Lecture 18. Herbivory: pollination vector s use plants for food Bees, bats, birds, beetles as cross-pollinators Homology, analogy, homoplasy, synapomorphy High-crown teeth Ruminants.
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Herbivory: pollination vector s use plants for food
Bees, bats, birds, beetles as cross-pollinators
Homology, analogy, homoplasy, synapomorphy
Vector: there are many ‘vectors’ including force vectors, but in this case ‘ the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization’ (Wikki).
[Posterior] inner face of honeybee hind leg showing basitarsalbrush, auricle & rake of pollen press. (Rake teeth are visible beyond the auricle on the leg’s outer [anterior] surface).
Bee with pollen baskets loaded.
Pollen is groomed from branched setae (hairs) of head, thorax and abdomen with other setae (differently shaped), formed as a brush on the enlarged basal tarsal segment. Cross-body use of an opposing leg rakes the pollen from the brush of each hind leg, drawing it up into a specialized segment joint called a pollen press: flexing compacts and squeezes pollen up into the pollen baskets (corbiculae) on the hind leg’s external surface.
Some bat (Chiroptera) species are pollen vectors: they specialize in feeding on nectar and pollen. Compared to insectivorous bats they have reduced dentition and unusually long tongues, often up to 1/3 the length of their body; they hover in flight and lap nectar from flowers.
M. Brock Fenton. Just Bats. Univ. of Toronto Press
“Three scanning electron micrographs of the hairs of bats: Geoffroy’s Tailless Bat (GTB) a), Woermann’s Long-tongued Fruit Bat (WLF) b) and a Mexican Bulldog Bat (c). “Note that the scales on the shafts of the hairs of the GTB and WLF bats protrude more than those of the insect-eating bat. Protruding scales in flower-visiting species have been proposed as pollen traps, permitting the bats to carry pollen more efficiently. The similarity of the hairs of the two flower bats is striking, given that they are from different families (a) New World Leaf-nosed bats (b) Old world fruit bats. The hairs are about a tenth of a millimetre in diameter.” [Brock Fenton]
Convergent evolution - describes similar forms that evolve in different lineages through similar selection pressures. A common ancestor of bats and birds was the first terrestrial quadruped, but then each lineage independently evolved powered flight via adaptations to their homologous forelimbs. They "converged" on a useful trait, forelimb flight: the cladistic term for this is homoplasy: it means ‘similarity of form’.
light shows the branched hairs of bees
bat hais SEM
In like manner flowers pollinated by bats may use something appropriate to the bat’s acoustic specializations: to its echolocation by ultrasonic sounds.
Simon R., Holderied M.W., Koch C.U., von Helversen O. 2011. Floral acoustics: conspicuous echoes of a dish-shaped leaf attract bat pollinators. Science 333: 631-633.
A leaf specialized to attract bat pollinators .
A bat-pollinated rainforest vine Marcgravia, has evolved a dish-shaped leaf (sometimes 2) with the upper concave side facing oncoming bat pollinators. This ‘dish’ reflects back the echoes made by the bat as a ‘echo signature’, localizing the flower within the sensory surround of the bat.
Flower shape may be adapted to only allow access by hummingbirds, e.g., long-deep corolla requiring a long thin beak to access nectar. Selection discriminates between the effective pollinator and other species that are less so.
Unlike bees, hummingbirds don’t collect pollen; they go for the nectar with specially modified tongues and beaks; but they do pick up pollen and transfer it. From the plant’s point of view its a good thing the bird doesn’t groom as much as bees.
Anderson B., Cole W.W., Barrett S.C.H. 2005. Specialized bird perch aids cross-pollination. Nature 435: 41-42.
In South Africa a plant, Babiana, is called ‘rat’s tail’ because it grows a “curious sterile infloresence axis”. The function of this plant part is to provide a perch for visiting , and pollinating, sunbirds. The paper by Anderson et al. describes experiments demonstrating perch removal reduces the plant’s fertility.
Many animals (beetles, ants, hawkmoths) feed on the pollen and nectar rewards of plants. It is a widespread and critical relationship.
Contrasts in TEETH between carnivory, herbivory
Caninesand incisors, in a Schnauzer dog (‘Ogden Gnash’) [the stick
is being held behind by his carnassial teeth (see below) and not by the visible canines]. The teeth of herbivores (right) are very different: broad and blocky, designed to grind the cell walls of plants rather than to penetrate.
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
Carnassial teeth in a sabretooth
From web: Steven M. Carr
Snake teeth are all of one functional type: recurved conical – one-way teeth for swallowing prey such as frogs; there are no molars and no bone-shearing carnassials.
In primitive land vertebrates (amphibians) the quadrate bone was part of the skull. It was immovably joined to the skull posteriorly, forming part of the fulcrum of the lower jaw/mandible.
Seymouria: Permian amphibian
Both a snake and a rorqual whale need a large gape but for different reasons.
Michel Laurin Tree of Life
The jaws are flexible in the anterior
midline and can ‘walk’ forward
on the prey, by alternating right and
left, loosening and setting the recurved teeth.
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.
Hypsodontteeth: high-crowned, protrude well above the gum, creating length for wear during a lifetime of grazing (grasses with silica being an adaptation of the plant to protect itself from such 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.
Greece Museum Paleontology Geology
Diastema: tooth series separation;
function in the horse?
(modified from Wikki) Selenodont teeth are the kind 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.
Cows: Holstein cow (below left), Angus cow (right) have been artificially selected to give milk or to be meat. Artifical selection has been affecting 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
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.
Treeless grasslands spread across much of the world by the end of the Miocene. Evolving ruminant mammals exploited prairie grasses as a source of food.
Cellulose is a polysaccharide, a linear chain of several hundred to over ten thousand glucose units. It is an important structural component of the primary cell wall of green plants.
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.
You already know many common names.
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.
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.
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.
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.
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.
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.
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 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.
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.
Another example of 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.