Lecture 6 Feeding & Digestion in animals

Lecture Context and Scope

  • Current lecture: Feeding & Digestion across animal kingdom.
  • Next lecture (preview): Human evolution to understand feeding/argument development.

Human Diet Overview

  • Humans = flexible, “optimistic” omnivores.
    • Eat nearly anything, incl. culturally extreme items.
    • Examples:
    • Tapeworms considered delicacy in parts of New Guinea.
    • Raw oysters (perceived as “swallowing elephants” by some outsiders).
    • Civet-coffee beans (beans excreted by civet; very costly but sought after).

Major Feeding Strategies (Animals)

  • External digestion + suction (centipedes, spiders – liquefy prey externally).
  • Internal digestion categories (three macro-guilds):
    1. Herbivores (plant eaters).
    2. Carnivores (animal-flesh eaters).
    3. Omnivores (consume both; opportunistic).

Herbivores – Sub-Guilds & Issues

  • Sub-groups:
    • Frugivores (fruit).
    • Granivores (seeds & grains).
    • Folivores (leaves).
  • Plants vs. Herbivores – Evolutionary arms race.
    1. Cellulose cell wall barrier → most animals lack cellulase.
    • Solution: recruit protistans & microbes in gut to digest cellulose.
    1. Silica/phytolith deposition in tissues – mechanically abrades teeth.
    • Example: Little rock-wallaby (Peridorcas concinna) survives on fern that is 25%25\% silica.
    • Adaptation: endless molar conveyor; only kangaroo with >4 molars; new teeth erupt at rear, worn teeth shed at front.

Structural Needs of All Herbivores

  • Cropping tool (incisors, beak, lips, prehensile tongue etc.).
  • Milling / grinding apparatus (molars, gizzard stones in birds/dinosaurs, stones swallowed by crocodiles).
  • Long, compartmentalised gut (cecum, multi-chambered stomachs) to allow microbial fermentation.
  • Symbiotic organisms producing cellulase.

Coprophagy & Microbial Inoculation

  • Purpose: re-ingestion recovers nutrients & microbes.
  • Rabbits: visible example – prays → defecates → re-eats.
  • Koala:
    • Backward-opening pouch despite arboreal life.
    • Mother produces special soft “pap” feces; joey protrudes head, consumes pap → gut inoculation with eucalyptus-digesting microbes.
  • Ringtail possum anecdote:
    • Captive orphan refused leaves until fed adult ringtail scat; afterward digested foliage successfully.

Carnivores – Core Features

  • Must: capture & kill prey, cut prey into ingestible units, short/simple gut (protein digests quickly).
  • Size-scale spectrum:
    • Small insectivores (e.g., ningaui observed biting legs off grasshopper first).
    • Apex specialist: Marsupial lion, Thylacoleo carnifex
    • Mass ≈ 160 kg160\,\text{kg} (lioness-sized).
    • Lower dentition mostly gone except bolt-cutter premolar pair.
    • Enlarged thumb claw for ripping.

Ingesting Large Prey – Techniques

  • Shaking to dismember: cinematic T. rex / real crocodiles.
    • Saltwater croc demonstration: goat shaken into ≈8 pieces (death roll), then swallowed fragments.
  • Egg-eating snake:
    • Skull loosely hinged; furthest jaw parts “walk” over egg.
    • Vertebral hypapophyses crack shell internally; snake later regurgitates shell shards.
  • Over-ambitious predation mishaps:
    • Australian green tree frog tried ingesting larger snake; reversed process, snake survived → ate frog.
    • Malaysian newspaper photo: python around human shoulders; autopsy revealed prior homicide & staged feeding (snakes don’t break bones; constrictors tighten on exhalation, maintain bone integrity to swallow smoothly).

Anti-carnivore Defenses (Intra-kingdom War)

  • Make yourself inedible (echidna spines; goanna lodged on echidna, both died → museum display).
  • Out-run carnivores – speed.
  • Out-think carnivores – escalating brain size ‘arms race’ over 65 Ma65\,\text{Ma} (Cenozoic):
    • Herbivore brains enlarged; carnivore brains followed in slightly delayed lock-step.
    • Hypothetical: had big-brained maniraptoran dinosaurs (Sinnanigines) not been wiped out by K-Pg impact, mammals might never have dominated.

Omnivores – Flexibility & Australian Context

  • Still need prey-capture tools (hands in primates, tongues in nectar-feeding bats) but only short gut.
  • Consume easy items (fruit, nectar) when available.
  • Opportunism vital in nutritionally harsh/variable Australia.
    • Many “carnivores” or “herbivores” are actually opportunistic omnivores.
    • Pet western quoll: ate mice instantly but also rotting cucumber from trash.
    • Swamp wallaby: classic folivore yet stole & ate cooked chicken from fridge; also ingests insects on leaves.

Tooth Function, Sharpening & Behaviour

Types of Tooth-Maintenance

  1. Thegosis (Greek “to sharpen”)
    • Wear one set of teeth against the other in non-eating direction to hone edges.
    • Occurs across mammals, reptiles, many herbivores & carnivores.
  2. Continuous replacement (polyphyodonty)
    • Seen in crocodiles, many snakes; worn teeth shed, new sharp ones erupt.

Boar Example (Classic Thegosis)

  • Upper canines = stubby “whetstones”.
  • Lower canines slide behind uppers every jaw cycle, stripping enamel; produce razor edges → formidable tusks feared by lions/humans.

Human Incisor Thegosis

  • Feel lower incisors:
    • Lingual surface smooth-rolled.
    • Labial surface knife-edge—product of night-time grinding.
  • Developmental wear:
    • Juvenile cusped teeth → adult horizontal blades.
  • Jaw action while eating apple:
    • Protrude mandible, bite, retract—scissor slice (horizontal segmented bite).
  • Night-grinding sound triggers innate danger response (analogous to hearing predator sharpening tusks).

Teeth as Weapons & Social Signals

  • Facial expressions display tooth status:
    • Aggressive: lower jaw offset, lips retracted (show sharpened edges); “growl” acoustic proxy for grinding when lateral movement impossible (dogs, large canines).
    • Friendly/smile: teeth precisely intercuspated centrally (signal “not sharpening – safe”).
    • Primate “fear grin” & human smile share ancestry – submissive non-threat.
  • Beards:
    • Throat protection during close combat; obscure jawline cues; moustache compromises—advertises maleness w/o full coverage.

Extreme Human Bites – Legal & Sporting Anecdotes

  • 1837 British ruling: woman may lawfully bite man’s nose if kissed without consent.
  • Mike Tyson vs. Evander Holyfield (ear bite).
  • Rugby scrum incident: hand bitten off.

Guinea-pig Demonstration

  • Repeated poking → tooth-sharpening squeak; clear acoustic thegosis signal.

Dental Practice Critique – Bruxism vs. Thegosis

  • Dentists label grinding “bruxism” → pathologise natural sharpening.
  • Expensive splints (“very costly aspirin”) don’t stop grinding; patient will wear through, need replacement.
  • Hyper-thegosis arises from chronic stress (excess nighttime battle-preparation mental state).
    • Solution: stress management / psychological intervention, not dental appliance.
  • Advice to patients:
    • Ask dentist about “thegosis”.
    • If ignored or misunderstood, consider switching practitioner.
  • Speaker presented yearly at University of Sydney Dental school; initial scepticism → gradual curriculum adoption.

Key Numerical & Temporal References

  • Silica content of fern diet for rock-wallaby: 25%25\%.
  • Mass of Thylacoleo carnifex: ≈160 kg\approx160\,\text{kg}.
  • Cenozoic “age of mammals”: last 65 million years65\,\text{million years}.

Practical / Philosophical Implications

  • Omnivory + opportunism = core survival trait in fluctuating Australian ecosystems.
  • Human dental health and behavioural ecology intertwined; modern dentistry must integrate evolutionary function.
  • Ethical/legal norms historically recognise teeth as legitimate defensive tools.