Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 40.4: Energy Requirements Are Related to Animal Size, Activity, and Environment
Animals need energy to survive, grow, move, and reproduce. The metabolic rate—the rate of energy expenditure—varies with body size, activity level, and environmental factors. This scaling relationship reveals key insights about animal physiology and ecology.
Measuring Metabolic Rate
- Metabolic rate: energy used per unit time
- Measured via:
- Heat loss (calorimetry)
- O₂ consumption or CO₂ output
- Units: kcal/day, Joules/hour, etc.
Two Baseline Measurements:
- BMR (Basal Metabolic Rate): for resting, fasting endotherms
- SMR (Standard Metabolic Rate): for ectotherms under similar conditions
- BMR examples (adults):
- Males: ~1600–1800 kcal/day
- Females: ~1300–1500 kcal/day
- SMR for alligator-sized ectotherm: ~60 kcal/day
➝ Endothermy is far more energy-demanding
Activity and Maximum Metabolic Rates
- Activities like running or flight increase metabolic demand
- Maximum metabolic rate: peak sustained energy output
- Inversely related to duration—short bursts only
- Typical daily energy use: 2–4× BMR/SMR
Influence of Size on Metabolism
- Larger animals use more total energy, but less energy per gram
- Follows a ¾ power law: metabolic rate ∝ mass¾
- Smaller animals (e.g., mice) must eat more per unit weight than larger animals (e.g., elephants)
Influence of Environment and Activity
- Temperature and activity impact metabolic rate:
- Ectotherms: strongly affected by external conditions
- Endotherms: stable rates, but increase metabolism in cold
- Example: bears increase metabolism to store fat pre-hibernation, then decrease it during dormancy
In a Nutshell
Metabolic rate reflects an animal’s energy needs and is shaped by its size, activity level, and environment. While large animals consume more energy overall, small animals require more per unit of mass. These energetic demands influence diet, behavior, and evolutionary adaptations.