The metabolic theory posits that physical constraints on energy uptake and allocation drive biological processes. This theory predicts broad ecological patterns such as the observed allometric scaling relationship between animals’ metabolic rate and body size.
Monash University scientists have challenged the conventional wisdom that physical constraints explain biological patterns. They created a new mathematical model of animal growth that describes how animals devote energy to growth and reproduction as they age and increase in size.
Lead study author Professor Craig White from the Monash University School of Biological Sciences and the Center for Geometric Biology said, “Despite the fact that living organisms cannot break the laws of physics, evolution has shown itself to be extraordinarily adept at finding loopholes.”
“An unexplained problem in biology concerns the non-proportional (allometric) relationship between energy metabolism and size.”
“Finding that an animal’s metabolism can be explained without invoking physical constraints means that we’ve been looking in the wrong place to find answers for why this widespread pattern occurs.”
“We believe that physical constraints don’t drive as much of the biology we observe as previously supposed, and that evolution has a wider range of options than previously thought.”
During growth or evolution, a gain in size is often followed by a less-than-proportional rise in energy needs so that more giant creatures consume less food and expend less energy than smaller ones.
For example, small mammals such as shrews might need to consume as much as three times their body weight in food each day, whereas the most giant—baleen whales—eat just 5–30% of their body weight in krill each day.
Professor White said, “Our study argues against the conventional wisdom that biological patterns such as allometric scaling occur because of physical constraints.”
“We devised a mathematical model of animal growth that describes how animals shift their energy allocation from growth to reproduction as they increase in age and size, and show that lifetime reproduction is maximized when metabolism scales out of proportion with size.”
“Many models presented since the early nineteenth century have used physical or geometric constraints to explain this pattern, but ours does not. Simply put, classic theories argued that animals have the metabolism they have because they must; we find they have the metabolism they have because it’s the best.”
“The study showed that allometric scaling does not have to result from physical or geometric limits. Instead, natural selection, not physics, favors allometric scaling.”