Editing Digestion (section)

===Specialised organs and behaviours===
To aid in the digestion of their food, animals evolved organs such as beaks, [[tongue]]s, [[radula]]e, teeth, crops, gizzards, and others.

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| caption1  = A [[Bird Hybrids|Catalina Macaw]]'s seed-shearing beak
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| caption2  = Squid beak with ruler for size comparison
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====Beaks====
[[Bird]]s have bony [[beak]]s that are specialised according to the bird's [[ecological niche]]. For example, [[macaw]]s primarily eat seeds, nuts, and fruit, using their beaks to open even the toughest seed. First they scratch a thin line with the sharp point of the beak, then they shear the seed open with the sides of the beak.

The mouth of the [[squid]] is equipped with a sharp horny beak mainly made of cross-linked [[proteins]]. It is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species.<ref name=Miserez>{{cite journal |last=Miserez|first=A|author2=Li, Y |author3=Waite, H |author4= Zok, F  |year=2007|title=Jumbo squid beaks: Inspiration for design of robust organic composites |journal=[[Acta Biomaterialia]] |volume=3 |issue= 1|pages=139–149 |doi=10.1016/j.actbio.2006.09.004 |pmid=17113369 }}</ref> The beak is the only indigestible part of the squid.

====Tongue====
{{Main|Tongue}}
The '''tongue''' is skeletal muscle on the floor of the [[mouth]] of most vertebrates, that manipulates [[food]] for chewing ([[mastication]]) and [[swallowing]] (deglutition). It is sensitive and kept moist by [[saliva]]. The underside of the tongue is covered with a smooth [[mucous membrane]]. The tongue also has a touch sense for locating and positioning food particles that require further chewing. The tongue is used to roll food particles into a [[Bolus (digestion)|bolus]] before being transported down the [[esophagus]] through [[peristalsis]].

The [[sublingual]] region underneath the front of the tongue is a location where the [[oral mucosa]] is very thin, and underlain by a plexus of veins. This is an ideal location for introducing certain medications to the body. The sublingual route takes advantage of the highly [[Blood vessel|vascular]] quality of the oral cavity, and allows for the speedy application of medication into the cardiovascular system, bypassing the gastrointestinal tract.

====Teeth====
{{Main|Teeth}}
Teeth (singular tooth) are small whitish structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, milk and chew food. Teeth are not made of bone, but rather of tissues of varying density and hardness, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if we were to bite into something too hard for our teeth, such as a chipped plate mixed in food, our teeth send a message to our brain and we realise that it cannot be chewed, so we stop trying.

The shapes, sizes and numbers of types of animals' teeth are related to their diets. For example, herbivores have a number of molars which are used to grind plant matter, which is difficult to digest. [[Carnivore]]s have [[canine tooth|canine teeth]] which are used to kill and tear meat.

====Crop====
A [[Crop (anatomy)|crop]], or croup, is a thin-walled expanded portion of the [[alimentary tract]] used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the [[gullet]] or throat. In adult doves and pigeons, the crop can produce [[crop milk]] to feed newly hatched birds.<ref name=Gordon>{{cite web | title=The Alimentary Canal in Birds | url=http://www.earthlife.net/birds/digestion.html | author=Gordon John Larkman Ramel | date=2008-09-29 | access-date=2008-12-16}}</ref>

Certain insects may have a crop or enlarged [[esophagus]].

[[Image:Abomasum-en.svg|150px|thumb|Rough illustration of a ruminant digestive system]]

====Abomasum====
{{Main|Digestive system of ruminants}}

[[Herbivore]]s have evolved [[cecum]]s (or an [[abomasum]] in the case of [[ruminant]]s). Ruminants have a fore-stomach with four chambers. These are the [[rumen]], [[Reticulum (anatomy)|reticulum]], [[omasum]], and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material.  Solids clump together to form the cud (or [[Bolus (digestion)|bolus]]). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.

Fibre, especially [[cellulose]] and [[hemi-cellulose]], is primarily broken down into the [[volatile fatty acids]], [[acetic acid]], [[propionic acid]] and [[butyric acid]] in these chambers (the reticulo-rumen) by microbes: ([[bacteria]], [[protozoa]], and fungi). In the omasum, water and many of the inorganic mineral elements are absorbed into the blood stream.

The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach (e.g., those in humans or pigs), and digesta is processed here in much the same way. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine.  Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine.

====Specialised behaviours====
[[File:Flesh fly concentrating food.jpg|thumb|A flesh fly "blowing a bubble", possibly to concentrate its food by evaporating water]]
[[Regurgitation (digestion)|Regurgitation]] has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop of [[Columbidae|pigeons and doves]] with which the parents feed their young by regurgitation.<ref name="Levi">{{cite book |last=Levi |first=Wendell |title= The Pigeon|year= 1977|publisher= Levi Publishing Co, Inc|location= Sumter, SC|isbn=978-0-85390-013-9 }}</ref>

Many [[Physical characteristics of sharks|sharks]] have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents (perhaps developed as a way to reduce exposure to toxins).

Other animals, such as [[rabbits]] and [[rodents]], produce [[Cecotrope|cecotropes]] to re-digest food, especially in the case of roughage. Capybaras, rabbits, hamsters, and other related species do not have a complex digestive system as ruminants. They instead extract more [[nutrition]] by giving their food a second pass through the [[gut (anatomy)|gut]]. Soft cecotropes of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten.

Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation. When they are born, their intestines do not contain these bacteria (they are completely sterile). Without them, they would be unable to get any nutritional value from many plant components.