Energy

Energy

Energy = the capacity of a physical system to perform work.

Energy can exist in many forms:

• Mechanical

• Chemical

• Light

• Heat

Food Energy

Food energy = chemical energy that animals obtain from nutrients in food.

Energy comes from:

• Carbohydrates

• Proteins

• Fats

During metabolism, nutrients are broken down and energy is released or absorbed.

Heat of Combustion

Heat of combustion = energy released as heat when a substance (food) is burned.

Complex Molecules

Examples:

• Starches

• Proteins

• Lipids

⬇ Catabolic reactions

Break complex molecules into:

Simple Molecules

• Monosaccharides

• Amino acids

• Glycerol

• Fatty acids

Energy is transferred to ATP.

ATP → ADP

ATP stores/transfers energy that can be used by the body.

Anabolic Reactions

• Build complex molecules from simpler molecules.

• Energy is transferred from ATP → complex molecules.

 What Does Food Energy Support?

Food energy is needed to support:

Maintenance

• Keeping the body functioning

Growth

• Producing new body tissue

Reproduction

• Gestation

• Lactation

Physical Activity

• Movement and activity

Key Idea

Energy requirements depend on what the animal needs its metabolism to support and for how long.

Thermoregulation

Animals need to regulate their body temperature.

Two important categories are:

Endotherms

Body temperature is primarily determined by heat produced through metabolism.

Ectotherms

Body temperature is primarily determined by heat from the environment.

Endotherms

Body temperature is primarily determined by heat produced from metabolic activity.

Endotherms have:

• Higher mitochondrial density per cell than ectotherms

• Higher metabolic rates

• Greater food requirements

How Does an Endotherm Cool Down?

• Eat less

• Reduce physical activity

• Panting/sweating → evaporative cooling

• Vasodilation

Vasodilation

Blood vessels widen → helps with heat loss.

How Does an Endotherm Warm Up?

• Eat more

• Increase physical activity

• Shivering → muscle contractions

• Some non-shivering thermogenesis

  • Brown adipose tissue

• Insulation:

  • Fur/pelage

  • Adipose tissue

• Vasoconstriction

Vasoconstriction

Blood vessels narrow → reduces heat loss.

Ectotherm

Body temperature is primarily determined by heat from the environment.

Characteristics:

• Body temperature changes with environmental temperature

• Metabolic heat production is minor/negligible

• Lower metabolic rate than endotherms

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

 Ectotherms — Pros and Cons

Advantages

• Require less food

• Can support a larger carrying capacity in a given environment

Disadvantage

• Less functional or dysfunctional when cold

 Endotherm vs. Ectotherm

Endotherm	Ectotherm
Heat mainly comes from metabolism	Heat mainly comes from environment
Higher metabolic rate	Lower metabolic rate
Requires more food	Requires less food
Body temperature is less dependent on environment	Body temperature changes more with environment

 Homeotherms vs. Poikilotherms

These terms describe how much body temperature changes.

Homeotherm

Maintains a relatively constant body temperature, regardless of environmental temperature, up to a point.

Colloquially called:

• "Warm-blooded"

Mostly:

• Mammals

• Birds

Normal Body Temperatures

• Birds → ~40°C

• Placental mammals → 37–38°C

• Marsupials → ~34°C

• Monotremes → ~30°C

Important

Scientifically, there is no such thing as "warm-blooded" and "cold-blooded."

Also:

Homeotherm ≠ always endothermic

Example given in lecture:

• Some fish in tropical waters or very deep waters can be homeothermic.

Poikilotherm

Body temperature fluctuates considerably, usually according to environmental temperature.

Colloquially called:

• "Cold-blooded"

Mostly:

• Fish

• Reptiles

• Amphibians

Important Example

The naked mole rat is the only mammalian poikilotherm.

Thermoneutral Zone

The range of environmental temperatures where an endotherm does not need to actively warm or cool itself.

Within the TNZ:
➡ The animal does not need extra energy for thermoregulation.

Factors Affecting TNZ

Many factors can influence an animal's TNZ:

• Body composition

  • Example: adiposity

• Fur/pelage

• Life stage

• Body size

Body Size

Smaller animals have:
➡ Higher metabolic rates

Surface Area to Volume

Body size affects heat loss because of the relationship between surface area and volume.

Example: Large Cube

Sides = 2 × 2

• Surface area = 24

• Volume = 8

Example: Small Cube

Sides = 1 × 1

• Surface area = 6

• Volume = 1

Upper Critical Temperature (UCT)

UCT

The environmental temperature at the upper limit of the TNZ.

Above the UCT:
➡ The animal must actively cool itself.

Heat Stress

If the temperature stays above the UCT for too long:

• Anorexia → reduced food intake

• Excessive sweating/panting

• Dehydration

• Cellular dysfunction if severe

• Death if severe and long-lasting

Lower Critical Temperature (LCT)

LCT

The environmental temperature at the lower limit of the TNZ.

Below the LCT:
➡ The animal must actively warm itself.

Cold Stress

If the temperature stays below the LCT for too long:

• Excessive shivering

• Bodily processes may slow

• Cellular dysfunction if severe

• Death if severe and long-lasting

 Basal Metabolic Rate (BMR)

BMR

The minimum amount of energy required by an endothermic animal each day to maintain homeostasis.

This is measured when factors that affect energy expenditure are minimized.

Conditions for Measuring BMR

The animal is:

• At rest, but awake

• Post-absorptive

  • 12-hour fast

• In a thermoneutral environment

  • Within TNZ

• Without physical or psychological stress

 Standard Metabolic Rate (SMR)

SMR

The minimum amount of energy required by an ectothermic animal.

Conditions:

• At rest, but awake

• Post-absorptive

  • 12-hour fast

• At a specific environmental temperature

• Without physical or psychological stress

BMR vs. SMR

Endotherm → BMR

Ectotherm → SMR

Metabolic Body Weight

The mass of an animal that is made up of metabolically active tissue.

Important

Energy requirements do not increase directly with body weight.

For example:

100 kg animal ≠ 2× the energy requirement of a 50 kg animal

In most cases:

➡ Smaller animals have a greater energy requirement per unit of body weight.

 Kleiber's Law

The relationship between body weight and energy requirements is not linear.

Max Kleiber

Developed an equation to estimate BMR based on metabolic body weight.

For mature, non-growing endothermic animals:

Kleiber's Law

Y = 70 × BW⁰·⁷⁵

Where:

• Y = kilocalories per 24 hours

• 70 = constant

• BW = metabolic body weight

Important Relationship

Metabolic body weight = BW⁰·⁷⁵

➡ Metabolic body weight follows the ¾ power-law of body mass for most animals.

Daily Energy Expenditure (DEE)

DEE

The energy used to support:

• Essential body functions

• Digestion

• Metabolism

• Physical activity

Equation

DEE = BMR + metabolic heat production + physical activity