Adaptation of the digestive system

Carnivores

Predators/scavengers

High energy content

Easily digestible

Long interval between short meals

Herbivores

Omnivores

Pig (&~bears)

Large species variation in digestive tract

Bears range from herbivore to carnivore

Degree of fermentation occurs in LI

Browsers or grazers

Low energy content

Microbial digestion

Digestive tract adapted for fermentation

Continuous eating

2 strategies (fore-gut/hind-gut)

Fermentation

Non-hydrolysable carbohydrate

Requires microbial degradation in absence of oxygen

Long, slow process

Fore-gut fermenters

Cattle/sheep = Ruminants

Stomach modified into fermentation chamber

Very efficient fermentation

Fermentation products absorbed in stomach

Microbial protein can pass into abomasum & small intestine for digestion/absorption

Hind-gut fermenters

Horses (& rabbits)

Large intestine modified into fermentation chamber

Moderately efficient fermentation

Fermentation products absorbed in colon

Microbial protein lost in faeces - rabbits practice coprophagy

Comparative fermentation

Hind-gut fermentation 70% efficient as fore-gut (lower degree of amylolytic fermentation, hind-gut lose microbial protein in faeces)

Hind-gut fermenters can alter transit time dependent on rate of fermentation

Ruminants more efficient on good quality forage, hind-gut more efficient on abundant poor quality forage (if short supply, ruminants predominate)

Ruminants nutritional strategies

Stems, blades, older leaves high in fibre

Ruminants have 3 substrategies

Fruit, seeds, buds, young shoots & leaves high in hydrolysable carbs

Grazers (grass/roughage eaters)

Intermediates

Browsers (concentrate selectors)

Ruminant adaptation

Browsers

Grazers

  • Narrow muzzles
  • More mobile lips/tongue
  • Smaller fore-stomach
  • Ruminant browsers retain functional oesophageal/ventricular groove
  • SI glucose absorption remains high
  • Wide muzzles
  • Less mobile lips/tongue
  • Larger fore-stomach
  • Ruminant grazers lose functional oesophageal/ventricular groove
  • SI intestinal glucose absorption declines

Oesophageal/ventricular groove

Present in all young ruminants

Retained only in browsers (to allow normal digestion hCHO

Enables milk to bypass fermentation chamber

Herbivore glucose absorption

Small intestine site of glucose absorption (via SGLT1)

SGLT1 regulated by diet

  • High in young, declines in grazers at weaning, remains high in browsers

Comparative digestion

Carnivores - No salivary amylase, high levels of enzymes from stomach/pancreas for proteins & fat

Omnivores - High levels of salivary amylase, high levels of enzymes from stomach/pancreas for proteins, fat & carbohydrate

Simple-stomached herbivores - Moderate levels of enzymes for carbohydrate prior to fermentation in large intestine, exploited in feeding concentrate diets high in hydrolysable carbohydrate

Comparative absorption

Carnivores/omnivores - Monosaccharides/peptides/monoglycerides and fatty acids absorbed in SI

Simple-stomached herbivores: Monosaccharides/peptides absorbed in SI, VFA in LI

Ruminant herbivores: VFA absorbed in fore-stomach, peptides absorbed in SI

Ruminants - Low levels of enzymes for carbohydrates after fermentation in fore stomach. Moderate levels of enzymes for protein digestion of microbial protein in abomasum/small intestine