One if by Land,
Two if by Sea

In the race
for mammal efficiency,
the results are in, 
and it’s a tie.

 By Mark Schrope

 Illustrated by Kathleen McKeehen

   A dolphin’s travel through the water is, by all accounts, a beautiful sight. Their seemingly effortless grace might lead you to conclude that a dolphin could not be better suited to its task. To some extent you’d be right, or so says Terrie Williams, a researcher in California.

    In January of this year, a landmark paper written by Williams, a biologist at The University of California, Santa Cruz, was published in the prestigious Philosophical Transactions of the Royal Society of London. She combined the results of her 25 years of research on the efficiency of swimming mammals with findings from other researchers on running and flying mammals to challenge a long-held assumption of biology. Her conclusions also add to our understanding of mammal evolution.

     Williams’ research involves a specific measure of how efficiently an animal is operating as it travels called the total cost of transport. This measure refers to the energy an animal is using in terms of calories divided by the distance it goes. It helps to think of calories as gasoline—Williams’ work revolves around the gas mileage achieved by numerous mammals.

     For many years scientists have known that fish are the most efficient travelers, followed by birds in flight, then running animals such as the horse. According to McNeill Alexander, a biologist at the University of Leeds in England who pioneered the study of animal movement, biologists generally assumed that this rule applied to mammals—that a swimming dolphin would be more efficient than either a flying bat or a running cheetah.

     Williams is the first person ever to test that assumption by comparing the efficiencies of the championship athletes of the mammal world—those evolved to specialize in one form of travel, be it swimming, flying, or running. What she found was that the hierarchy of travel efficiency for the animal world as a whole did not hold for mammals. Instead, Williams’ paper presents her remarkable discovery that champion swimming, running, and flying mammals have about the same high level of efficiency. This is particularly remarkable given that they have varied speeds and use different percentages of their energy meeting the two basic needs of mammals while traveling—keeping warm and moving limbs. So, a seal and a cheetah of the same weight will go about the same distance on 1,000 calories. Williams puts that another way. “If you think of a cheetah as a BMW,” she says, “this research 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.”

     In her paper, she goes on to speculate just why it is that all three types of mammals have ended up with basically the same efficiency. Her suggestion is that they have reached an evolutionary peak—the bodies of the specialized mammals can’t evolve to travel any more efficiently than they already do. Some researchers disagree with this speculation and say that what she has found is nothing more than a coincidence. As with any new idea, particularly one that is difficult to prove or disprove, the debate is likely to continue for some time.

     Williams’ research has not been confined to champion marine mammal athletes like dolphins. Over the course of her career she has worked with all kinds of mammals that get themselves soggy on a regular basis. This includes mammals such as the mink or sea otter that are considered semi-aquatic. The distinction between semi-aquatic and marine mammals is a little fuzzy. Though some will be angrier than others, just about any land animal thrown into the water will swim back to shore. And some specialized mammal swimmers like sea lions, though awkward on land, can get around using their body and flippers. Minks and sea otters, on the other hand, have legs that allow them to walk fairly easily on the land. Their bodies are not streamlined for swimming, but they spend a large amount of time in the water. Unlike fully marine mammals, which can migrate great distances across oceans, the semi-aquatics tend to stay closer to shore.

     Because semi-aquatic mammals live in a sort-of evolutionary limbo between land and sea, spending time in both but not fully adapted to either, Williams feels they can help us understand how marine mammals evolved from land mammals.

      Williams, it seems, was destined to spend her life studying how mammals move. As a five-year-old paging through National Geographic, she remembers being mesmerized by a series of photographs showing leopards kicking up a cloud of dust as they tore a baboon apart. “The thing that struck me more than anything was the power and motion of the animals,” she says of the photos she has kept now for almost 40 years.

     That fascination with movement grew through the years. As a teenage lifeguard she watched swimmers with the same interest she had as a child for the magazine photos. She recalls thinking that, unlike the leopards, the swimmers looked awkward as they made their way back and forth across the pool. “I realized that humans were just pathetic in the water,” she says, and she wanted to figure out why.

     At first, Williams thought her fascination with mammal motion could best be pursued studying human physiology. She attended medical school at Rutgers University in New Jersey but became disenchanted within a year. The illness and poverty she witnessed while working in the public hospital there was so grim that she concluded there was nothing enjoyable about the work and left.

     She decided it was the physiology she was most interested in, not people, so she turned her attentions to animals that were a bit less depressing to work with. She grew up in Norfolk, Virginia, surrounded by rivers and the Chesapeake Bay, as well as the Atlantic Ocean a few miles east. Her family’s favorite activities from swimming to fishing centered around water, so it was no surprise that she decided to focus her attention on aquatic mammals.

     After leaving medical school, Williams stayed at Rutgers but switched to the biology department where she did her doctoral research on minks. Williams explored whether the minks’ ability to get around in water and on land made them good at both, one, or neither form of travel. One way to answer that question would be to look at the minks’ total cost of transport when swimming or running. Though that is a measure of how many calories they use to go a given distance, the exact number of calories an animal takes in as food is difficult or impossible to measure. Even if you could calculate those calories, you wouldn’t be able to tell exactly how many of them were spent on a specific activity during an experiment. The amount of oxygen, however, can be measured directly during experiments. Because animals need a specific amount of oxygen to put a given number of calories to use, Williams used oxygen measurements as a gauge of the minks’ efficiency.

     Williams’ conclusion? “They do everything crummy.” Like a four-wheel-drive truck, they can go just about anywhere but they pay a high price for this versatility—horrible efficiency. Williams’ later research on other semi-aquatic mammals such as sea otters has confirmed this conclusion as a general rule for semi-aquatic mammals. She has found that these mammals use up to five times as much energy as the specialized mammals to get around.

     Williams says that humans can be thought of as semi-aquatic mammals in one sense. Although any mammal will swim if forced to, humans enter the water on a regular basis by choice. Of course, we do even worse than the real semi-aquatics. The same amount of energy that carries a dolphin 60 miles will take us a whopping 5 miles. In terms of efficiency, we do best when we stay dry. Having evolved for travel on land, that’s where we can compete as specialized athletes in the mammal world. Our total cost of transport while running is similar to that of a champ like the cheetah.

     The problem with jumping from land to water is that limbs like legs, which propel creatures efficiently on land, are terrible for travel in the water and vice-versa. The body that can do double-duty reasonably well has to be a compromise, so it uses more energy at either form of travel than do the specialists. “That is why being a triathlete is so bloody difficult,” Williams says.

     Her conclusion that semi-aquatic mammals are terribly inefficient got her thinking about the evolutionary history of the marine mammals. Scientists believe that mammals first evolved on land but that some, the ancestors of dolphins and whales, started the transition to marine mammals about 60 million years ago. The first step would have been for some land mammals to start braving swampy waters near shore seeking food that was more plentiful or easily obtained there. Little by little, over the course of millions of years of natural selection, some of these animals would have become better adapted to moving in the water, allowing them to spend more time there.

     In simple terms, natural selection means that animals with adaptations that give them an advantage over others of their species will survive longer, and so have more babies. Thus, a helpful adaptation can in time become common to the species. The source of these adaptations is random changes that crop up in an animal’s genetic code that lead to some new trait.

     The idea is that the evolving characteristic, such as an animal’s hearing, improves because such adaptations increase the chance of survival. Better hearing might help an animal detect an attacker sooner and escape, leaving another animal to be the attacker’s dinner.

     In the case of animal travel, two potential improvements would be to get faster or to increase efficiency. But switching from travel on land to swimming provides some difficult challenges. Water is much denser than air, making it harder to move. It also sucks heat away from the body faster than air, making it harder to stay warm. Of course, life on land is no picnic either. There an animal constantly battles gravity, unlike the swimmer suspended in water. This suspension is good news for a streamlined animal, such as a dolphin, which can overcome water’s density, but what about those ancient land mammals dabbling in the wet life?

     Most scientists are used to thinking of evolution as a process that results in obvious improvements in the characteristics or efficiency of an animal’s body. For instance, some light-colored moths evolved to dark to blend better with dark trees and avoid being seen. The tuna’s tail has improved in increments through evolutionary time to become the ideal propeller. This sped tuna up to catch more food while avoiding becoming food themselves. But the transition of land mammals to water would have meant a drastic loss of efficiency, says Williams. Taking today’s semi-aquatic mammals as a model for the ancient transitional ones, Williams thinks travel in water would have been about as efficient as taking your car for a spin in a lake.

     At first glance it appears that, in terms of natural selection, mammals never should have taken the path to aquatic travel because it would have meant giving up an advantage in efficiency. “This situation meant overcoming a hurdle. Things were not getting progressively better, things got worse,” Williams says.

     Frank Fish, a professor at West Chester University in Pennsylvania who also studies mammal movement and evolution, says Williams raises a good question: “If you are compromising yourself, how do you manage? What is the advantage?” But if the move to water was a true disadvantage, those animals would not have survived. Williams’ solution to this puzzle is simple. “You have to rethink what is being optimized,” she says. Although the physical cost of moving around in the water was probably higher than on land for the transitional mammals, a food advantage would have balanced this out.

     Once mammals made it into the water, evolution appears to have proceeded in the way scientists commonly think—with adaptations leading to faster, more efficient swimming. Having shed new light on the evolutionary path of marine mammals, Williams went on to study where that path led. She applied the techniques she developed for minks on specialized swimmers such as seals and dolphins to see how much efficiency they had evolved.

     To determine the efficiency of swimming seals, Williams had the animals swim against flowing water in a tank, while she measured their oxygen consumption. She took measurements with the animals swimming at a variety of speeds. The speed at which the seal swam most efficiently was used to determine its total cost of transport for swimming. Williams compares this level of peak efficiency to a car getting its best gas mileage at sustained highway speeds. By establishing this peak efficiency for all the mammals she has studied, Williams is able to make comparisons between them.

     Dolphins are harder to study than the seals because researchers can’t make the water in a tank flow fast enough to challenge them, Williams says. She had to invent a new way to measure their oxygen use while allowing them to swim freely. The system she developed involves using controlled experiments to measure how much oxygen dolphins use at a given heart rate while resting or exercising with their snout against a rubber pad (see illustration). She then attaches a heart rate monitor to the dolphins when they are in the open ocean. Oxygen use can be estimated by comparing the heart rates of dolphins swimming freely against those achieved during the controlled experiments. The dolphins’ peak efficiency includes the high-speed jumps, called porpoising, for which they are famous. Williams says these maneuvers are a more efficient way for fast-swimming dolphins to get a breath of air. When they just come to the surface for a breath, the drag there makes them work harder than if they leap for a breath.

     After years of gathering this kind of total cost of transport data, Williams started comparing her results for different mammals. She found that the amount of energy used by a seal and a dolphin of equal size while swimming was about the same. They had both evolved to about the same level of efficiency.

     Intrigued, Williams wondered what would happen if she compared efficiencies for other mammals against those of dolphins and seals. By searching through journals and speaking to other researchers, she assembled comparable data from a zoo of other mammals, including runners from rodents to elephants, flyers like bats, and other marine mammals such as whales. Wiliams found that the specialized mammal runners, flyers, and swimmers had all evolved to reach the same efficiency, though they travel at different speeds. “That was a real shocker,” she says, given the previous assumption by scientists that the general rule of swimming being the most efficient form of travel followed by flight, then running, would apply to mammals. Instead she says, “It looks like a mammal is a mammal is a mammal.”

     This relationship between the different forms of travel is not simply a matter of all mammals using the same amount of energy to make their fins, legs, or wings go. Making those limbs move is only one way mammals use energy. Because they are warm-blooded, they also use a fair amount of energy just to keep warm. This is called a maintenance cost.

     The amount of energy needed for maintenance and travel costs can be very different in different places. Water carries heat away from an animal’s body so much faster than air that mammal swimmers use nearly three times more energy to stay warm compared to runners and flyers. On the other hand, swimmers are not fighting gravity as much, so it takes less energy for the streamlined swimmers to move their limbs and body.

     So each form of travel has essentially one advantage and one disadvantage. The amazing thing, Williams discovered, is that despite the great range in energies spent on maintenance and travel costs, different mammals still managed to end up with that same efficiency. So the ancient mammals that successfully entered the sea and began the process of specializing as swimmers eventually came right back to the same efficiency they had on land.

     Williams thinks this information shows that the level of efficiency reached by the specialized mammals represents an evolutionary peak, which means that there is not really any room for improvement in travel efficiency given the constraints of a mammal body. “It is not just coincidental,” agrees Fish, the Pennsylvania biologist. “It means there is some overriding physical constraint that dictates how much energy a [mammal] is going to spend to move.”

     One possible explanation for why mammals have hit this peak is that it is a mechanical limitation. But, if this were the case, one would expect air, land, and water travel to lead to different evolutionary peaks.

     Because all three forms of travel have led to the same peak for mammals, Williams believes the limit is imposed by physiology, something all three forms of travel have in common. Her suggestion is that the lungs are the source of the limitation. This conclusion is where, she admits, she is “most out on a limb.”

     "I think I’m known for having a little creative interpretation,” she says. “That doesn’t necessarily mean it’s right. I’d just rather have fun trying to speculate.”

     Williams’ conclusion wasn’t just an arbitrary guess. All bodily functions of animals are indeed driven by oxygen. Williams suggests that because mammals’ lungs are in charge of passing oxygen into the bloodstream, where it can be used to supply the muscles, the limitation probably stems from the lungs’ ability to do their job.

     Other researchers have determined that fish gills are the most efficient at taking in oxygen, followed by the air sacs in birds, and finally the lungs of mammals. Supporting Williams’ theory, the efficiency in the animal world follows the same pattern. Fish says that while her explanation will be difficult to prove it is a “seed of an idea” that could answer the question of why specialized mammals seem to have reached a wall in terms of efficiency. “I think her ideas are really bearing fruit and being taken notice of all over,” he says. “The work she has done is just fantastic.”

     Alexander, the British mammal movement pioneer, is one who has taken notice. He says the results of Williams’ research are intriguing, and notes that they will interest many biologists. However, he just wrote an analysis of her research for the journal Nature, in which he disagrees with her conclusion that mammals have hit an evolutionary peak because of their lungs. Instead he feels that the only safe conclusion is that the similarities between efficiencies for different forms of travel in mammals are a coincidence.

     Alexander offers little explanation for his rejection of the lung idea. He does say the conclusion that the cost of transport is about the same for bats as it is for mammal runners and swimmers is wrong. His reasoning is that the efficiency of bats is about as close to birds as it is to the other specialized mammals. Williams agrees that because bats fall between birds and the rest of the mammals, one could compare them in either direction. She says that at one point she considered only comparing runners and swimmers in her paper, but decided the bats were close enough to make the comparison interesting and worth considering.

     As with any idea about evolution, Williams’ conclusions can never be fully tested without a time machine or a multimillion-year attention span. A reasonable next step will be for other researchers to see if what Williams has learned about mammals is a universal phenomenon. One of the best ways to do that will be to look at other animals with the same oxygen delivery system but different modes of travel.

    Scientists think bird evolution was similar to that of mammals in that they began as runners before evolving to specialize in a new form of travel. Because there are still bird runners, an interesting comparison could be made. The ostrich, an efficient runner without flight capability, is so much larger than most birds that comparisons become messy. The New Zealand kiwi bird, however, is a runner with a size similar to the average bird. Williams is hoping that bird researchers will eventually make the same comparison among birds that she made among mammals. “I’m dying to see a kiwi on a treadmill,” she says, referring to the method used to measure the cost of transport for runners. If the same pattern of similarity in the total cost of transport were found, it would lend strong support to Williams’ idea that oxygen delivery systems can determine what the peak efficiency of an animal will be.

     Williams says that other than Alexander’s commentary, response to her research has been positive. As more scientists become aware of her research, new discussions will be sparked. Her ideas throw a wrench in the way scientists have thought about evolution and the hierarchy of efficiency for more than 25 years. It could take years for these ideas to be fully accepted even if further research supports them. Williams is patiently excited about the debate process and research to come. As she says, “I can’t even say where it’s going to go.”

 

BIOs

 

WRITER Mark Schrope

B.S., biology, Wake Forest University; M.S., Chemical Oceanography, Florida State University.
Internship: Popular Science magazine, New York.

ILLUSTRATOR Kathleen McKeehen

B.A., English, University of WA, 1974.
Internship: Curtis Botanical Magazine, Kew Gardens, United Kingdom.