Basal metabolic rate does not scale in proportion to body mass — it scales with mass raised to roughly the 3/4 power, an empirical relationship known as Kleiber's Law (1932):
BMR = a · M^0.75 (mammals, a ≈ 70 kcal/day)
BMR = a · M^0.72 (birds, a ≈ 78 kcal/day)
SMR(T) = a · M^0.76 · Q10^((T-20)/10) (ectotherms, a ≈ 4.9)
Dividing by mass gives the mass-specific metabolic rate a·M^(b-1) — since b−1 is negative, every kilogram of a shrew burns far more energy per day than a kilogram of an elephant. That single exponent explains why small endotherms must eat almost continuously while large ones can survive on relative scraps.
- Body mass slider — logarithmic, spans roughly 5 g to 6 t; moves the glowing marker along the log-mass axis of the plot and re-evaluates BMR from the active taxon's power law.
- Mammal / Bird / Ectotherm — swaps the (a, b) coefficients above. Only ectotherms answer to the temperature slider: mammals and birds are homeotherms, so their internal metabolic machinery is buffered against ambient temperature by thermoregulation.
- Ambient temperature — for ectotherms, metabolic rate follows a Q10 = 2.5 relationship: roughly a 2.5× change in rate per 10 °C, referenced to 20 °C. This is why a lizard's energy budget swings with the weather while a mammal's does not.
- Pulse rate — the glow/heartbeat animation frequency is driven by the classic allometric heart-rate relation HR ≈ 241·M^-0.25 bpm for mammals (Stahl, 1967), an analogous form for birds, and a temperature-scaled analogue for ectotherms — visualising that smaller, faster-burning bodies literally pulse faster.
- Surface-area law toggle — overlays the older (and wrong) 19th-century hypothesis that metabolism should track body surface area, M^0.667. Kleiber's actual M^0.75 line sits visibly above it at large body sizes, which is the historical reason the 3/4 exponent was considered surprising.
- Drag to pan, scroll/pinch to zoom — the plot is a real log–log axis; the mass axis runs left→right, the metabolic-rate axis runs bottom→top, and both are drawn to true logarithmic scale so slopes on screen equal the power-law exponents.
The reference species are positioned by their true published mass and metabolic rate on the same log–log axis — together they trace the same curve as the formula, exactly as Kleiber's original mouse-to-elephant plot did.