Draft:Lingual feeding

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Lingual feeding (Latin lingua, meaning “tongue”) in zoology is a form of feeding behavior first named by David D Crespo (born in 2003) when an animal uses a oral muscular organ, most notably the tongue that's normally enlarged for foraging and hunting and acts as a tool to obtain certain food items that take up a significant part in the animals’s diet. These foods may not be as easily accessible to other species that live in the same habitat. A popular example of Lingual Feeding is the Giant anteater, which has the longest tongue in comparison to its body size of any mammal, up to two feet long. Being a strict insectivore, it uses its long tongue that is covered in a sticky saliva to collect colonizing insects like termites and ants from nests it has prised open with its large, curved claws.

Invertebrates[edit]

Because snails and slugs lack developed mouth parts or jaw muscles to chew, they have a specialized anatomical, oral structure known as a radula, which is often compared to a tongue. It is a thin ribbon that’s constructed out of Chitin and is covered in microscopic teeth that is mainly used to scrape or slice up food matter before it’s passed through the esophagus and is used by both carnivorous and herbivorous snail and slug species, as well as most other types of mollusks except the Bivalves, who instead use Cilia, which are filaments that they wave back and forth to collect small organisms and transport them to the mouth.[1]

Most insect species (especially true bugs) also feed through a special mouth structure, called a proboscis that’s long, thin and straw-like, allowing insects to suck up liquids. Other insects like flies however, tend to feed on solid, decaying substances and so they must regurgitate digestive stomach acids to help dissolve and break down the solids, liquefying them and therefore making sucking up the contents much simpler. Pollinating insects, such as a butterfly or a bee is able to curl and uncurl its proboscis at will to drink the sweet nectar from flowering plants. Butterflies have a much longer proboscis to have access to feed from narrow, deep flowers that bees wouldn’t usually be able to reach, ensuring that every single part of every single type of flower gets effectively pollinated.

The assassin bugs (Reduviidae) also harbor a long, curved and pointed proboscis (sometimes called the rostrum), but they use it in a more predatory manner. They use it for stabbing their prey and injecting them with a venom-filled saliva. The bugs may also use it as a defense mechanism and to deter potential enemies. Both the nymphs and adults are highly carnivorous and hunt insects and other arthropods. The assassin bug is an active hunter but when prey is spotted, they patiently wait motionless for the insect to come closer within range. It then pounces on the prey item, catching it with its raptoral (prey-grasping) front limbs to pin it down, whilst it injects its deadly saliva. Once the insect is dead, it uses its sharp proboscis to pierce the exoskeleton and suck up the fleshy fluids of its victim's flesh and internal organs.[2]

Vertebrates[edit]

Amphibians[edit]

Many amphibian species have a long, sturdy tongue with a adherent tip to seize their prey. Bizarrely, the tongue itself is actually attached to the front of the mouth, instead of the back. In some frog species, the tongue can be as much as a third of the frog’s overall body length and has to be folded down inside its voracious maw. Terrestrial salamanders also capture their prey by flicking out their sticky tongues, of which can fire out in less than half a second. A frog’s tongue is made up of two distinct muscle groups; the extender and the retractor. The extender pushes the tongue out the mouth at astonishing speeds of up to four meters per second. As the frog opens its mouth, its opening jaws rotate the tongue’s position as it fires. The tip of the tongue is covered in a mucus-like saliva, which is a non-Newtonian fluid (it can behave as both a solid and a liquid) to grab onto an unsuspecting prey animal. A frog’s tongue is also about 10 times softer than a human’s, about as malleable as a human brain, making it a lot more flexible and able to wrap around food more easily. The retractor muscles then pull the tongue back into the frog’s jaws, along with the prey in less than 0.07 seconds. As well as being extraordinarily quick, a frog’s tongue is also exceedingly strong. A study in 2014 discovered that the powerful tongues of horned frogs could lift 1.4 times their own body weight. This combined with the horned frog’s large size, allows it to drag larger prey such as mice, small lizards and even other frogs. Other large frogs like the American bullfrog (Lithobates catesbeianus) also have a rapid tongue strike, large size and powerful jaws to break open the shells of crayfish, ram’s horn snails (Planorbis) and the exoskeletons of diving beetles (Dytiscus).[3]

The Tiger salamander (Ambystoma tigrinum) has been shown to position itself before striking its tongue at its prey. Then, the mouth opens widely, while the lower jaw stays stationary and the tongue swells and bulges, changing its shape as it's shot forward. The protruded tongue has a concaved centre and the brim of this caves in inwards as the prey is hit, trapping it in a mucus-filled dent. Here, the animal flexes the neck, retracting the tongue and closing the jaws as soon as the prey enters.[4]

The salamander genus of Hydromantes has some of the fastest tongue strikes on the planet, fully extending their tongues and capturing their insect prey in just 20 milliseconds. The salamanders of this genus can be identified by their extremely elongated tongues that can project up to 6 cm from the mouth, 80% of their body length, the longest tongue to body ratio of any animal. The whole salamander family of Plethodontidae (the lung-less salamanders) is capable of doing this because it has an advanced muscle system in its throat, called the Subarcualis Rectus (SAR). These powerful muscles compress the tongue, squeezing it with such force that the entire tongue skeleton can protrude forward.[5]

Reptiles[edit]

Perhaps the most famous reptiles who use their tongues for hunting are the chameleons (Chamaeleonidae). All 160 species of chameleon posses extensive, modified tongues and use them for the same reason. The hyiod bone has a long, parallel-sided projection, known as the entoglossal process, over where the tube-shaped accelerator muscles rests. The accelerator muscle contracts around the entoglossal prosess and is accountable for creating the work to power the tongue’s projection. The retractor muscle, the hyoglossus, connects with the hyiod and accelerator muscle and is what makes the tongue retreat back into the chameleon’s mouth after being fired. The tip of the tongue is a thick ball of muscle that is covered in a sticky, adhesive substance. Before impact, a pair of muscles called the pouch retractors contract to turn the tip into a concave suction cup to envelope prey more effectively. A chameleon has one of the fastest tongues in the while Animal Kingdom and can shoot it out in as little as 1/100th of a second and can fire out in a speed of 0-60 mph. The exact acceleration behind the launch of the tongue has been known to exceed 41 g. It is the second fastest tongue on Earth, second only to the salamander. Smaller species tend to have faster and more powerful tongues than the larger ones, as the Rosette-nosed pygmy chameleon (Rhampholeon spinosus) was measured to have a total power of 14,040 Watts per Kg. As a result, chameleons also have the most powerful tongue muscles found in any animal.[6]

The smaller chameleons also appear to have a longer tongue to body ratios. For example, the Meller’s chameleon (Triceros melleri), Parson’s chameleon (Calumma parsonii) and the Malagasy giant chameleon (Furcifer oustaleti) are some of the largest species in the world and have some of the longest tongues found in chameleons in terms of sheer length, due to there large size. The meller’s chameleon’s tongue reaching at up to 20 inches (51 cm) long, over 80% of its body length, which averages at about 24 inches (61 cm). On the other hand, the Bearded leaf chameleon (Rieppeleon brevicaudatus), rosette-nosed pygmy chameleon and von Hohnel's chameleon (Triceros hoehnelii) all have a tongue that are over twice the total length of their bodies, which are on average only 90 mm or less. Being around 3 inches (8 cm), the Rosette-nosed pygmy chameleon may attain tongue lengths of 2.5 times its body size, a maximum of nearly 14 cm (5.51 inches).[7]

Agamid lizards also use their tongue to catch their prey. The Common agama (Agama agama) has a long, sticky tongue that’s covered in mucous glands to help the lizard grab onto smaller prey, such as insects and even small mammals and reptiles. The Bearded dragon also uses its tongue to collect food in much the way as the chameleons, although a lot shorter and therefore having a shorter firing range.[8]

Birds[edit]

Hummingbirds (Trochilidae) are known for feeding from the nectar of flowers and they do this by harnessing a prolonged, thin beak and an elongated tongue, which can be double the length of the beak. This extraordinary length allows hummingbirds to access nectar that may be located deep inside the interior sections of flowering flora. The birds drink with these lengthy tongues by quickly sipping up the liquid. The tongues have tubes inside them which run down their length, to act as a straw. These tubes can open down their sides as they go into the sugary liquid and close around the nectar, trapping it so it can be pulled back into the beak. The tongue itself is actually forked and is compressed until it reaches nectar, then it is spread out into two points. This incredibly fast action traps the nectar and the liquid then travels up the grooves.[9]

Several species of parrot use their tongues for gathering nectar as well, including the Rainbow lorikeet (Trichoglossus moluccanus). Rainbow lorikeets from Australia have their uniquely brush-like tongues equipped with a Papillae appendage, of which is covered with hairy projections called Papillae that the birds have adapted for naturally soaking up pollen and nectar off of native flowering vegetation such as Bottle brush, Grevilleas, Pittosporum, Spathodea campanulata, Metroxylon sagu and most famously Eucalyptus trees. These lorikeets are important pollinators of coconuts in Melanesia. They may also use their tongues to lap up the juices of different kinds of fruits like figs, mangoes and papayas that have been previously eaten by other animals such as fruit bats.[10]

Woodpeckers use their ribbon-like, protruding tongues to capture insects that are crawling beneath the tree bark. The European green woodpecker (Picus viridus) has one of the longest at up to 4 inches (10 cm), being as much as a third of the bird’s total length. It’s so long that like hummingbirds, it has to be recoiled behind the skull, over the cranium and eye sockets and finally into the right nostril, in order to comfortably fit inside the woodpecker’s head. It lacks the barbs that allow it to latch onto insects, which are found in the genus Dendrocopos, as well as the Black woodpecker from Eurasia. Instead, it has a sticky surface caused by secretions of the enlarged salivary glands. This results in the bird being capable of sticking its tongue into soft tree bark, its stickiness collecting ant species of the genera Lasius and Formica, the bird’s main food source.[11]

Mammals[edit]

Mammals that feed using their tongues are surprisingly common and species frequently evolve specialized tongues for similar feeding purposes in acts of convergent evolution. A famous example of this is the giraffe (Giraffa) and its closest living relative, the Okapi (Okapia johnstoni). Both species only inhabit the African continent and are browsers with long, prehensile tongues. However, okapis live in the rain forests of Central Africa and giraffes live in the savannas and grasslands of Sub-Saharan Africa, resulting in the okapis filling the ecological role of giraffes in the rain forests, where giraffes are mostly absent. Of the two, the giraffe has the slightly longer tongue, being up to 20 inches (50 cm long), whereas the okapi has one that’s 18 inches (45 cm) long. Both their tongues are blackish purple in color, possibly to prevent them from getting sunburns. Since the giraffe’s tongue is particularly dexterous, they can be used to maneuver around thorns and wrap around leaves of the Acacia tree, of which giraffes often feed from. The giraffe has the most adapted tongue of any browsing animal and the surface is rough, prickly and rugged and covered in thick, glue-like saliva to prevent the thorns from stabbing it.[12]

Another example of convergent evolution with lingual feeding are the pangolins (Pholidota), anteaters (Vermilingua) and Aardvarks (Orycteropus afer). The Giant anteater has the longest tongue not only of the following animals, but also out of all mammal species. It can fully extend as long as 24 inches (60 cm) and is covered in backwards-facing papillae and coated with adhesive, thick saliva that’s secreted by enlarged salivary glands, which is how insects become stuck to the tongue and are then brought to the narrow, toothless mouth. When feeding, a giant anteater can flick its tongue in and out 160 times in a minute (nearly three times per second). Biologist, Karen Reiss states that the tongue has no attachments to the hyoid, which is what allows the anteater to move its tongue at such high speeds. All these adaptations enable the giant anteater to eat an average of 30,000 ants in a single day.[13]

The Sun bear (Helarctos malayanus) holds one the longest tongues of all carnivores and can grow up to 10 inches (25 cm) long. The tongue is also protrusible, meaning it extend far outwards from the bear’s mouth. Sun bears use these remarkably extensive tongues for foraging on a variety of different foods, such as extracting termites and ants from nests, beetles and beetle larvae out of bark, licking honey and bee larvae out of hives and even for picking some types of fruit off tree branches, as this species is also arboreal.[14] Predators like lions, tigers, leopards and other members of the cat family (Felidae) use their tongues to lick and scrape away any remaining bits of meat left on bones, but also use these tongues for grooming and cleaning their fur. Tiny, sharp spines called papillae create an extremely jagged and prickly surface on the tongue. So much so that it only takes a few licks before a section of the victim is left without any skin. These minuscule barbs also help to remove hair or feathers off of prey and tenderize the meat, resulting in it assisting in the animal’s digestion.

The Common vampire bat (Desmodus rotundus) feeds exclusively on blood, mainly from large mammalian fauna. It slits open a small hole on the victim’s skin with its razor-sharp incisor teeth and then laps up the pouring droplets of blood, using lateral grooves on its tongue designed for this feeding habit. These grooves are located on the underside of the tongue, this forms a straw-like structure to let the bat suck up double its weight in blood.[15] Another bat species has an even more amazing tongue, the Tube-lipped nectar bat (Anoura fistulata) from Ecuador. It uses this remarkably long and attenuated tongue to drink nectar from flowers but it may also occasionally feed on pollen and small insects. With the tongue being up to 3.5 inches (8.5 cm) long and having an overall body length of 2.5 inches (6.4 cm), this little bat has the longest tongue in proportion to its size of any living mammal, as much a 150% of its actual body size. Like the pangolins and anteaters, the tube-lipped nectar bat has its tongue detached from its hyoid bones and it stretches past the pharynx, going deep into the thorax. The extension lies between the trachea and the sternum.[16]

David Crespo’s theory on Lingual heat gathering[edit]

Crespo hypothesized that the dark bluish-black color of both the giraffe and its relative, the okapi's tongue is adapted for gathering heat from their hot, humid environment, using that heat to break down toxins of which are often present in their plant based diet, which can make up over 100 different kinds of plants. Both the giraffe and the okapi’s blackish tongue is the result of the density of melanin pigments in their skin, which are the pigments that give some humans darker colored skin, eyes or hair than others. The more melanin pigments packed together in a body region, the darker the color of that area will become.The bluish or purplish-black tinge of the tongues would provide a great conductor of heat,as darker shades of color tend to gather more heat and faster than lighter colors.[17] To avoid any burns on the tongue, the pigments of melanin are able to disperse and dissipate over 99.9% of UV rays.[18]

This would explain why the giraffe and okapi both evolved in the continent of Africa out of all others, the practically omnipresent heat that constantly surrounds them. But the melanin that causes the blackish color can also prevent sunburns from UV rays and may also aid in blocking the processes of the body that can lead to the cancer of melanocytes known as Melanoma, for example the skin of dark-colored humans, which is caused by the increased production of melanin per cell, is able to withstand much greater heat than the skin of whiter humans, without getting many negative effects on their skin. In fact, people of European ancestry are up to 10 times more likely to develop melanoma cancer if exposed to intense UV radiation than African-Americans.[19] Therefore, the darker an organism’s skin is, the more heat it can bare, as multiple big game animals in Africa are a plain grey to dark brown color like elephants and rhinoceroses. The dark tint added with the tough, scaly papillae that covers the dorsal side of the tongues of both animals that creates a rough, leathery surface protects their tongues from sunburn, as well as their thick, sticky saliva which is kept relatively moist on the tongue because the overlapping papillae protect it from evaporating or burning under the intense African sun. The papillae however, are only found on the top side of the tongue, giving it a more bluish grey color and also making the dorsal side surface a lot more rugged and prickly, similar to a ca's tongue. This is to do with the top side of the tongue obviously being more exposed to the sun more frequently than the underside, where it's sheltered from the heat. Both the giraffe and okapi receive most of the moisture they need from the food that they consume. All this allows both the okapi and the giraffe to absorb the sun’s heat without getting sunburns on the tongue or without even developing any signs of skin cancers like melanoma by persistent UV light.

The giraffe lives in mostly scorching, dry grasslands, savannas and even sometimes deserts, where heat is ever present. In Africa, the savannas alone can reach average temperatures of around 68-86 degrees Fahrenheit (20-30 degrees Celsius). However, the okapi lives in the much more wet and humid climate of the rain forests of central Africa, mostly in the North-Eastern parts of the Democratic republic of the Congo, in the Ituri forests. The thick, dense vegetation of the canopy stops the sun’s light and heat rays from reaching the forest floor, where the okapi inhabits (being the only native mammal species that exclusively lives on the forest floor) and the majority of the air will be damp and humid, with a considerably lower temperature of about 77 degrees Fahrenheit (25 degrees Celsius) on average. This could explain why okapis prefer to feed in tree-fall gaps and other forest clearings, where the falling of one or several trees leaves a substantial gap in the canopy, allowing the heat and light of the sun to penetrate to the rain forest ground. This heat from the sun will dry up most of the forest floor and will leave the air much hotter and dryer than the rest of the surrounding environment, reaching a closer temperature that's akin to that of the savannas. Larger tree-fall gaps hold more light and heat than smaller gaps and some can be well over 10 meters (30 feet) in diameter. The okapi would then find one of these tree-fall gaps by following the pathways of past generations of okapis.[20] Once it’s within the sun’s range, it will begin to feed on the leaves of different plants, many of which are poisonous to most other animals and humans. Another good reason for okapis to visit tree-fall gaps is because the plants that they feed on will have more sunlight to absorb in those gaps through photosynthesis than plants shaded by the canopy. The plant will get much more nutrients from this sunlight and so having more energy to grow produce more leaves for the okapi, who is the only fully leaf-eating ungulate out of the 13 or so species native to the Ituri forest.[21] They are also the only known species of mammal that feed solely on the vegetation of the under story.

The okapi could also use its blackish-blue colored tongue to absorb the sun’s heat without getting burnt. The two related animals might already have some sort of immunity to the toxins of the vegetation, but once the tongue and mouth in general gets hot enough for a particular toxin, all this heat might be able to help the breaking down or dissolving process of various toxic substances which are found in multiple plants of the okapi’s diet, as well as in the giraffe's. Many of these poisons are alkaloid like tannins[22], terpenes, glycosides and even extreme poisons like Strychnine, Curare[23] and Ricin, found in Ricinus communis.[24] Giraffes feed mainly on acacia trees that hold high quantities of tannins as well. The condensed tannins in the plants that giraffes browse on increase in the foliage as a chemical defense against browsers like the giraffe and limit the accessibility and nutrition of the vegetation, so giraffes avoid parts of the tree that may hold higher levels of tannins.[25] The heat will collected by the tongue, as both species are often spotted with their tongues hanging out of their mouths for most of the time, since they spend the majority of their day feeding in heavily sunlit areas. The heat could spread throughout their mouths via saliva and as soon as it has been exposed to sufficient Ultra Violet rays from the sun, the increased temperature inside their mouths could also kill off any unwanted or potentially harmful bacteria, aiding in digestion. This acts as a first step in the dissolving of the toxins, before the plant matter carrying them gets transported to their four chambered stomach, where it will mashed up into cud. Both the okapi and giraffe are ruminants and as such, after hey have chewed and swallowed the plants they have previously consumed, they will regurgitate the cud back into their mouths to further neutralize the toxins. Like most browsing herbivores, saliva found in the giraffe and okapi can also contain proteins to counteract some of the alkaloids, mainly tannins. These proteins secreted along with the saliva could maybe bind the themselves with the tannins in a chemical bond to counterbalance them.[26] Most alkaloids can be degraded or decomposed with enough heat, except for strychnine and caffeine, which are sublimable and can be be turned from their typical solid form, into a gas without becoming a liquid first. But the okapi will only eat specific plant species only so often, so that no one type of toxin will dominate its diet and build up inside the animal's gut, resulting in poisoning the animal. Plants that may contain traces of strychnine or curare, of which belong to the genus Strychnos, will be eaten less frequently than plants who carry milder toxins. Having a pan tropical distribution, Strychnos plants can be regularly found in the Congo rain forests, but would most likely be avoided by okapis, especially the young. The two species have also been seen occasionally licking animal bones or eating clay or charcoal from burnt trees, presumably for supplementing their herbivorous diet.

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