A New Look at an Old Fable:The Tortoise and the Hare.
August 31, 2018
We all know Aesop’s iconic fable "The Tortoise and the Hare.” Aesop tells of a race between a fast, but often-distracted hare and a slow but relentless tortoise. The tortoise wins because of his steady determination, with the lesson being that “slow and steady wins the race.” True, readers might be surprised by the outcome. But according to a new analysis by Adrian Bejan, a professor of mechanical engineering at Duke University’s Pratt School of Engineering, it’s really not surprising at all.
“The fable of ’The Tortoise and the Hare’ is a metaphor about life, not a story about a race,” said Bejan. “We see in animal life two starkly different lifestyles—one with nearly steady feeding and daily sleep and another with short bursts of intermittent feeding interspersed with day-long siestas. Both of these patterns are the rhythms of living that Aesop taught.”
In a study published in the journal Scientific Reports, Bejan analyzed the reported speeds of animals based on land, air and water. The results show that some of the world’s fastest animals are actually some of the slowest when their movements are averaged throughout their lifetimes. Bejan developed what he calls “constructal theory” to explain basic characteristics of locomotion for every creature -- how fast they get from one place to another and how rapidly and forcefully they step, flap or paddle in relation to their mass.
Essentially, the theory says that animal locomotion is no different than other flows, both animate and inanimate: they all develop in space and in time such that they optimize the flow of material. That means that from simple physics and based only on gravity, density and mass, you can explain within an order of magnitude many features of flying, swimming and running. It doesn't matter what type of animal it is or how many legs it has or even if it swims; the basic characteristics can be explained.
In the case of animal locomotion, this means that animals travel the greatest distance while expending the least amount of energy. In fact, constructal theory shows that all animals’ speeds tend to increase along with their body mass and adhere to a similar ratio. For example, the stride frequency of running vertebrates bears the same relationship to the animals' mass as does the rate at which fish swim. Similarly, the velocity of runners conforms to the same principles as the speed of birds in flight.
“When I give speeches on this topic, somebody always brings up outliers such as the cheetah, which is the fastest land mammal,” said Bejan. “But this study shows ‘outliers’ are to be expected and, when looked at over their lifetimes, are not so different from their lumbering cousins after all, because the cheetah spends a lot of its time resting.”
So the fastest animals are neither the biggest nor the fastest over a lifetime. And over the long run, the race does indeed go to the slower steadier animals.
Frank Graff is a producer/reporter with UNC-TV, focusing on Sci NC, a weekly science series that airs Tuesdays on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!