Liza Shapiro, a physical anthropologist at The University of Texas at Austin, is trying to work out an evolutionary mystery.
The mystery: Why do primates who walk on all fours do it differently than most other animals who walk on all fours?
Primates who walk on all fours do it in a diagonal sequence. They put down a foot on one side and then a hand on the other side, continuing that pattern as they move along.
Most other mammals move in a lateral sequence, moving a foot and then a hand on the same side and then moving in the same sequence on the other side.
"We assume it evolved early (in primate evolution) but I want to know is it really unique to them?" Shapiro said. "Does it have to do with habitat? Does it have to do with the size at which they evolved?"
Shapiro, a professor in the Department of Anthropology, thinks it has to do with both: The arboreal habitat of early primates and the small size of those primates.
A hypothesis, called the fine branch niche hypothesis, is that the diagonal sequence pattern helped the first primates get out on thinner, flimsier branches of their arboreal habitat, where insects and fruit were to be had. It also was where bigger predators would be unable to follow.
In the most fundamental sense, the ability of early primates to live on fine branches might have been a key factor in their survival. Finding food and refuge on the outer branches would have helped them grow, multiply and eventually evolve into today's primates, including lemurs, monkeys, apes and humans.
The fine branch niche hypoethesis is established, but has not reached consensus among physical anthropologists. Shapiro thinks previous work in this area is missing a piece of the puzzle--primate body size relative to branch size.
Most explanations for the way primates walk have focused on maintaining balance in small branches, a problem if you are large relative to the branch. But if early primates were very small, the term "fine branch" takes on new meaning, and some of the old explanations for the way primates walk may need to be reassessed.
Shapiro is testing her small-size theory in the laboratory with the world's smallest primate, the mouse lemur; a small arboreal marsupial, the sugar glider; and a small terrestrial marsupial, the gray short-tailed opossum from Brazil.
She is testing the animals as they grow from babies to see how their size affects their ability to negotiate the fine branches.
The animals are small, even as adults weighing from 60 to 130 grams.
The animals walk on rods of various diameter (5 millimeters to 2.5 centimeters) and flexibility, to mimic how they might negotiate branches of trees.
In the early going, the experiment has produced an intriguing result.
"The most terrestrial of the three in my sample, the gray, short-tailed opossum, can move on a small branch as an infant, but has trouble as an adult," Shapiro said.
The opossum is not built for branch walking. It does not have grasping hands or feet, it has no nails and does not use the diagonal sequence gait.
"But simply because it's smaller, it can navigate a small branch," Shapiro said. "This implies to me that primate anatomical and locomotor features could have evolved in small branches, but the size of the branches per se may not be the key selective pressure leading to these features."
She also is testing whether the rod going up or down might play a role. In other experiments, Shapiro said that animals use a diagonal sequence gait to go up an incline, but use the lateral sequence gait to do downhill.
In the lab, Jesse Young, a postdoctoral student, stands at one end of the pole and releases the sugar glider and Shapiro waits at the other end to catch it.
The sugar gliders are gently awakened, placed on a pole on which they are expected to walk, scurry or scamper.
Shapiro and her students call the animals by name: Brigitte, Susan, Jack and Bill. She named them after her former professors at the State University of New York-Stony Brook.
The whole time four bright lights are shining for the four cameras that record the sugar glider's every move.
Shapiro and the students patiently work with the sugar gliders, coaxing them with sweet talk and fruit and mealworms and a cloth that has their scent on it to get them across the pole.
The sugar gliders will be outfitted with reflective markers and then videotaped. Then their movements will be digitized. That allows three-dimensional position points and quantification for detailed analysis of their movements.
Later, Shapiro will study the raw tape and the digitized, quantified version looking for the minutest detail of the sugar glider's stride. Which limb moves first? Which next? How far does the front limb extend? And more.
Yes, physical anthropologists are that detailed oriented. They make exact note of the distance between hip sockets, the length of strides, the placement of fore- and aft-limbs down to the millimeter.
And that attention to detail can pay off.
"Looking through the video frame-by-frame sometimes I get insights that I wouldn't get just from the numbers," Shapiro said.
From studying video of baby baboons, she noticed that they use a gait that adult baboons never do. She wrote a research paper on it.
Sometimes, she said, it helps to follow in her subjects' footsteps.
"Yes, I've been known to crawl around on my office floor," Shapiro said. "It is hard to get your mind around it sometimes because it's not just the sequence. There are subtle differences in the timing."
Shapiro's interest in quadrupedalism grew out of her earlier work on how primates differ in how they move and in their behavior since they shared a common ancestor.
"My (current) work on quadrupedalism takes a broader perspective in the sense that it addresses not how primates differ from one another, but how primates differ from other vertebrates," she said.
The National Science Foundation sponsors her quadrupedalism project with a $226,192 grant.
Shapiro's career started with a love of animals when she was young. She started college with thoughts of becoming a veterinarian, but she took a course in physical anthropology in the first semester of her freshman year.
"I really liked anatomy and I really liked animals, so it really came together," she said. "It's a thrill for me to be able to work with animals."
Shapiro studies ancient fossils to see how primates were built and then tests modern animals with similar characteristics in the lab.
"The problem is that they have combinations of anatomical features that don't exist in any living animals so we just have to piece it together," Shapiro said. "We really don't know because we've never seen that combination in primates."
The sugar gliders along with the mouse lemurs and opossums are stand-ins for those early primates in her project.
"They (the sugar gliders) happen to fit the bill for what I need," she said. "It's a small arboreal marsupial that walks like a primate."
And maybe it will help her solve the evolutionary mystery.