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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
A dolphin’s travel through the water is, by all accounts, a
beautiful sight. It’s 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.”
-
- BIO
-
- WRITER
Mark Schrope
- B.S., biology,
Wake Forest University; M.S., Chemical Oceanography, Florida State
University.
Internship: Popular Science magazine, New York.
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1999 Mark Schrope
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