Lecture 18
Teeth
Ruminants
Lysozymes and molecular homoplasy
Hind-gut fermenters
Hard ticks smell cow burps
Gary Larson cowphiliac
Ennos page 214
• Defenses of leaves
• There are more than 5000 spp. of grass covering >30% dry land:
successful.
• Basal meristem: leaves of grass grow from their base – even if the
top of the stem is eaten they can survive.
• Reinforcement of the leaves comes from fibres (woody
sclerenchyma fibres) running longitudinally along the leaf. Lateral
cracks are stopped when they reach a new fibre “just as in
composite material”. Herbivores have trouble ‘snipping’.
• Some grasses use SILICA (glass) as a defense. “incorporation of
silica bodies, effectively small lumps of glass, into the epidermis of
the leaf”.
• Horses and grasses arms race: hypsodont teeth vs silica abrasion.
Selenodont teeth are typical of even-toed ungulates such as deer
(Cervidae) and cattle (Bovidae); the cusp’s enamel cutting surface is
increased by elongation into a crescent shape.
• [Selene (moon goddess): major cusp has a half-moon shape.]
• High-crowned teeth, hypsodont of horses, simply means lots of tooth
protruding above the gumline; low crowned teeth have less crown:
selenodont teeth are low crowned.
• Brachydont teeth are the opposite of hypsodont: human teeth are
brachydont, i.e., low-crowned.
• Selenodont teeth are adapted for grinding and the occlusal surface
is not covered in enamel (as e.g., in humans); rather, the layers of
enamel, dentine, and cement are all exposed, with alternating layers
of enamel and dentine wearing unevenly to create an enamel cutting
edge.
• [for further detail: Univ. of Michigan, The Animal Diversity Web]
Function of cheek teeth in ruminants
•
Cheek teeth are premolars and
molars used to masticate (ten
$ word for ‘chew’) and triturate
(grind or chew thoroughly).
Chewing reduces food to a
pulp by compression and shear
forces. The lower jaw describes
a small circle, pushing
mandible teeth up against the
maxilla and then shearing them
laterally The tooth crowns wear
unevenly because of the
greater hardness of 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 per day!
(R. Bowen)
yakkhapadma
Grasslands National
Park Saskatchewan
Treeless
grasslands
spread across
much of the
world by the
end of the
Miocene.
Evolving
ruminant
ungulate
mammals
exploited prairie
grasses as a
source of food.
Some Taxonomy
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.
Suborder Ruminantia, a suborder of Artiodactyla, comprises the deer family
(Cervidae) and cattle, sheep and antelopes (Bovidae); these are the most
recently evolved ungulate group; they all have a fully developed 4-chamber
stomachs. They have lost the upper incisors and often the canines and have
selenodont molars.
Cellulose: a problem to chew, a
problem to digest.
Evolving hoofed mammals
needed other organisms to
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
teeth quickly.
Diverticulae and valves
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 control speed of
travel of the ‘disassembly line’. The
muscular walls engage in peristalsis
and churning to mix the digestant.
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 ruminoreticuluar fold.
Rumen and reticulum are eccentric, located
toward 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.
Ruminoreticular 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 is formed there
into a bolus, this facilitated by the reticulum’s rough walls. The cow
regurgitates this bolus into its mouth and chews, i.e. RUMINATES.
High-risk Ingestion is separated in time from chewing, permitting chewing when risk is low.
ideasinfood.com
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.
rumenHealth.com
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 reticuloomasal opening. If the lips
remain open the material
goes into the rumen.
shunt
College of Veterinary
Medicine Univ. of Minnesota
Fistula: opening in the
animal’s side giving access to
rumen contents for study
Cooperative extension system
Food is ingested (while exposed to prairie predators) and swallowed
quickly into the rumen. The rumen houses bacteria, ciliate protozoa, fungi – microorganisms with capacity to make cellulases. Rumen content is maintained at a
steady pH and constant temperature – ideal conditions for anerobic fermentation.
Saliva adds 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.
Some nutrient absorption does occur in the rumen via the epithelium of the
papillae in its walls.
In the rumen micro-organisms digest the carbohydrates of 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; absorption of acids of course shifts the pH up.
The rumen microbes also synthesize protein and rumen-living bacteria can
use ammonia as a source of nitrogen to make the protein (this is the basis of being
able to add urea as a supplement to cattle feed). To gain this protein which is part of
the bacteria, the cow must pass the microbes on into the abomasum and digest
them. It then treats the protein in a more ‘normal’ nonruminant fashion: absorption
within the small intestine.
Adaptations ‘explain themselves’ when studied in the context of animal behaviour
and ecology and they occur within ‘adaptive systems’. Animal herbivores that
evolved fermentation chambers to exploit micro-organism enzyme systems also
have selenodont grinding teeth to mechanically break down cellulose cell walls.
And they have cursorial (running) adaptations: lengthened legs, extrinsic muscles,
hooves adapted for fast running. Limb structures have been selected for good
distance advantage at the expense of poor force advantage so hooves can move
relatively fast. They have good binocular vision and high sensitivity to objects
moving at a great distance in their field of view. There is an integrated package of
different adaptations – a system -- explained by a way of life.
Significance of ruminant
adaptation
Wallaby: cud-chewing ruminant and a marsupial that evolved its
rumination separately from the artiodactyls: like cervids and bovids
though, green plant tissue, eaten in haste, is regurgitated in leisure
and masticated (chewed) further.
National Geographic
Similar stomach chamber
features, including an
oesophageal
groove/shunt evolved in
wallabies independently
of the evolutionary events
that led to the rumen of a
cow.
Kangaroos & wallabies (Setonix):
An example of homoplasy
[convergence]: independent
adaptation resulting from
similar selection processes
(think also bat hairs and bee
hairs).
Stomach diagram 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 skip chamber 1.
Exploiting the digestive capacities of micro-organisms for
herbivory
•
•
•
•
•
The trouble with herbivory is cellulose.
Mammals lack the necessary enzymes to deal with the cellulose of plant cell
walls.
Ungulates evolved a chambered stomach housing ciliate protozoa, yeasts
and other micro-organisms which are able to digest cellulose.
This ruminant structural adaptation is a fermentation vat diverticulum of the
gut, housing unicellular creatures optimally [giving them an ideal habitat];
the mammal exploits the digestive capacities of the micro-organisms.
When these have made more of themselves the ungulate passes them
further along its gut, kills and digests the micro-organisms to nutritive
benefit.
Langur
monkeys
offer another
good
example of
homoplasy.
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
lacking cellulases that
benefitted from
digesting cellulose.
Stewart C-B., Schilling J.W., Wilson A.C. 1987. Adaptive evolution in the
stomach lysozymes of foregut fermenters. Nature 330: 401-404.
An example of adaptive evolution at the protein (sequencing) level.
Lysozymes are special enzymes evolved by animals to deal with bacteria that
invade the body. In the gut of mammals there are typical lysozymes that keep
the bacterial fauna in the gut space under control: the old function of the
lysozyme: lysozymes throughout the body fight invading bacteria (present in tears
for example).
In two separate lineages, a new function arose independently as a gut lysozyme
was recruited to deal with digesting the bacteria progressing onward into the
stomach. The ruminant ungulate lineage and the line leading to colebus
monkeys: lysozyme was recruited to a new function that of digesting the bacteria
that enter the stomach from the fermentation region – these lysozymes are
specialized to deal with the higher pH of the stomach.
Amino acid sequence of langur stomach lysozyme compared to that of cow: 130
amino acids: the resulting unusual similarity attests to homoplasy.
Ixodes ricinus European Sheep tick: vector of Lyme disease,
tick-bourne encephalitis, no eyes, 3 mammalian hosts in life
cycle
Wikki
Donze G.
Rumen metabolites serve ticks to exploit large mammals. J. exp. Biol. 207: 4283Hard ticks react to volatile rumen end-products – short-chain carboxylic acids (acetic,
proprionic, butanoric etc.) in ‘cow burps’: eructions.
The hoatzin, living in mangrove swamps of the Amazon drainage,
is the only known foregut fermenting bird.
Hindgut
fermentation
*horses
rabbits
rodents like
beavers and
porcupines
Like cattle, hInd-gut herbivore fermenters* have a chamber of micro-organisms
protozoa, bacteria, exploited to break down vegetable matter; this chamber 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 re-eaten: caecotrophy is when food is passed twice through the
gut.
Vispo C., Hume I.D. 1995. The digestive tract
and digestive function in the North American
porcupine and beaver. Can. J. of Zoology 73:
967-974.
In the beaver the caecum and proximal colon
function together as a fermentation chamber. In
the porcupine fermentation is confined to the
caecum.