Food Intake

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Food Intake
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Food Intake

• When evaluating the nutritional environment of an
  animal, usually measure nutrient concentration of
  food
• Animal may be able to exist on poor quality food
  if it can eat enough
• Intake rate is as important as food quality
• Can be considered on many temporal and spatial
  scales
• Influenced by internal external processes

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Food Intake

• External Influences Short Time Scale
• Early research considered intake rate a function
  of searching and handling time. Relationship
  between these determines shape of functional
  response curve.
• Intake increases with increasing density up to an
  asymptote
• Works for predators

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Food Intake

• Fig. 1 Holling 1959

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Food Intake

• Situation different for herbivores
• Early attempts to predict intake rate for
  herbivores based on plant biomass
• Decent relationship for large grazers in
  grasslands
• Intake and biomass not related for browsers
• Intake more closely related to bite size in many
  situations

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Food Intake

• Figs. 3 and 4 Spalinger and Hobbs (1992)

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Food Intake

• Models proposed to predict intake rate by
  herbivores (Spalinger and Hobbs 1992)

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Food Intake

• Internal influences on intake rate
• Factors that influence the size of a meal and how
  often an animal feeds
• Food quality can determine the mechanism that
  acts to determine intake

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Food Intake

• Physical regulation - distention
• Studied most in herbivores, but also relevant to
  carnivores that kill large prey items
• Physical capacity can change over time
• Physical capacity related to rate at which food
  clears the digestive tract
• Digestion and absorption
• Chemical composition of food. Nectar digested
  quickly, high fiber foods could be digested for
  several days
• Rate of passage through the digestive tract

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Food Intake

• Physical regulation Passage Rate
• In ruminant, passage rate determined by
• Particle size rumination rate and fiber
  content. Grinding feeds will increase passage
• Flexibility in passage of food out of the rumen.
• Technique time out Passage rate
• Plastic, dyes, glass beads (not as good)
• Dye on hair of prey for carnivores
• Chromium mordanted to plant fiber
• Rare earth elements (e.g. Yt)
• Liquid passage using Co-EDTA and Cr-EDTA

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Food Intake

• Fig. 15.1 Robbins (1993)

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Food Intake

• Physiologic Regulation
• Many mechanisms tied to energy status
• Short term many interacting factors
• Neuropeptides and neurotransmitters
• Hormones (e.g. pituitary, pancreatic)
• GI-tract peptides (e.g. cholecystokinin)
• Nutrients (e.g. glucose, lipids, VFAs)\
• Ammonia concentration in the rumen

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Food Intake

• Physiologic Regulation
• Long term
• Set points. Body appears to have targets for
  body size and composition
• Regulation not fully understood, but the
  hypothalamus involved
• Leptin is hormone produced by fat that appears to
  be related to food intake, but mechanism is
  complicated

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Food Intake

• Set points
• May vary seasonally
• Rigidity of set points varies by species
• Some animals (e.g. pigs, zebras, bears) will
  become obese with ad lib food. Other species
  regulate body composition well.
• Indeterminant growth in some species also
  suggests a lack of set point for body size
• Secondary plant compounds may also limit intake

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Food Intake