Food and Energy


Hill et al., Ch. 4-7



Updated 12 October 2005


Students: If you rely on printed copies of these notes, make sure that you have the latest copy (check date stamp) and remember to study the linked figures and graphs online.


Nutrition, Feeding, and Digestion (Hill et al., ch 4)


A.   Three “musts” for every animal species:

1.     Harvest energy (heterotrophic; requires energy and structural/regulatory components)

2.     Allocate energy (energy budget)

a.      maintenance - energy to power and maintain existing tissue

b.     production - energy to produce new tissue (growth and reproduction)

3.     Avoid dying (requires energy)


B.     Components of harvesting energy

1.     Nutrition – provide materials for normal functioning: energy (caloric) and structural/regulatory (caloric/noncaloric)

2.     Feeding – acquisition and ingestion of food

3.     Digestion and absorption – break down complex compounds to simpler compounds that can be absorbed

4.     Costs – energy used to acquire and process food and maintain digestive tract

5.     Defense – make yourself inedible (diet choice); component of avoidance of death


C.   Nutrition - body composition reflects diet (HFig. 4.1)

1.     Essential caloric components

a.        proteins: polymers (amino acids, peptide bonds, nitrogen)

-       functions: structural, regulatory

-       20-22 “standard” aa (required for protein synthesis in all organisms); HFig. 4.21; >200 total aa known in organisms

-       only source of nitrogen (H2N, amino group) is other proteins; atmospheric N2?

-       ~10 aa “essential” - cannot synthesize, must ingest or obtain them from nitrogen-fixing symbionts (HTab. 4.1)

-       ~50% of organic material in mammals is protein; no storage (HFig. 4.22); aa at a given time constitute functional proteins (body composition is not static!)

-       high aa turnover rate + nitrogen commonly limiting in nature ® problem!

b.       lipids: glycerol + fatty acids (>50; HFig. 4.32, HFig. 4.31)

-       triglycerides (fats, oils), waxes, phospholipids, sterols (HFig. 14.1)

-       functions: storage, membranes, reduce EWL (terrestrial animals)

-       animals synthesize most (exception: omega-3/omega-6 FA – “essential”); FA requirements may differ among spp. (e.g., cats vs. dogs)

c.        carbohydrates: polymers of monosaccarides (HFig. 4.4)

-       functions: storage (starch, glycogen), structural (cellulose, chitin)

-       cellulose and chitin are the most abundant organic compounds on earth, yet many animals lack the enzymes (cellulose, chitinase) to digest them; chitin >50% dry weight of many arthropods; cellulose >75% energy flow in forests (decomposition)

-       no “essential” carbs

d.       PLC oxidation yields primarily carbon, hydrogen, oxygen, and nitrogen (proteins only); generally interchangeable for supplying caloric nutrition (exception - brain requires carbohydrates); relative energy yields (HTab. 4.2)

2.     essential non-caloric components; necessary for maintenance and growth; variable requirements in different species

a.        vitamins (organic compounds required in small amounts; obtain from food, cannot synthesize)

-       water-soluble (HTab. 4.3); general function; essential for most

-       lipid-soluble (HTab. 4.4); more specialized function; not universally required

b.       minerals: >20 elements, e.g., calcium, iodine, phosphorus; many essential; common limiting factor in soils (HFig. 4)


D.   Feeding

1.     Basic feeding strategies; if energy available, species will harvest it (LFAW); important in maintaining biodiversity by permitting species coexistence (ecology); three fundamental categories:

a.      individually attack and ingest; huge diversity of structural devices

b.     suspension feeding (mostly aquatic); HFig. 4.10

c.     symbioses (in gut)


E.    Digestion and absorption – major determinants of the nutritional quality of foods (ingest but not digest/absorb!)

1.     Major digestive pathways

a.      ingestion®extracellular digestion®absorp®blood®cells; muscular movement

-       vertebrates (HFig.4.20)

-       arthropods (HFig. 4.16)

b.     ingestion®extracellular digest®absorption®intracellular digest®blood®cells; ciliary movement (sort particles)

-       mollusks (HFig. 4.17, HFig. 4.17.2)


2.     Function – breakdown large, complex food molecules (polymers) into smaller, simpler substances

a.      mechanical - muscular activity (gut motility):

-       peristalsis

-       segmentation

b.     hydrolytic reactions (uptake water, release heat)

-       reaction rate regulated by enzymes

-       enzyme activity affected by pH and temperature (optimal responses)

3.     Mechanisms of digestion

a.      proteins; easily digested

-       break peptide bonds with appropriate enzymes resulting in smaller AA chains (HFig. 4.18)

-       stepwise hydrolysis results in free AA (absorbed)

-       proteins have the greatest diversity of chemical bonding types

-       to hydrolyze a particular bond requires a specific enzyme working within a specific pH range

-       animals have all necessary digestive enzymes in (e.g., pepsin/stomach, trypsin/pancreas)

b.     fats: insoluble, not easily digested, facilitated by mechanical and chemical (bile) emulsification

-       digestion involves hydrolysis with appropriate enzymes (lipase/pancreas)

-       results in glycerol and free FAs

-       wax lipids: important in marine food chains; most terrestrial animals cannot digest (no wax lipase); exceptions: wax moth, some birds (e.g., honey guides, warblers that eat waxy berries)

c.     carbohydrates: easily digested except for structural carbs

-       monosaccharides (e.g., glucose, fructose) absorbed directly (no digestion necessary)

-       disaccharides (e.g., sucrose) and polysaccharides (e.g., starch, glycogen) involves hydrolysis with appropriate enzymes (e.g., amylase/mouth & pancreas) ultimately resulting in monosaccharides

-       structural polysaccharides (cellulose, chitin) - most animals cannot digest; requires symbiotic microorganisms (bacteria and protozoans) that produce cellulase and chitinase

4.   Herbivore digestive strategies

a.      prolong digestion time

-       lengthened gut

-       retrograde peristalsis (e.g., stomach-mouth, small intestine-stomach, cloaca-large intestine)

-       pass digesta through >1 time (coprophagy)

b.     let symbiotic organisms do it for you (anaerobic microbial fermentation); numerous functions:

-       synthesize “essential” B vitamins and amino acids

-       digest cellulose (cellulase)

-       recycle excretory nitrogen into new protein; importance for low-grade forage (camels/sheep excrete little/no urea when fed protein-free food)

-       also synthesize sterols for arthropods (essential) and facilitate digestion of blood in blood-feeding animals (e.g., leeches, ticks, lice, vampire bats)

5.        Vertebrate fermentation: foregut (HFig. 4.14,1; HFig. 4.14,2) or hindgut (HFig. 4.15)

a.      foregut fermentation (esophagus, stomach)

-       ruminant ungulates (e.g., cattle, sheep): rumen, posterior stomach (secretory)

-       rumen produces ~70% of total energy requirements (fermentation); methane (10% energy lost)

-       lose nutrients to microbes before absorption®periodically flush stomach into intestine to reclaim digestive products

-       other foregut fermenters: sloths, kangaroos, colobus monkeys, many rodents, hippos, hyraxes, few birds (e.g., hoatzin)

b.     hindgut fermentation (small and large intestines, caeca)

-       widespread: many rodents, rabbits, horses, gallinaceous birds, herbivorous fish and turtles, iguanas, many arthropods (termites, shipworms)

-       defecate some microbial products (vitamins, amino acids); cannot absorb (hindgut); reclaim by coprophagy (rabbits, many rodents, some deer, some apes)


F.       Absorption (assimilation) – two fundamental processes

1.     Hydrophilic molecules

a.      monosaccharides, amino acids, water-soluble vitamins

b.     cannot dissolve in cell membranes

c.     require specific transporter proteins (e.g., vertebrate glucose transporters); absorb by facilitated diffusion and active transport

d.     absorption of a given molecule restricted to the gut region where the specific transporter protein is made

2.     Hydrophobic molecules

a.      glycerol, fatty acids, lipid-soluble vitamins

b.     dissolve in cell membranes (absorb by simple diffusion)

c.     more widespread absorption


G.     Absorption (digestive) efficiency (AE, DE)

1.        Methods of study (bomb calorimetry)

2.        Values (Table)


Feeding strategy








































































H.      Responses to feeding (review response levels, HTab. 1.2)

1.     Acute

a.      coordinated mechanical and chemical actions (involving regional muscle contraction/relaxation, acid secretion, and specific regional enzyme secretion; HFig. 4.20)

b.     cf between-meal digestive physiology

2.   Digestive physiology may be adjusted in various time frames

a.      chronic – physiology acclimates (days/weeks timscale) to particular foods (e.g., relative amount of protein/carbs in diet)

-       opportunistic feeding (up/downregulation; e.g., snakes; energetic benefits)

-       accompanying acclimation in gut morphology; structurefunction

b.     clock-driven – endogenous biological rhythms (circadian, circannual, lunar); e.g., hibernation (HFig. 4.21x)

c.     ontogenetic – genetic changes between birth and adulthood (e.g., tadpole-frog)

d.     evolutionary – digestive physiology subject to changes in gene frequency among populations/closely-related species (e.g., vertebrate glucose transporters; frequency of human lactase gene)


I.         Defense (deter predators; derived in part from diet)

1.     Plant poisons

a.      plants cannot run away, resist predators with chemical defenses (extremely common, relatively large part of energy budget): develop a large variety of toxins, e.g., akaloids, glycosides, tannins, oils/resins, oxalic acid, enzyme inhibitors, hormone effectors and hormone mimics

b.     why are plant poisons of interest to animal physiologists?

-       animals must counter plant toxins in diet (e.g., koalas, squirrels)

-       animals may incorporate plant toxins into their own tissues for defense

2.     Animal poisons

a.      common in invertebrates (e.g., millipedes, hymenopterans, arachnids, beetles) and each vertebrate class – fish (many), amphibians (all), reptiles (some lizards/snakes), birds (pitohui), mammals (platypus, some shrews)

b.     toxin composition may be highly complex and variable in time, space, and ontogeny (related to diet variation; e.g., captive dendrobatid frogs, geographic variation in viperid venoms)

c.     carnivores must counter animal poisons in diet (e.g., toad, newt eaters)



Energy Metabolism (Hill et al., ch 5)


A.   Introduction

1.     Forms of energy in animals

a.      chemical, electrical, mechanical (all capable of doing physiological work)

b.     energy transformations are inefficient (efficiency = output energy/input energy); eventually degraded to heat energy (1-way irreversible flow); heat energy cannot do physiological work

2.     Energy metabolism – def: sum of all the processes by which energy (primarily chemical) is acquired, transformed, channeled into useful functions, and dissipated; 2 basic processes:

a.      catabolism – break down organic molecules to release energy; organismal®cellular (cellular-level energy source - ATP)

b.     anabolism – use energy to construct molecules

3.     Aerobic vs. anaerobic processes

4.     In what ways do animals use energy? (HFig. 5.2); absorbed energy used for biosynthesis (=production), maintenance, and external work


B.    Metabolic rate – measures the rate at which energy is converted to heat and external work; significant for 3 reasons:

1.     Determines how much food an animal must ingest (costs)

2.     Represents an index of intensity of living (ectothermy - low energy strategy vs. endotherm - high energy strategy)

3.     Measures the energetic effect on an animal’s environment (ecological implications, e.g., distribution)


A.   Measuring metabolic rate

1.     Methods

a.      monitor caloric input (ingestion) and output (feces, urine); assumes no growth and no change in body composition (e.g., storage)

b.     measure total heat production (calorimeter) and external work (or minimize)

c.     measure metabolic water produced (isotope labeling)

d.     measure gas exchange (CO2  production or O2 consumption); standard method for aerobic respiration (if substantial anaerobic respiration, other methods needed, e.g., measure lactic acid concentration)

2.     Important consideration: energy content of food is variable



kcal/l O2


(CO2/ O2)

Metabolic water (g/g food)











Protein (urea)





a.      mixed diet (not eat pure food types); conventional value used for calculations = 4.8 kcal/lO2

b.     how determine what kind of food is being metabolized?; respiratory quotient (RQ); extremes (carbs, fat); intermediate values determined by amount of protein metabolism (measure nitrogen excretion)


B.    Kinds of metabolic rate measurements

1.     Standard (basal) MR (SMR/BMR) – rate for fasting animals in resting condition (i.e., no activity, no digestion, no stress)

a.      ectotherms (SMR; temperature-dependent; must specify temperature)

b.     endotherms (BMR; temperature-independent if measured in thermoneutral zone)

2.     Active MR (AMR) – rate for animals active at a specified level (often forced maximum)

3.     Field MR (FMR) – rate of normally active animals (in nature); most useful parameter for characterizing an animal’s normal energy expenditure (=ADMR, active daily MR); measure with doubly-labeled water (DLW) techniques

a.      inject DLW (mixture of 3HHO and H218O) with known ratio of isotopes into animal; monitor animal (SREL racers 1, 2)

b.     take periodic blood samples (SREL racers 3, 4)

c.     measure change in ratio of isotopes (3H and 18O); related to CO2 production (PFig. 12-3)


C.   Factors affecting metabolic rates (>10 known)

1.     Activity (external work); large effect (more later)

2.     Temperature (large effect; ch 8)

a.        rate of change (Q10) - increase in physiological rate when Tb increases 10C

b.       normal values: Q10 = 2-3 most animals; obscured in thermal neutral zone of endotherms

3.     Digestion

a.      measure as O2 consumption above SMR (acute response; specific dynamic action = SDA; HFig. 5.52)

b.     extent and duration of SDA highly variable and depends in part on size and composition of meal (protein highest)

c.     occurs after absorption; partly reflects cost of synthesizing nitrogenous waste products from digesting proteins

d.     SDA for a given food type is independent of taxon (e.g., cost of digesting 1g of protein is similar in all animals)

e.      long-term overeating induces some individuals to raise their metabolic rates and not gain weight (diet-induced thermogenesis; DIT® chronic response); important to understand for human obesity problem

4.     Body size (scaling); less than proportional (WFig. 6.20)

a.        weight-specific MR (HFig. 5.101)

b.       most animals, not just endotherms, (SNFig. 5.11) have slopes ~0.75 suggesting general biological principle

c.        differences in intercepts in permit direct comparison of MR among groups at any body size

-     marsupials (MR = 0.409 M0.75) vs. eutherians (MR = 0.676 M0.75); SNFig. 5.13; eutherians >60% higher

-     non-passerine birds have similar rates as eutherian mammals (MR = 0.676 M0.75)

-     passerine birds (MR = 1.11 M0.724) are >60% higher (weight-specific; HFig. 5.9) than eutherians

d.       why common slope of 0.75? (still unknown; current research pp. 142-144; READ)

e.        functions of metabolic scaling models

-       description of observed data

-       prediction (provides null hypothesis, HFig. 7.9)


D.   Energy storage

1.     For many animals, input calories = output calories (regulate intake); mostly frequent-feeding endotherms; feedback loop involving hypothalamus, levels of blood sugar, and gastric fill (also regulated in some ectotherms, e.g. goldfish); for most ectotherms (infrequent feeders), intake regulation poorly known

2.     If input < output, consume body componens (carbohydrates first, fats, then proteins)

3.     If calories input (from proteins, fats, or carbohydrates) > calories expended:

a.      most animals store excess as fat (regardless of food type ingested)

-       light in weight (migratory birds store up to 50% body weight as fat)

b.     some store as carbohydrates (glycogen)

-       lower relative energy content; 10X heavier - requires additional 3-5 g water for each g glycogen stored

-       sessile animals (weight not hinder locomotion)

-       provide carbs quickly for anaerobic metabolism in hypoxic environments and burst activity in otherwise aerobic animals


E.    Growth efficiency (HFig. 5.2)

1.     Gross (ingested) vs. net (assimilated) efficiency; variation due to:

a.      ontogeny: usually changes through life (HFig. 5.13)

b.     thermal strategy: ectotherms (~50%) vs. endotherms (<2%); Eff.jpg

c.     important consideration in certain human industries (e.g., agriculture/aquaculture; cost/benefit) and ecology (life history theory – allocation of energy)



Aerobic and Anaerobic Metabolism (Hill et al., ch 6; pp. 150-160 - Biol. 259 review)


A.        Kinds of cellular-level respiration

1.     Aerobic pathway (glycolysis/Kreb’s cycle/ETS/oxidative phosphorylation; complete oxidation of carbs, proteins, lipids):

a.      Glucose + O2 ® ATP + CO2 + H20  

-       36-38 net ATP from 1 glucose molecule (60-70% efficient)

-       may use O2 from external or internal environments (e.g., myoglobin)

2.     Anaerobic pathways (incomplete oxidation; occur only in certain cells; e.g., vertebrates, only in cells that produce LDH, catalyzes pyruvic acid®lactic acid; not in brain!)

a.        anaerobic glycolysis: Glucose/glycogen ® ATP + lactic acid

-       most vertebrates; 2 net ATP from 1 glucose (<10% efficient)

-       may accumulate lactic acid (disrupt blood acid-base balance); must regulate release of lactic acid into blood

-       buffering capacity of muscle is greatest in muscles used in intensive burst activity or prolonged low-level anaerobic activity

-       fate of lactate? – retain, eventually oxidized in Krebs/ETS (primarily endotherms; produces ATP) or converted to glucose (gluconeogenesis - ectotherms and endotherms; uses ATP)

b.       phosphagen pathways (HFig. 6.5)

3.     Comparative pathway properties (HTab. 6.1.1; HTab. 6.1.2)


B.      Interplay of aerobic and anaerobic respiration during vertebrate activity

1.     Behavior reflects and is limited by underlying physiology (e.g., ontogenetic changes in Nerodia)

a.      high level activity usually involves >1 catabolic process; transition phases due to O2 delivery lag (HFig. 6.7.2)

b.     variation in transition phases with level of activity, no steady-state at extreme levels (HFig. 6.8.3)

c.     variation in aerobic capacity among spp. (frog vs. toad) and individuals (genetics, ontogeny, aerobic fitness)

2.     Variation in behavioral performance important component in ecology and ultimate fitness (e.g., foraging, escaping predators, mating, raising young)

a.      physiology®behavior®ecology®evolution


C.      Effects of low oxygen (hypoxia, anoxia)

1.     Examples

a.      internal parasites (e.g., intestinal)

b.     aquatic/marine animals in bottom sediments (e.g., benthic invertebrates, hibernating turtles)

c.     eutrophic ponds (diel cycle)

d.     diving vertebrates

e.      animals at high altitudes

2.     Responses

a.      tolerate regional body anoxia, preferentially supply oxygenated blood to brain (mammals, birds, crocodilians, some turtles)

b.     lower metabolic rate (metabolic depression); many invertebrates, turtles

c.     some turtles tolerate total body anoxia and lower metabolic rate (comatose, no brain activity); accumulate large amounts of lactic acid (buffered from lethal levels by shell)

d.     increase ability to extract O2 (oxygen regulation; HFig. 6.123); no increase in tolerance

e.      anaerobes - many invertebrates (worms, mussels), few fish (goldfish, some carp); mechanisms?

-       metabolic depression

-       more efficient anaerobiosis (cf anaerobic glycolysis)® higher ATP yield; use carbs/amino acids/lipids; less pH drop

-       fish: lactate®ethanol, diffuses out of blood through gills (benefit-avoid disrupting acid-base balance; cost-lose energy)



Activity Metabolism (Hill et al., ch 7)


A.      Energetic costs of routine daily life (sustained aerobic activity) vs. burst activity (sprinting: escaping predators, chasing prey - anaerobic); methods:

1.     Directly measure O2 consumption

a.      fit mask over head; lab or field (radiotelemetry – humans )

-       radiotelemetry on other animals; must transmit index of O2 (e.g., heart/respiration rate)

b.     force sustained controlled activity on treadmill, wind/water tunnel (control speed)

2.     DLW studies of free-ranging animals

3.     Calculate from time-energy budgets

a.      generalized total energy budget (allocation of energy is fundamental for all organisms - big 3 musts; often adaptive (life history theory)





Efood & drink =

(Efeces + Eurine + Erestmetab + Eactmetab + Edigestion) +

(Egrowth + Ereproduction)


b.     example (HTab. 7.2); calculate from known hourly cost values (lab studies)


B.      Energetic cost of locomotion (major part of Eactmetab)

1.     Effect of speed

a.      running (linear, HFig. 7.4)

b.     swimming (J-shaped, HFig. 7.3)

c.     flying (U-shaped, HFig. 7.61)

2.     Effect of grade and gait

a.      running vs. walking in humans (SNFig. 5.14)

-       intersection of running/walking plots coincides with natural changes from walking to running (reduce cost of locomotion)

b.     effect of gait (SNFig. 5.16)

-       animal selects speed and gait; chooses a narrow range of speeds within each gait (choice coincides with minimum cost of transport for a given distance)

3.     Effect of body size and shape (HFig. 7.4; HFig. 7.8 )

a.      slope steeper (costlier) with decrease in size

b.     directly compare different-sized and shaped organisms by calculating mass-specific rates (SNFig. 5.20)

-       cost of moving a given distance terrestrially similar for all taxa (despite structural differences)!

4.     Effect of minimizing time vs. distance (HFig. 7.61; HFig. 7.62); which is “better”?

a.      ecological context - predatory attack vs. migration

b.     also migration – go faster (at greater cost) if increase rate of encountering feeding patches

5.     Cost of transport (energy to move 1 kg 1 meter) for different locomotor modes (HFig. 7.7)

a.      runners use lever systems to support body weight (high cost of postural control)

b.     swimmers move in a medium of high resistance and viscosity (fish are streamlined, move at low speeds, and have near neutral-buoyancy; low cost of postural control)

-       cf non-streamlined surface swimmers (e.g., duck, man); cost ³ terrestrial

-       note cost (to fish) similar to human bicyclist

c.     fliers streamlined but move at high speeds (air low resistance, low viscosity) which provides lift

-       drag reduced in smaller animals (e.g., insects not streamlined)

-       many long migrators are fliers (e.g., terns)

d.     migratory line

-       migration selected for only if cost of transport is below a minimum value

-       note cost of transport for a large migratory mammal (e.g., reindeer) similar to small fish


C.      Capacity for activity (aerobic scope)

1.     Calculate: AS = max AR (VO2max) – SMR (BMR)

a.        roughly (for vertebrates),  AS = 10X resting MR; scales with body size (VO2max; HFig. 7.9)

b.       variable between major phyletic lines

-       SMR ectotherm = 0.1-0.25 BMR endotherm (SNFig. 5.11); therefore, max MR ecto ≈ min MR endo! (HTab. 7.3)

-       why endothermy?  (hypo: permitted faster sustained locomotor ability)

c.     variable among species (HFig. 7.9)

-       identify adaptations (deviation from regression line)

-       pronghorn (fastest sustained runner);  cf. cheetah (sprinter, anaerobic; cannot sustain)

-       what limits VO2max?; hypo: ratio mitochondria/contractile fiber within cell

d.     variable among individual of a species

-       reflects (in humans) genome (50%), training (e.g., increase oxygen-delivery, enzyme activity, glucose transporters; 10-30%), and unexplained (20-40%)

-       basis for world-class athletes in sustained, high-energy activities (humans and other)

-       little known re non-human animals; variability likely basis for some selection pressures (e.g., foraging success, predator escape)

2.     AS champs

a.        hummingbird; 42 ml O2/g/h (most energetically active vertebrate; hovering flight)

b.       large flying insects (e.g., butterfly/bumblebee); 100 ml O2/g/h


D.      Average daily energy expenditure (FMR; ADMR)

1.     2.5-3X resting MR; scales with body size

2.     what is max sustained?

a.      mammals/birds rearing young® 2.4-6.7X BMR

b.     mammals at -10C® 3.7-6.1X

c.     human Tour de France® 4.5X

3.     max ≈3-7X resting; what limits? (current research)