Mammalian Feeding Strategies: Herbivores and Carnivores

Herbivore Food and Adaptations

• Topics for Part Two:
  • Herbivore food and adaptations (teeth, cellulose digestion).
  • Carnivores: predator strategies, cranial and dental characteristics.

Mammalian Feeding Strategies

• Mammals have a high metabolic rate (5-10 times higher than reptiles of the same size).
• Three main feeding strategies:
  • Herbivores: feed on plant material.
  • Carnivores: feed on other animals.
  • Omnivores: feed on a mix of plant and animal material.
• There can be mixing of strategies, but the main strategy defines the category.

Herbivores

• Herbivore foods:
  • Leaves, grass, seeds, fruit, nectar, etc.
  • Different plant foods have different energy levels and ingredients.
    • Fruit and nectar: Rich in sugar.
    • Seeds: Rich in protein and fat.
• Focus on leaves as food.
  • Low nutritional value for the following reasons:
    • Contain a lot of indigestible material, like cellulose.
    • Comparisons:
      • Water: Grass and lean meat have similar percentages.
      • Protein: More in lean meat than in grass.
      • Fat: More in meat.
      • Carbohydrates: More in grass, primarily cellulose.
• Cellulose:
  • Long chains of glucose molecules.
  • Most animals lack the enzymes to break it down.
  • Plant cell walls make leaves tough and abrasive.
  • Phytoliths (plant stones made of silica) can damage teeth.

Adaptations of Herbivores

• Spend a lot of time feeding and processing food.
  • Example: Pronghorn spends nearly 80% of its time feeding or digesting.

Teeth

• Herbivore teeth are different from carnivore teeth.
• Carnivore teeth:
  • Incisors, canines, premolars, and molars.
  • Molars and premolars: Grinding.
  • Canines: Ripping.
  • Incisors: Chiseling.
• Herbivore teeth:
  • Generally lack canines.
  • Many lack incisors (or only have a few on the lower jaw).
  • Gap between incisors and molars/premolars.
  • Enlarged incisors in some animals (e.g., beavers).
• Lobed teeth:
  • Ridges of teeth run together.
  • Premolars and molars look similar.
  • Grinding area to macerate tough cell walls.
  • Enamel worn down, exposing dentine -> sharp enamel blades for shearing.
• Dental Durability:
  • Rough food wears down teeth.
  • Mammals have only one set of replacement teeth.
  • Teeth need to be strong:
    • Crowns extend deep into jaw bones.
    • Deep lower jaws.
    • Cement covers the whole tooth.
  • Herbivore tooth:
    • Cement covers the whole tooth from root to top.
  • Worn-out teeth cause feeding problems.
  • Exception: Elephants have six molars that replace worn-out ones.

Digestion of Cellulose

• Mammals cannot break down cellulose themselves.
• Herbivores rely on symbiotic microorganisms that can break down cellulose.
• Two main solutions:
  • Foregut fermenters: Microorganisms in the upper part of the digestive system.
  • Hindgut fermenters: Symbionts in the latter parts of the digestive system.
• Foregut fermenters:
  • Ruminants: Four-chamber stomach.
  • Pseudo-ruminants: Enlarged stomach, no chambers.
• Hindgut fermenters:
  • Simple stomach, enlarged cecum and intestine.
  • Examples: horses, elephants, howler monkeys, koalas, rabbits.
• Hindgut Fermentation:
  • Rely on chewing.
  • Nutrients absorbed in the stomach and small intestine.
  • Cellulose broken down in the cecum and large intestine.
  • Energy lost in feces.
  • Coprophagy: Eating feces to regain energy.
    • Cecal pellets: Contain broken-down material from the cecum.
    • Smaller herbivores do this because they need a lot of energy.
• Foregut Fermentation (Ruminants):
  • Large stomach divided into four chambers (three non-absorptive, one true stomach).
  • Do not need to chew thoroughly.
  • Regurgitate and swallow food multiple times.
  • Slow passage of food.
• Ruminant Stomach:
  • Rumen: Contains microorganisms.
  • Abomasum and intestine: Absorption.
  • Cow eats grass -> ingests cellulose.
  • Microorganisms produce cellulase -> breaks down cellulose into glucose.
  • Microorganisms use glucose for energy.
  • Microorganisms produce fatty acids (absorbed by the cow for energy).
  • Microorganisms provide protein (amino acids).
  • Microorganisms produce gases (methane, $CO_2$), contributing to climate change.

Carnivores

• Carnivores: Meat-eaters.
• Carnivore: A taxon including dogs, cats, bears, seals, and sea lions.
• Predators: Find, catch, kill, and digest prey.

Predator Strategies:

• Sit-and-wait predator:
  • Sit still for a long time (e.g., puma).
  • Profitable if prey is common.
• Search-and-chase strategy:
  • Hunt in packs or solitarily (e.g., grey wolf).
  • Requires strength and stamina.

Carnivore Adaptations:

• Teeth:
  • Incisors: Grasping and tearing.
  • Canines: Puncturing.
  • Premolars and molars: Shearing blades; carnassial teeth (only in Carnivora).
• Strong jaw muscles:
  • Temporal muscle is enlarged.

Scavengers:

• Feed on dead animals or plant material.
• Opportunistic: Hunt or feed on carrion.
• Examples: brown hyena (solitary), spotted hyena (hunt in packs).

Insectivores:

• Insects are abundant and a good source of protein.
• Examples: bats, shrews, aardvarks.
• Horseshoe bat: Uses echolocation to capture moths.
• Anteaters: Eat ants and termites.
  • Adaptations: Huge claws for digging, elongated jaws, reduced or absent teeth, horny palates, large salivary glands.

Convergent Evolution:

• Different species develop similar traits.
• Animals that feed on insects have generalized dentition.
• Ancestors of most mammals were likely insectivorous.