals commonly have long whiskers (vibrissae) on each side of the mouth. These structures have sensory roles, providing their bearers with exquisite tactile sensitivity. They seem not to have, however, a role in ingestion.

A possible exception might be rictal bristles which are hair-like feathers that arise around the base of the beak and project anteriorly. These are mostly found in insectivorous species like nightjars, swallows, and flycatchers (Fig. 4). Nightjars are medium-sized birds in the family Caprimulgidae, characterized by long wings, short legs and very short bills. They are mostly active in the late evening and early morning or at night, and feed predominantly on moths and other flying insects. They beak is surrounded by stiff vibrissae arranged in parallel and pointing anteriorly, forming a “basket” around the beak. It was been suggested that the birds may used them as a net, to direct into the mouth flying prey that would have otherwise escaped predation. This hypothesis has not been experimentally tested and a valid alternative role for the vibrissae would be that of a tactile receptor.

Cheek and floor of the mouth

Cheek pouches are pockets on both sides of the head of some mammals between the jaw and the cheek. They can be found on mammals including the platypus, some rodents, and most monkeys, as well as the marsupial koala. The cheek pouches of chipmunks can reach the size of their body when full. Cheek pouches allow the rapid collection of food, serving for temporary storage and facilitating transport. In monkeys of the subfamily of Cercopithecinae, they collect predigested food and allow the animals to carry their food to safer locations.

Pelicans are well-known for having a large pouch that can contain several liters of water. The animal opens the beak and scoops a volume of water containing small fish into the pouch. Specialized muscles then contract, expelling the water from the pouch and forcing the fish down the throat.

Jaws and Teeth

An amazing variety of jaws and teeth arrangements has evolved in vertebrates, as a result of their radiation and occupation of a broad range of ecological niches. The position, size and strength of jaws and teeth evolves to match diet and foraging mode. As an example, both in fishes and tadpoles, species that feed on the surface, midwater and bottom tend to have their mouths oriented upward, forward and downward, respectively.

Some fishes use their mouths as an anchorage device to attach to a host. This can be used for dispersal, non-invasive feeding or invasive feeding. The candiru (vampire fish) is a species of parasitic freshwater catfish in the family Trichomycteridae. It is native to the Amazon Basin where it is found in Bolivia, Brazil, Colombia, Ecuador and Peru. It attacks larger fishes by swimming under the operculum and into the gills, where it latches onto aortic arteries and feeds on blood for 1-4 min. After feeding, the candiru leaves the host and remains buried at the bottom of the river while digesting the meal.

Half of the known species of lamprey are also parasitic fish. They are jawless and have the mouth organized as a suction cup, with series of rasping teeth arranged in circles. Parasitic lampreys feed on prey as adults by attaching their mouths to the target animal's body, then using their teeth to cut through surface tissues until they reach blood and body fluid. Secretions in the lamprey's mouth prevent the victim's blood from clotting. Victims typically die from excessive blood loss or infection. After one year of hematophagous (blood) feeding in salt water, lampreys ascend a river to spawn and die.

Concentric circles of sharp teeth can be found around the suction cup shaped mouth of the jawless lampreys. These parasitic fishes use their teeth to harvest lymph and blood from their host until it is killed or badly debilitated. The keratinized teeth of lampreys are thought to have evolved independently from the enamel covered teeth of most other vertebrates.

In contrast with the candiru and lamprey, remoras (family Echeneidae) do not use their mouths to attach to their hosts. Instead, their first dorsal fins are modified into suction organs. While remoras move around attached to the body of the host, they are not parasites. Remoras feed on food particles of feces of the host in a commensal relationship that may even be somewhat mutualistic as the remoras may remove some ectoparasites and loose flakes from the host's skin.

Baleen

Baleen whales have evolved baleens which are series of parallel plates of calcified keratin attached to the upper jaw and used for filter feeding. The whale opens its mouth widely and scoops in dense shoals of prey (such as krill, copepods, small fish and sometimes birds that happen to be near the shoals), together with large volumes of water. It then partly shuts its mouth and presses its tongue against its upper jaw, forcing the water to pass out sideways through the baleen, thus sieving out the prey which it then swallows. This is a form of suction feeding and it allows the whale to capture multiple prey items at once. Whales are mammals and as such they were terrestrial animals before they moved into the sea. The availability of food as many small prey items is thought to have driven the loss of teeth and the evolution of suction feeding and baleen. The baleen is a keratinous structure that bears no resemblance with the structure of teeth. It has most likely evolved from the epithelium of the gums, in a similar way as to how the epithelium of the mammal skin produces nails, horns and hair, which are all made of keratin.

 Tongue

Many animals use the tongue as the primary way of taking food from the environment. Chameleons project a sticky tongue that adheres to the prey, whereas woodpeckers impale pray items with their barbed tongues. Giraffes have long and versatile tongues with which they harvest leaves from trees as they feed. The general structure of the tongue, specializations, and its role in gustation are discussed in the chapter Tongue and Gustation.

Summary

A wide diversity of facial adaptations that facilitate food ingestion can be found in animals. Examples include trunks, beaks, tentacles, whiskers, cheek pouches, suction cups, keratin denticles, and a baleen. These structures vary with phylogenetic history but also with prey type and feeding behavior.

Key terms

Trunk, proboscis, beak, tentacle, barbel, whisker, vibrissa, cheek pouch, throat pouch, suction cup, keratin denticle, suction feeding, hematophagy, baleen, tongue

Figure credits

Figure 1 by Fir0002 - Own work, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=1391195

Figure 2 by Didier Descouens - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=10900952

Figure 3 by John Gould (14.Sep.1804 - 3.Feb.1881) - From "Voyage of the Beagle" as found on http://darwin-online.org.uk/converted/published/1845_Beagle_F14/1845_Beagle_F14_fig07.jpg]; also online through Biodiversity Heritage Library at http://biodiversitylibrary.org/page/2010582., Public Domain, https://commons.wikimedia.org/w/index.php?curid=3918303

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Figure 5 by Ian Beveridge, Rodney A. Bray, Thomas H. Cribb and Jean-Lou Justine - Beveridge, I., Bray, R. A., Cribb, T. H. & Justine, J.-L. 2014: Diversity of trypanorhynch metacestodes in teleost fishes from coral reefs off eastern Australia and New Caledonia. Parasite, 21, 60. doi:10.1051/parasite/2014060, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=36855176

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