As biologists strive to unravel the profound mysteries of life, we also tend to engage with physics. However, a new study published in the journal Science reveals that sometimes physics—the study of the material world—also “throws in the towel” when confronted with certain biological issues.
For centuries, scientists have questioned why, in terms of kilograms, larger animals burn less energy and require less food than smaller ones. Why does a tiny shrew need to consume food three times its body weight, while a gigantic whale can sustain itself on just 5-30% of its body weight in daily intake?
Why does a tiny shrew need to consume food three times its body weight?
While previous attempts to explain this relationship have relied on physics and geometry, scientists believe that the true answer lies in evolution. This relationship maximizes an animal’s reproductive capacity.
How Much Do Physical Constraints Shape Life?
The earliest explanation for the disproportionate relationship between metabolic rate and size was proposed nearly 200 years ago.
In 1837, French biologists Pierre Sarrus and Jean-François Rameaux argued that energy metabolism should scale with surface area rather than body mass or volume. This is because metabolism generates heat, and the amount of heat an animal can dissipate depends on its surface area.
Since Sarrus and Rameaux presented their ideas, numerous alternative explanations for the observed scaling of metabolic processes have been proposed over the past 185 years.
American researchers Geoff West, Jim Brown, and Brian Enquist published a model in 1997 that describes the transport of essential materials through networks of branching tubes, such as the circulatory system.
They argued that their model provides “a theoretical, mechanical basis for understanding the central role of body size in all aspects of biology.”
These two models are philosophically similar. Like many approaches proposed over the past century, they attempt to explain biological patterns by citing physical and geometric constraints.
Evolution is the Answer
Living organisms cannot defy the laws of physics. However, evolution has proven remarkably adept at finding ways to overcome physical and geometric limitations.
In their new study, biologists decided to explore what happens to the relationship between metabolic rate and size when such physical and geometric constraints are disregarded.
Living organisms cannot defy the laws of physics.
Thus, they developed a mathematical model of how animals utilize energy throughout their lifetimes. In this model, animals devote energy to early-life development and subsequently allocate an increasing amount of energy to reproduction as they mature.
The biologists used the model to identify which characteristics of animals lead to the largest reproductive output over their lifetimes. They found that the animals predicted to be the most successful at reproducing are those that exhibit precisely the asymmetric metabolic scaling with size observed in real life!
This discovery suggests that asymmetric metabolic scaling is not an inevitable consequence of physical or geometric constraints. Instead, natural selection shapes this scaling because it is favorable for lifelong reproduction.
American biologist Theodosius Dobzhansky once stated: “Nothing in biology makes sense except in the light of evolution.”
The scaling of metabolism can arise even in the absence of physical constraints, indicating that scientists may have been looking in the wrong place for explanations. Physical limitations may be less of a driving force in biological models than is often believed. The possibilities for evolution are broader than we appreciate.
Why have we historically been so ready to invoke physical constraints to explain biological phenomena? Perhaps it is because we feel more comfortable in the safe haven of seemingly universal physical explanations than in the relatively uncharted biological wilderness of evolutionary interpretations.
