March 7,12

Feeding mechanisms continued

Teeth and jaws

Rumination

March 7, 2013

Teeth

Mollusc radula

Fluid feeders

Cibarial pump

Crops

Teeth: canines and incisors in a Schnauzer (‘Ogden Gnash’): the stickis being held by his carnassial teeth (see below) not the canines.

Silliness in previous lecture about felineshaving canine teeth and why cats should resent this.

Adaptive packages: vertebrate teeth evolve together with jaws (lower chordates (e.g., Branchiostoma) have no teeth or jaws). Jaws and teeth function together arising as adaptations for capture and restraint of prey.Teeth are conical to start with (the most primitive shaped tooth in the dog’s mouth is its canine which plays a role in bringing down prey). Conical teeth (fishes and reptiles), function in restraint through penetration: used with the jaw to gain a hold upon prey and arrest it. As later specializations, the conical tooth may bend into a one-way restraining recurved tooth (snakes) or broaden in one plane with a tapered cutting edge on one side (sharks) creating a tooth that cuts as it inserts.

Snake teeth are all of one functional type: recurved conical – one-way teeth; there are no molars and no bone-shearing carnassials.

Carnassial teeth: carnivores

• Premolar teeth specialized for breaking bones: during biting the inner surface of the fourth upper premolar (blue) passes outside the outer surface of the first lower molar (red). This upper and lower-jaw tooth combination is characteristic of both feliform (cats) and caniform (dogs) carnivores [it is a synapomorphy* for these groups].

Carnassial teeth in a sabretooth cat (Smilodon)

From web: Steven M. Carr

The forces generated by upper and lower-jaw carnassials create shear strain force in prey bones. Upper and lower teeth don’t line up, so instead of compression the bone is stressed as a shear force. The cusps are shaped to cradle and stop any roll of the cylindrical bone. ‘Shears’ is a good mechanical name for scissors

*

Teeth specialized with a crown for grinding.

Major tooth materials: 1) enamel, shinny hard thin layer over the tooth surface: almost completely inorganic. “Two-thirds of its substance consists, as seen in mammals, of long prisms of calcium phosphate, arranged with their axes at right angles to the surface...” (Romer); 2) dentine, softer, wears more easily, forms main bulk of tooth 3) cement. These materials, differing in wear rates, keep tooth from becoming smooth through wear.

Hypsodont teeth: “height is attained by a skyscraper-like growth of each cusp or ridge on the tooth; ...as wear takes place, it grinds down through a resistant complex of layers of all the tooth materials: enamel, dentene, cement.

Grazing hoofed mammals need to grind down cellulose of plant cell walls

Hypsodont teeth: high-crowned and protrude well above the gum, providing length for wear during grazing (grasses with silica being an adaptation of the plant to protect itself from grazers. Ruminants and horses have hypsodont dentition. The opposite condition is called brachydont. Hypsodont teeth may also be selenodont (see next slide). A cow’s teeth continue to grow through life (?).

prehistoric gazelle

Greece Museum Paleontology Geology

Diastema: tooth series separation;function in the horse?

What is a selenodont molar? modified from Wikkipedia

• Selenodont teeth are the type of molars and premolars commonly found in ruminant herbivores. They are characterized by low crowns, and crescent-shaped cusps when viewed from above (They differ from human molars in that the occlusal surface is not covered in enamel; rather, the layers of enamel, dentine, and cement are all exposed, with cement in the middle, surrounded by a layer of enamel, then a layer of dentine, all wrapped in a second outer layer of enamel. Viewed from the side, selenodont teeth form a series of triangular cusps. The combination of triangular profiles with ridges formed by the exposed layers makes the sideways jaw-motion of ruminants (think of a cow chewing) an effective way to break-up tough vegetable matter

Web sources for more information on ruminant herbivores

• http://www.vivo.colostate.edu/hbooks/pathphys/digestion/herbivores/lysozymes.html

• http://www.vivo.colostate.edu/hbooks/pathphys/digestion/herbivores/index.html

• http://www.vivo.colostate.edu/hbooks/pathphys/digestion/herbivores/rumen_anat.html

Information posted by Prof. Richard Bowen, Colorado State Univ. College of Veternary Medicine

Cows: Holstein cow (below left), Angus cow (right) have been artificially selected to produce milk or to produce steaks. Artifical selection has been affected a

historical genome created by natural selection; they are less well adapted to dealing with predators.

These animals are descendants of animals with paws (?) and claws that evolved hooves in association with a grazing lifestyle.

Gazelle, only naturally selected

Will Cravens

Which is the more effective ‘cursorial adaptation’ (design for running) hoof or paw? Hypothesis: hooves could be more effective than paws because selected mainly for locomotion, while paws have to be selected not just for locomotion but for bringing down prey.

Adothwolf

Treeless grasslands spread across much of the world by the end of the Miocene. Prairie grasses were exploited as a source of food by evolving ruminant mammals. Cellulose cell walls of green plants present a challenge: a herbivore must use grinding teeth to break the cellulose (glucose chains). And they needed other organisms to actually access the sugars. Most multicellular animals cannot synthesize cellulases: only micro-organisms can make the necessary enzymes. And the grasses evolved other protections as well, building silica into their wall tissue, a hard crystalline mineral that abrades mamalian teeth quickly.

Grasslands National Park Saskatchewan

Order Perissodactyla are odd-toed ungulates: horses, zebras, tapirs, rhinos.

Order Artiodactyla are even-toed ungulates. Artiodactyls include about 180 species: pigs, camels, hippos, deer, giraffes, sheep, cattle, antelopes, goats, bison, musk ox, caribou.Ruminantia, a suborder of Artiodactyla, comprises the deer family (Cervidae) and cattle, sheep and antelopes (Bovidae); these are the most recent ungulate group, characterized by full development of a rumen-based feeding system (4-chamber stomach). They have lost the upper incisors and often the canines and have selenodont molars.

Function of high-crowned and selenodont teeth in ruminants

• Cheek teeth are premolars and molars that usually function in food processing rather than prey capture. They are used to chew: masticate and triturate: they reduce food to a pulp by compression and shear. Watch a cow’s lower jaw describe a small circle, pushing mandibular teeth up against the maxillar and then shearing them laterally relative to each other. The tooth crowns wear unevenly because of the different hardness of dentene, cement and enamel.

• Mastication is not the same thing as maceration: maceration means to soften and separate food parts by the addition of a solvent, in this case saliva. Ruminants produce large quantities of saliva: 100-150 litres of saliva per day! (R. Bowen)

yakkhapadma

Digestive tract of the cow viewed from the right side; the cardia is a weak valve where the oesophagus enters the rumenoreticulum; the whole of the rumenoreticulum is a diverticulum of the gut – a huge fermentation vat for microbes. The omasum connects to the abomasum or true stomach; the pylorus (valve) separates this from the small intestine.

Within the rumen ingested plant material separates into three zones: gas produced by microbes rises to occupy the upper regions (methane and carbon dioxide); yesterday’s hay sinks to the bottom, and newly arrived roughage floats in a middle layer.;

Guts are muscle-invested tubes organized into a linear sequence of chambers, separated by valves, with sidebranching blind-ending diverticulae. Valves (like the pyloric), control speed of travel of the contents. The muscular walls engage in peristalsis and churning to move the digestant and to mix it. The rumen and reticulum are an example of a (very large) blindly ending sidebranch of the cow gut; oesophagus empties into the rumen which is semiseparated from the reticulum by a low fold, the rumino-reticuluar fold.

Grass is ingested by ruminants without chewing, going down the oesophagus past the cardia (valvular entrance into the stomach) and into the rumen. Later this unchewed material moves forward into the reticulum and there is formed into a bolus by the reticulum’s rough walls. The cow regurgitates this bolus up into its mouth and chews: triturates/masticates.

Rumen and reticulum are eccentric, occupying the cow’s left side.

The oesophageal groove/reticular groove runs in the wall of the reticulum and ends at the reticulo-omasal opening leading into the omasum. It is a special kind of valve functioning as a shunt.

Tripe: lining of the reticulum, criss-crossing ridges and pits, functions in forming the grass/hay boluses that return to the mouth for further chewing; see the oesophageal groove.

ideasinfood.com

rumenHealth.com

College of Veterinary Medicine Univ. of Minnesota

The oesophageal groove functions as a shunt. Reswallowed food material can reach the omasum via this groove. The groove is formed by two heavy, muscular folds in the reticulum wall; these can close to create a passage and this passage redirects materials to the reticulo-omasal opening. If the lips remain open the material goes into the rumen.

Fistula: opening in the animal’s side giving access to rumen contents for study

Cooperative extension system

Food is ingested rapidly out on prairie (while relatively exposed to predators) and swallowed directly into the rumen. The rumen houses bacteria, ciliate protozoa, fungi – micro-organisms with cellulases. The rumen ingesta is maintained at a steady pH and constant temperature – ideal conditions for anerobic fermentation. Saliva provides water and mineral ions, e.g, bicarbonate, to this fermentation vat, the latter helping to buffer the contents to maintain the best pH for the microbes. Nutrient absorption does occur in the rumen via the papillae in its walls.

The rumen provides a site where micro-organisms can digest carbohydrates, protein and fibre (plant cell walls). Carbohydrates both structural (cellulose cell walls) and sugars and starches, when they undergo microbial fermentation produce volatile fatty acids (VFA): e.g., acetic, propionic, butyric etc. The vast majority of VFAs are passively absorbed through the rumen wall. (So while the saliva buffers the pH up digestion is producing acids moving the pH down; and absorption of acids of course shifts the pH up.)The rumen microbes also synthesize protein and ruminal bacteria can use ammonia nitrogen as a source of nitrogen (this is the basis of being able to add urea as a supplement to cattle feed). To gain this protein the cow must pass the microbes on into the abomasum and effectively digest them. It then digests and absorbs the protein in a more ‘normal’ nonruminant fashion within the small intestine. The process of rumination is specifically ingesting rapidly and completing chewing at a later time. The regurgitated material is called a bolus or cud.

National Geographic

Wallaby: cud-chewing ruminant and a marsupial that evolved its rumination separately from the artiodactyls: like them though, green plant tissue, eaten in haste, is regurgitated in leisure and masticated (chewed) further.

Similar stomach chamber features, including an oesophageal groove evolved in wallabies independently of the evolutionary events that led to the rumen of a cow.

Homoplasy [convergence]independent adaptation resulting from similar selection processes

See the section (above left re the stomach diagram) which illustrates the basis of the oesophageal groove of a wallaby. When the muscular folds are drawn together a separate tube is created which permits chewed material to be reswallowed and to pass through chamber I and directly into chamber II.

Some animals have a chamber of micro-organisms for breaking down vegetable matter that occurs later in the digestive tract: horses and rabbits use a modified caecum, part of the large intestine. Since this occurs after the absorptive region of the small intestine the material must be eaten: caecotrophy is when food is passed twice through the gut.

Langur monkey

Langurs are monkeys that feed on the leaves of trees. They have an anterior gut chamber that houses bacteria that can break down cellulose. They cannot be called ‘ruminants’ as they don’t chew a cud. But they are obligate herbivores. Lysozymes are proteins evolved by mammals to defend against bacteria as pathogens. For example lysozymes evolved to attack bacteria infecting the eye. In the gut of mammals there was always a need, even in those that did not try to exploit cellulose as food, to keep their resident bacterial fauna under control. The primitive gut lysozymes of mammals, those that control the bacteria in their intestine are not however, found in their stomach. The stomach is too low in pH for these lysozymes and contains the enzyme pepsin which would hydrolyse normal (protein) lysozymes.In the course of their evolution ruminant animals evolved special lysozymes that are resistant to the stomach’s (abomasum) acidic conditions and resistant to the presence of protein-destroying pepsin.Langur monkeys show convergence in the sequence of amino acids making up the lysozymes of their stomach with those of animals such as a cow or a moose.

LYSOZYMES IN THE STOMACH OF TWO DISTANTLY RELATED TAXA SHOW AMINO-ACID SEQUENCE CONVERGENCE.

Reading: Stewart C-B. et al. 1987. Nature 330: 401-

Significance of ruminant adaptation

Adaptations make sense only in the context of the lives of the animals that have them. This is how one understands adaptations. And many structures on an animal are affiliated in adaptation toward the same ends (e.g., teeth and jaws and limblessness in snakes). The ungulate animals that evolved their stomach into chambers that house ciliate protozoa, yeasts and other micro-organisms, that use the enzyme systems of these unicellular creatures to digest the cell walls of grasses, also have high-crowned grinding teeth to successfully chew cellulose cell walls. And they have cursorial (running) adapatations: long legs, muscles and bones and hooves, adapted for fast running: limb structures selected for distance advantage imparted to hooves. They have good depth vision and high sensitivity to objects moving at a great distance in their field of view. Knowing that such animals evolved in a prairie/grassland habitat to exploit grass as food, allows us to understand how adaptations for cursorial limbs could affiliate with adaptations such as their oesophageal grooves. Ungulates adapted to exploit grass as food and to run from pursuing predators in open spaces where there is nowhere to hide evolved both rumens and long legs. Grooves and hooves were selected in the same historical context of feeding upon grasses. Something upon which to ruminate.