Energy for Maintenance

Basal Metabolic Rate (BMR)

Basal metabolic rate (BMR) is the minimum amount of energy required by an endothermic animal each day to maintain homeostasis under specific conditions.

Energy is needed for essential body functions, including:

• Respiration.

• Circulation.

• Nerve function.

• Cellular maintenance.

• Kidney and liver function.

Endotherms vs. Ectotherms

Type	Definition
Endotherm	Body temperature is primarily determined by heat produced through metabolic activity.
Ectotherm	Body temperature is primarily determined by heat from the environment.

Remember: BMR specifically applies to endotherms in this lecture.

Conditions Required to Measure BMR

BMR is measured under controlled conditions. The animal must be:

• At rest, but not sleeping.

• In a post-absorptive state.

• In a thermoneutral environment.

• Free from physical or psychological stress.

These conditions help measure the energy needed for basic body functions without additional energy demands.

Thermoneutral Zone (TNZ)

The thermoneutral zone (TNZ) is the range of environmental temperatures in which an endotherm does not need to actively warm or cool itself.

Within this zone:

• The animal does not need additional energy to regulate body temperature.

• Basal metabolic heat production equals heat lost to the environment.

Heatproduced=Heatlost

BMR Is Not Always the Lowest Metabolic Rate

An animal's metabolic rate may fall below its BMR during certain conditions, including:

• Sleeping.

• Prolonged starvation and/or dehydration.

• Hibernation.

• Torpor.

Key point: BMR represents energy expenditure under specific conditions, but it is not necessarily the lowest metabolic rate an animal can reach.

 Estimating BMR in Mammals

BMR can be estimated using equations based on body weight. These equations use metabolic body weight, which accounts for the relationship between body size and energy requirements.

Kleiber's equation (1947)

Developed for mature, non-growing endothermic mammals.

Y=70xBW^0.75

Where:

• Y = BMR in kcal/day.

• 70 = constant.

• BW = body weight in kg.

• BW0.75 = metabolic body weight.

McNab's equations (1988)

McNab introduced more recent BMR estimates.

McNab's equations (1988) cont

Mammal group	BMR equation
Placental mammals	Y=57.2BW0.716
Marsupial mammals	Y=46.6BW0.737

Different equations are used because BMR can vary among mammal groups and feeding habits.

BMR Equations Based on Mammal Feeding Habits

The lecture provides additional equations for mammals with different food habits. Use the equation that matches the animal's group when applying these estimates.

Vertebrate eaters

Group	Equation	Weight range (kg)
Vertebrate eaters	Y=91.8BW0.813	0.05–175.0

Invertebrate eaters

Group	Equation	Weight range (kg)
Flying	Y=47.3BW0.797	Below 0.01–0.06
Burrowing	Y=31.7BW0.738	0.40–4.00
Small	Y=44.1BW0.597	Below 0.01–0.12
Large	Y=43.5BW0.719	0.20–7.70
Anteaters	Y=19.7BW0.753	1.20–30.6

Frugivores (fruit eaters)

Group	Equation	Weight range (kg)
Flying	Y=56.6BW0.691	Below 0.01–0.60
Arboreal	Y=50.1BW0.603	0.30–14.30
Terrestrial	Y=60.9BW0.744	0.50–9.20

Nectarivores (nectar eaters)

Group	Equation	Weight range (kg)
Nectarivores	Y=23.3BW0.451	Below 0.01–0.05

Grazers

Group	Equation	Weight range (kg)
Small	Y=62.5BW0.617	Below 0.03–0.11
Large	Y=69.1BW0.808	0.14–150.00
Burrowing	Y=42.5BW0.799	0.17–6.80

Folivores (leaf eaters)

Group	Equation	Weight range (kg)
Small	Y=42.7BW0.545	Below 0.05–0.32
Arboreal	Y=39.9BW0.657	0.86–7.00
Terrestrial	Y=52.6BW0.750	0.80–400.00

Estimating BMR in Birds

The lecture separates birds into two groups:

Bird group	BMR equation
Passerine birds	Y=114.7BW0.73
Non-passerine birds	Y=73.6BW0.73

Here, Y represents BMR in kcal/day and BW represents body weight in kg.

Alternative Method: Oxygen Consumption

For the major project, BMR can also be estimated by converting resting oxygen consumption into energy expenditure.

The equation from Thompson and Nicoll (1986) is:

Where:

• Resting O₂ consumption is measured in ml/hour.

• 0.0048 = kcal per ml of oxygen.

• 24 = hours per day.

The final answer is BMR in kcal/day.

Remember

1cm3ofO2=1mlofO2

Example calculation

Suppose an animal has a resting oxygen consumption of 100 ml/hour.

BMR=1000.004824

\[ \boxed{\text{BMR}=11.52\text{ kcal/day}} \]

This example demonstrates how to convert oxygen consumption per hour into daily energy expenditure.