SANTA CRUZ, CA -- As any swimmer knows, moving through water is nothing like moving on land. When the ancestors of modern marine mammals first ventured into the ocean some 60 million years ago, they had to adapt to a medium 800 times denser and 60 times more viscous than air. The spectacular success of their descendants illustrates the remarkable power of natural selection.
According to a new study comparing the athletic abilities of different types of animals, modern marine mammals are so well adapted to aquatic life that they are as efficient in swimming as specialized land mammals are in running. Terrie Williams, an associate professor of biology at the University of California, Santa Cruz, found that elite animal athletes, from horses to killer whales, achieve an optimal efficiency for locomotion that is determined more by their basic mammalian physiology than by their mode of transportation.
"The bottom line is that terrestrial and marine mammals expend similar amounts of energy to live and move in their respective environments," said Williams, who has been studying the exercise physiology of terrestrial and marine mammals for over 20 years.
In an article published this month in the Philosophical Transactions of the Royal Society of London, Biology, Williams presented a comprehensive analysis of the energetic costs of locomotion in terrestrial, aquatic, and semiaquatic mammals. She also extended her comparisons to include flying mammals (bats), as well as fish and birds.
Her findings indicate that for specialized mammals, whether they run across the plains, swim through the oceans, or fly across the evening sky, the energetic cost of moving through the environment is about the same. Some may achieve higher speeds than others, but their efficiency -- the amount of energy required to move a set distance -- appears to be constrained by a physiological limit for the mammalian way of life.
Energy requirements do vary with an animal's size due to metabolic factors. A swimming grey whale, for example, expends less energy per pound than a bottlenose dolphin. But Williams was surprised to find that the relationship between body mass and energy use is the same for mammals that run, swim, and fly, whereas mammals, fish, and birds show distinct differences.
Williams analyzed physiological data from a wide variety of animals, drawing on her own research for some animals, such as dolphins, and on published data for others. She based her comparisons primarily on calculations of each animal's "total cost of transport." The cost of transport for an animal is like the miles-per-gallon rating of a car, she explained.
"If you think of a cheetah as a BMW, this paper shows that if you put that BMW motor on a streamlined boat, it would use the same amount of gas to move a mile in the water as it did on land," Williams said. "The trick is the chassis has to be adapted to the environment, and that means specializing."
Semiaquatic animals that try to have it both ways, like muskrats and mink, pay a high price for their versatility, sacrificing energetic efficiency for the ability to move back and forth between land and water. Williams found that the energetic cost of swimming for semiaquatic mammals is two-and-a-half to five times higher than for fully aquatic marine mammals.
These higher costs would also have applied to transitional species in the evolutionary lineages that led to modern marine mammals. The fossil record includes primitive cetaceans (ancestors of whales and dolphins) and archaic pinnipeds (seals and sea lions) with skeletal characteristics suggesting a semiaquatic lifestyle.
The ancient oceans must have offered significant advantages to these transitional mammals, because the energetic costs associated with the move from land to water would have been high, according to Williams. Other researchers have cited an abundance of food in the marine environment and reduced competition from other predators as factors that may have made it worthwhile for the ancestors of marine mammals to venture into the aquatic realm. In addition, it is likely that these animals limited the high energetic costs of swimming by jumping in and out of the water, the way modern mink and river otters do, Williams noted.
The sea otter is an unusual case because it has the body of a semiaquatic mammal, but spends most of its time in the water. Williams views sea otters as living on an "evolutionary edge," meeting their energetic requirements by such a slim margin that they are highly vulnerable to environmental perturbations, such as oil spills and coastal pollution.
"In order to make it in the marine environment, these mammals have to spend inordinate amounts of time grooming a fur coat," Williams said. "They don't have insulating blubber like other marine mammals, so to counterbalance the heat loss they have an extraordinarily high metabolism. They eat 25 percent of their body weight in food every day and have to eat every few hours. Overall, there is very little margin of safety for the sea otter."
Williams includes humans in the semiaquatic category, too, although only the best human swimmers (trained athletes) are able to swim as efficiently as a sea otter. In fact, Williams said, her curiosity about how mammals swim began when she worked as a lifeguard in college and was struck by the poor swimming abilities of humans.
"I began studying a wide variety of mammals to find out just what makes an efficient swimmer. By comparing semiaquatic mammals with highly adapted marine mammals, I could follow the physiological and morphological trends that lead to swimming proficiency," she said.
Williams's research included gathering physiological data from trained dolphins swimming alongside a moving boat. She has also used video cameras strapped to the backs of seals and dolphins to view their swimming dynamics while portable monitors recorded their heart rates.
When she compared different species of highly adapted marine mammals, Williams found that swimming style has relatively little effect on the cost of transport. Sea lions use their fore-flippers to propel themselves, while seals use their hind flippers and dolphins and whales use a distinctive undulatory motion, but they all achieve comparable energetic efficiencies. Other researchers have made similar observations regarding the performance of two-legged and four-legged runners.
Comparing marine mammals with fish, Williams found that the overall cost of transport is higher for swimming mammals than for fish, primarily because mammals, being warm-blooded, have to expend energy to maintain their body temperature. When it comes to the energetic cost of swimming by itself, disregarding the baseline costs of maintaining a mammalian physiology, dolphins and sea lions swim just as efficiently as salmon.
To round out the analysis, Williams also took a look at flying creatures. The four species of bats she examined showed transport costs comparable to those of other mammals. For birds, however, transport costs fall in between those for mammals and fish.
It is not clear why the cost of swimming in fish should be lower than the cost of flying in birds, while both are energetically cheaper than running in mammals, Williams noted. But the fact that mammalian specialists in all three modes of locomotion have essentially the same transport costs suggests a physiological explanation. For instance, the respiratory systems of fish, birds, and mammals may differ in the efficiency with which they are able to deliver oxygen to muscles.
"For years, researchers have been focusing on the differences between runners, flyers, and swimmers," Williams said. "This study is exciting because it highlights the similarities between mammalian athletes of all kinds."
Materials provided by University Of California, Santa Cruz. Note: Content may be edited for style and length.
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