It is a truism in biology that smaller animals have higher metabolic rates than larger ones. Think of the shrew, and other such animals, that have to eat a large percentage of their body weight in order to survive. Recent research has qualified this picture somewhat. Researchers at the University of California have performed an extensive study and come up with some interesting results. A little background from Science Daily:
"...all living organisms have to transport energy obtained from food they eat to support the working of their internal organs, such as the brain or heart. The larger the organism, the further away are the organs from the body surface. “This makes energy supply more and more difficult for larger organisms,” he said. “For example, bacteria have to transport the obtained food over less than one micron, which is their body length, while the distance between an elephant's trunk and an organ such as its brain or heart is about ten million times longer."
With that in mind, the researchers compared metabolic rates in various groups of organisms ranging from bacteria (over 80 species were included) to elephants to test the idea that smaller animals are more metabolically active than larger animals:
But a new study led by Bai-Lian Li, professor of ecology at UC Riverside, shows that this is true only for organisms that are closely related evolutionarily and have body masses differing by no more than 6-7 orders of magnitude – about the difference in body mass between an elephant and a shrew.
For a pair of organisms that don't meet these conditions, that is, organisms that are not closely related evolutionarily and whose body mass difference exceeds the 6-7 orders of magnitude range, the researchers find that the small organism consumes about the same amount of energy per unit mass as the large organism: 1-10 watts per kilogram of body mass in the resting state of the organisms.
Since bacteria and elephants are not closely related, in evolutionary terms, and since an elephant is more than 6-7 times the size of bacteria:
"...elephants, ..., appear to be capable of supplying their tissues at a rate similar to that of tiny bacteria."
Even more interesting:
The researchers’ analysis also shows that the rate of energy consumption per unit body mass declines with growing body size in groups of evolutionarily close organisms, such as mammals. For example, one gram of an elephant’s body uses up 25 times less energy than does one gram of a shrew’s body, accounting for why shrews have to eat more often than elephants. On the other hand, a bacterium, which is not closely related to an elephant in an evolutionary sense, consumes approximately the same energy per unit body mass as the elephant.
Which may help explain trends towards large size in a lot of lineages throughout evolutionary history.
Finally, this is the most fascinating part:
So far, life scientists have held the view that the properties of living organisms are shaped by the changing external physical environment to which the organisms must continuously adapt. The new study posits, however, that living organisms are able to overcome the physical limitations imposed on them by their own physical properties and their external environment in order to maintain optimal, biochemical characteristics, such as the mass-specific metabolic rate the researchers studied.
Although, I'm not sure if the bolded sentence isn't just an alternative way of phrasing the first sentence. If organisms are continuosly adapting to the changing physical environment aren't they overcoming the physical limitations imposed on them by their own physical properties and their external environment in order to maintain optimal, biochemical characteristics?