Dogs Chase Efficiently, But Cats Skulk Counterintuitively
Science Daily
December 4, 2008
ScienceDaily (Dec. 4, 2008) — A Duke University study suggests that evolution can behave as differently as dogs and cats. While the dogs depend on an energy-efficient style of four-footed running over long distances to catch their prey, cats seem to have evolved a profoundly inefficient gait, tailor-made to creep up on a mouse or bird in slow motion.
"It is usually assumed that efficiency is what matters in evolution," said Daniel Schmitt, a Duke associate professor of evolutionary anthropology. "We've found that's too simple a way of looking at evolution, because there are some animals that need to operate at high energy cost and low efficiency."
Namely cats.
In a report published online Nov. 26 in the research journal Public Library of Science (PLoS), Schmitt and two former Duke co-researchers followed up on a scientific hunch by measuring and videotaping how six housecats moved along a 6 yard-long runway in pursuit of food treats or feline toys.
Long-distance chase predators like dogs can reduce their muscular work needed to move forward by as much as 70 percent by allowing their body to rise and fall and exchanging potential and kinetic energy with each step. In contrast, the maximum for cats is about 37 percent and much lower than that in a stalking posture, the report found.
"An important implication of these results is the possibility of a tradeoff between stealthy walking and economy of locomotion," the three researchers wrote in PLoS. "These data show a previously unrecognized mechanical relationship in which crouched postures are associated with changes in footfall pattern, which are in turn related to reduced mechanical energy recovery."
In other words, they found that when cats slink close to the ground they walk in a way that "the movements of their front and back ends cancel each other out," Schmitt said. While that's not good for energy efficiency "the total movement of their bodies is going to be even and they'll be flowing along," he added.
"If they're creeping, they're going to put this foot down, and then that foot down and then that one in an even fashion. We think it has to do with stability and caution, Schmitt said."
Walking humans recover as much energy as dogs, said Schmitt, who studies gaits of various mammals. "Our centers of mass rise and fall when we walk. And when we do that, humans and other animals exchange potential and kinetic energy. It's an evolutionary miracle in my view.
"But cats need to creep up on their prey. Most scientists think that energetic efficiency is the currency of natural selection. Here we've shown that some animals make compromises when they have to choose between competing demands."
The study was supported by the National Science Foundation. Kristin Bishop, a former postdoctoral researcher at Duke, was the lead researcher and first author. Another author was Anita Pai, a former Duke undergraduate who is now a medical student at Vanderbilt University.
While the dogs depend on an energy-efficient style of four-footed running over long distances to catch their prey, cats seem to have evolved a profoundly inefficient gait, tailor-made to creep up on a mouse or bird in slow motion. (Credit: iStockphoto)
http://www.nsf.gov/awardsearch/showAward.do?AwardNumber=0452217
NSF Org:
BCS
Division of Behavioral and Cognitive Sciences
Initial Amendment Date:
March 11, 2005
Latest Amendment Date:
May 5, 2008
Award Number:
0452217
Award Instrument:
Continuing grant
Program Manager:
John E. Yellen
BCS Division of Behavioral and Cognitive Sciences
SBE Directorate for Social, Behavioral & Economic Sciences
Start Date:
March 15, 2005
Expires:
February 28, 2009 (Estimated)
Awarded Amount to Date:
$119660
Investigator(s):
Daniel Schmitt daniel_schmitt@baa.mc.duke.edu (Principal Investigator)
Sponsor:
Duke University
2200 W. Main St, Suite 710
Durham, NC 27705 919/684-3030
NSF Program(s):
PHYSICAL ANTHROPOLOGY
Field Application(s):
Program Reference Code(s):
OTHR, 0000
Program Element Code(s):
1392
ABSTRACT
The origin of our Order-Primates-is believed to be associated with a fundamental change in locomotor behavior in response to the mechanical requirements of moving and foraging on thin flexible terminal branches of the forest canopy. The quadrupedal walking of primates differs from other mammals in footfall sequence, limb and joint excursions, muscle recruitment, and peak load distribution. Although these data already suggest that primates are unusual in their walking patterns, recent research has suggested a more fundamental way in which the walking of primates differs from nonprimate mammals.
During walking in most animals, the center of mass (COM) moves up and down as if it on the end of a stiff upside-down pendulum. By moving their center of mass up and down with appropriate timing during walking, most animals can conserve muscular energy through the exchange of potential and kinetic energy. In contrast, during running the COM moves like a bouncing ball and energy storage and recovery occurs in tendons and muscles. Virtually nothing is known about the movements of the COM in quadrupedal primates, but some research has suggested that primates use unusually compliant walking gaits. Such walking gaits involve deep elbow and knee yield, suggesting that the COM follows a flatter path with minimal exchange of potential and kinetic energy. If this is the case then this would represent a fundamental shift in the locomotor mechanics of primates compared to other mammals. However, no direct empirical data exist describing the movement of the COM in primates.
In this study five primate species (two prosimians, one New World monkey, and two Old World monkeys) and one nonprimate species (domestic cat) will be studied walking on terrestrial and, for the primates, arboreal supports across a force platform. Force data will be used to determine vertical and fore-aft accelerations, velocities, and displacements of the COM of the whole body and the forequarters and hindquarters. From these values the oscillations of kinetic and potential energy as well as the percentage of energy recovery can be calculated.
This study tests the hypothesis that walking gaits of primates are unusual among mammals in that they do not conform to an inverted pendulum model. This potential difference is not a trivial mathematical distinction nor is it simply a difference of degree but rather an abrupt adaptive shift. Understanding the mechanics of walking in primates will shed light on the origins of primate locomotion and the origin of specialized locomotor behaviors such as arm-swinging and bipedalism.
Aside from improving our understanding of primate locomotor evolution, this project benefits scientific education at two levels. This project will directly involve undergraduate and graduate students with a strong emphasis on female and minority student participation. In addition, the research techniques, data, and results developed from this project will be incorporated into general anthropology courses and advanced graduate and undergraduate courses.
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