Animal Metabolic Rate Calculator
Calculate basal and field metabolic rates using allometric relationships
Enter mass in kilograms (e.g., 0.02 kg = 20 g)
Results
Understanding Animal Metabolic Rate
Metabolic rate is the rate at which an organism converts stored chemical energy into usable energy for biological processes. It is fundamental to understanding animal energetics, ecology, and life history strategies.
Types of Metabolic Rate
Basal Metabolic Rate (BMR)
The minimum energy expenditure required to maintain basic physiological functions at rest, in a thermoneutral environment, post-absorptive state.
Kleiber's Law (Mammals): BMR = 70 × M^0.75 kcal/day
Alternative (Mammals): BMR = 293 × M^0.75 kJ/day
Birds: BMR = 78.3 × M^0.723 kcal/day (higher than mammals due to flight adaptations)
Field Metabolic Rate (FMR)
The average daily energy expenditure of free-living animals in their natural habitat, including all activities.
Mammals: FMR ≈ 3-5 × BMR (varies with activity level)
Birds: FMR ≈ 2.5-4 × BMR (flying is energetically expensive)
Ectotherm Metabolic Rate
Reptiles at 20°C: BMR = 10 × M^0.825 kJ/day
Temperature Effect (Q10): Rate doubles for every 10°C increase
Ectotherm metabolic rates are typically 5-10× lower than endotherms of similar mass at the same temperature.
From Oxygen Consumption
Energy Equivalence: 1 mL O₂ ≈ 4.48-5.05 kcal (depends on RQ)
RQ = 0.7: Fat oxidation (4.69 kcal/L O₂)
RQ = 0.85: Mixed diet (4.86 kcal/L O₂)
RQ = 1.0: Carbohydrate oxidation (5.05 kcal/L O₂)
Allometric Scaling: Kleiber's Law
Kleiber's Law describes the empirical observation that metabolic rate scales as body mass to the 3/4 power (M^0.75), not proportionally.
BMR = a × M^b
Where:
- a = Normalization constant (varies by taxon)
- M = Body mass (kg)
- b = Scaling exponent (≈0.75 for most animals)
Biological Significance: Larger animals have lower mass-specific metabolic rates. A 1 kg mammal uses ~10× more energy per gram than a 100 kg mammal.
Metabolic Rates Across Animal Groups
| Animal Example | Mass (kg) | BMR (kcal/day) | Mass-Specific |
|---|---|---|---|
| Mouse | 0.02 | ~4 | 200 kcal/kg/day |
| Rat | 0.3 | ~28 | 93 kcal/kg/day |
| Cat | 4 | ~200 | 50 kcal/kg/day |
| Human | 70 | ~1800 | 26 kcal/kg/day |
| Horse | 500 | ~7500 | 15 kcal/kg/day |
| Elephant | 5000 | ~42000 | 8.4 kcal/kg/day |
| Hummingbird | 0.003 | ~1.2 | 400 kcal/kg/day |
| Lizard (20°C) | 0.1 | ~2 | 20 kcal/kg/day |
Factors Affecting Metabolic Rate
Intrinsic Factors
- • Body Size: Scales with M^0.75
- • Body Composition: Lean mass more metabolically active
- • Age: Higher in juveniles (growth), lower in elderly
- • Sex: Males often have higher BMR than females
- • Genetics: Heritable variation in metabolic efficiency
- • Physiological State: Reproduction, molt, migration
Extrinsic Factors
- • Temperature: Critical for ectotherms (Q10 effect)
- • Season: Winter torpor, summer estivation
- • Food Availability: Fasting reduces metabolic rate
- • Activity Level: Exercise increases energy expenditure
- • Altitude: Hypoxia affects oxygen consumption
- • Climate: Cold adaptation increases BMR
Endotherms vs. Ectotherms
Endotherms (Birds & Mammals)
- • Generate heat internally
- • High, constant metabolic rate
- • Independent of ambient temperature (within limits)
- • Energy expensive (need frequent feeding)
- • Active in wide temperature ranges
- • Rapid sustained activity possible
Ectotherms (Reptiles & Amphibians)
- • Rely on external heat sources
- • Low, variable metabolic rate
- • Strongly dependent on ambient temperature
- • Energy efficient (can survive without food longer)
- • Activity limited by temperature
- • Behavioral thermoregulation essential
Ecological and Physiological Significance
- Energy Budgets: Determine food requirements and foraging time needed to meet metabolic demands
- Life History Strategies: High metabolic rates associated with shorter lifespans and earlier reproduction
- Geographic Distribution: Metabolic constraints limit species ranges (Bergmann's rule)
- Population Dynamics: Energy availability affects population density and carrying capacity
- Climate Change: Rising temperatures affect ectotherm metabolic rates and energy demands
- Conservation: Metabolic rates help estimate food requirements for captive and wild populations
- Comparative Physiology: Understanding adaptations to different environments and lifestyles
Measurement Techniques
Direct Calorimetry
Measures heat production directly in a calorimeter chamber. Most accurate but expensive and impractical for large animals or field studies.
Indirect Calorimetry
Measures O₂ consumption and CO₂ production. Most common method. Uses respirometry equipment (metabolic chambers, gas analyzers). Can be applied in lab and field.
Doubly Labeled Water (DLW)
Gold standard for measuring FMR in free-living animals. Injects isotopes (²H and ¹⁸O), then tracks elimination rates. Non-invasive but expensive.
Heart Rate Telemetry
Correlates heart rate with metabolic rate using calibration curves. Allows continuous monitoring in free-ranging animals via radio transmitters.
Important Considerations
- Standard Conditions: BMR requires specific conditions - post-absorptive, resting, thermoneutral zone
- Scaling Exponents: The 0.75 exponent is empirical; mechanistic explanations remain debated
- Taxon-Specific Equations: Different groups have different normalization constants and exponents
- FMR Variation: Field metabolic rates vary 2-10× among species of similar mass
- Torpor and Hibernation: Some endotherms can reduce metabolic rate 90-95% to conserve energy
- Surface Area vs. Mass: Historical debate - Rubner's surface law vs. Kleiber's 3/4 power law
- Phylogenetic Effects: Closely related species may deviate systematically from general equations
References
- Kleiber, M. (1947). "Body size and metabolic rate." Physiological Reviews, 27(4), 511-541.
- Schmidt-Nielsen, K. (1984). "Scaling: Why is Animal Size so Important?" Cambridge University Press.
- McNab, B. K. (2002). "The Physiological Ecology of Vertebrates: A View from Energetics." Cornell University Press.
- Nagy, K. A. (2005). "Field metabolic rate and body size." Journal of Experimental Biology, 208(9), 1621-1625.
- White, C. R., & Kearney, M. R. (2013). "Determinants of inter-specific variation in basal metabolic rate." Journal of Comparative Physiology B, 183(1), 1-26.
Recommended Calculator
Casio FX-991ES Plus
The professional-grade scientific calculator with 417 functions, natural display, and solar power. Perfect for students and professionals.
View on Amazon