L7: Energy

Energy

• capacity of a physical system to perform work

• exists in many forms (e.g. mechanical, chemical, light, heat)

• Heat of combustion: energy released (heat) during combustion of a

  substance (food)

Food energy

• chemical energy that animals derive from certain nutrients in food

• energy derived from carbohydrates, proteins, and fats

• nutrients are metabolized, during which energy is released/absorbed

Food energy is required to sustain an animal’s metabolism during...

• maintenance

• growth

• reproduction (gestation and/or lactation)

• physical activity

Endotherm and requirements

body temperature primarily determined by heat derived from metabolic activity

- Higher mitochondrial density per cell than ectotherms → ↑ metabolic rate → need more food

How does an endotherm cool down?

• eating less

• reduce physical activity

• panting/sweating (evaporative cooling)

• vasodilation

How does an endotherm warm up?

• eating more

• increase physical activity

• shivering (muscle contractions)

• (Some) non-shivering thermogenesis (brown adipose tissue)

• insulation (fur, pelage, adipose tissue)

• vasoconstriction

Ectotherm and characteristics

body temperature primarily determined by heat derived from the environment

• Body temperature fluctuates with environmental temperature

• Metabolic heat production is minor/negligible

• Lower metabolic rate than endotherm

How does an ectotherm cool down?

• Reduce physical activity

• Seek shelter/shade/water

• Vasodilation

• Mucus secretions (replaces sweating)

How does an ectotherm warm up?

• Increase physical activity

• Seek external sources of heat

• Countercurrent heat exchange

Pros and Cons of ectotherms

Pros

• require less food

• larger carrying capacity in a given environment

Cons

• less functional or dysfunctional when cold

Changes in Body Temperature in Relation to Environmental Temperature in endotherms vs ectotherm

Homeotherm, characteristics, and who

Maintains a relatively constant body temperature (regardless of the environmental temperature, up to a point)

• colloquially termed ‘warm blooded’ animals (no such thing scientifically)

• mammals and birds (mostly)

Normal body temperatures of homeotherms

• birds: 40 0C

• placental mammals: 37 - 38 0C

• marsupials: 34 0C

• monotremes: 30 0C

Are homeotherms endothermic?

HOMEOTHERMS are not always ENDOTHERMIC

(e.g. some fish in tropical waters or very deep waters)

Poikilotherms, characteristics, and who

Body temperature that fluctuates considerably (usually according to environmental temperature)

• Colloquially termed ‘cold blooded’ animals

• Fish, reptiles, and amphibians (mostly)

The only mammalian poikilotherm is:

The naked mole rate

Thermoneutral Zone (TNZ)

environmental temperature range where endotherms do not need to warm/cool themselves

Factors that influence an animal’s TNZ

• body composition (e.g. adiposity)

• fur/pelage

• life stage

• body size (SA / V)

• smaller animals have ↑ metabolic rates

Upper critical temperature

environmental temperature at the upper limit of an endotherm’s TNZ

If above the UCT for too long, heat stress ensues:

• anorexia

• excessive sweating/panting (dehydration)

• cellular dysfunction (if severe)

• death (if severe & long-lasting)

Lower critical temperature

environmental temperature at the lower limit of an endotherm’s TNZ

If below the LCT for too long, cold stress ensues:

• excessive shivering

• bodily processes may slow

• cellular dysfunction (if severe)

• death (if severe & long-lasting)

Basal metabolic rate

minimal amount of energy required by an endothermic animal each day to maintain homeostasis

*in the absence of factors that would affect energy expenditure

How can BMR be determined:

• at rest (but awake)

• in a post-absorptive state (12-h fast)

• in a thermoneutral environment (TNZ)

• without physical/psychological stress

Standard metabolic rate (SMR)

minimal amount of energy required by an ectothermic animal

How can SMR be determined

• at rest (but awake)

• in a post-absorptive state (12-h fast)

• at a specific environment temperature

• without physical/psychological stress

Metabolic body weight (BWx)

mass of an animal that is comprised of metabolically active tissue

Energy expenditure correlations with BW

• not linear

• 100kg animal does not require 2 x the energy as a 50 kg animal

• In most cases smaller animals have a greater energy requirement per unit BW

How to estimated BMR?

Max Kleiber developed an equation to estimate BMR:

• based on metabolic body weight

• for mature (not growing) endothermic animals

Kleiber’s Law: Y = 70 x BW 0.75 where,

Y = kilocalories per 24 hours

70 = constant

BW = metabolic body weight

Kleiber’s law on metabolic BW

metabolic BW follows the ¾ power-law of body mass (most animals)

Metabolic body weight = BW0.75

Daily Energy Expenditure (DEE)

energy used to support essential body functions (including digestion & metabolism) + physical activity

DEE = BMR + metabolic heat production + physical activity