B. Grossi et al/PLOS ONE 2014
It can take only a few seconds of video to make a great moment in science. A recent entry is a video that shows two chickens walking. One clucks along with a normal chicken strut, while the other has wide and heavy strides. And no wonder: He’s got what looks like a toilet plunger strapped to his tail.
The video wasn’t just playing chicken. The chicken and his wooden tail tramp along in the name of science.
Bruno Grossi and colleagues at the Universidad de Chile were looking to learn more about how dinosaurs may have walked. While one modern relative of dinosaurs, the crocodilians, walks on four legs, birds — feathery descendants of the Tyrannosaurus rex we know and love — walk on two. This means that birds might be a good model to study to understand how bipedal dinosaurs moved as they stomped across the Mesozoic landscape. But there’s a big difference between most modern birds and the great land-bound dinosaurs: Dinosaurs had tails.
So if you want to create a model for how dinosaurs may have walked, you have to give a bird a dinosaur tail. And in a study published February 5 in PLOS ONE, that is exactly what Grossi and colleagues attempted to do. The paper is based on an earlier study by Matt Carrano, a paleontologist now at the Smithsonian Institution in Washington, D.C. During his years as a graduate student, he attempted to see how a tail might change the way a bird walked. Birds do not walk like crocodilians or mammals. We mammals move our legs from the hip, and our thigh bones are vertical. But birds are configured very differently. The “knee” we can see is actually the ankle. The real knee is hidden up the feathers. And instead of a vertical thigh, birds’ thigh bones lie almost horizontal and don’t move when the birds walk. All of their movement comes from the knee down. Carrano says that scientists call it “Groucho Marx walking.” But with tails, dinosaurs would have had a shifted center of balance. Would that shift promote more movement from the hip?
To study whether or not a tail would turn a bird’s knee action into thigh action, Carrano purchased a bunch of chickens from a breeder. He attached a metal rod to each chicken as a “tail,” attaching it with veterinary tape. Unfortunately, he had little success. The weight wasn’t well distributed and the chicken merely squatted down more, resulting in more knee action than before.
Grossi and colleagues thought they could do better. They too started by buying some chickens. One-third of the chicks roamed wild and free. Another third had wooden tails mounted on modeling clay and attached to a Velcro jacket that fit on their hind ends. The final third had an equivalent mass strapped to the center of the back.
Determining the correct tail mass was a weighty problem. Scientists are not quite sure how much an average theropod dinosaur’s tail weighed. By looking at various reconstructions and measurements from different paleontologists, the scientists came up with a conservative estimate — 15 percent of the chick’s body weight. The chicks began wearing their wooden tails or weights three days after hatching, and the tails were switched out for new ones every few days to adjust the weight.
Chickens wearing tails or weights adapted quickly, moving around and interacting with other birds without a problem. But they moved very differently than the free-roaming chickens. Grossi and his colleagues observed that chickens with tails shifted their weight forward. With the weight forward, the thigh bone became more vertical, and the source of movement shifted from the knee to the thigh. Adding a tail made the chicken’s movement less birdlike, and more … dinosaur-like.
Putting a tail on a chicken can’t tell us what happened in evolution to turn a dinosaur stride into a chicken strut. But it does show that scientists can re-create some of the biomechanical changes that may have taken place. They show that the tail changes the center of gravity and alters the walking gait. Carrano notes that it is “an important proof of concept.” Putting tails like these on chickens, he explains, helps scientists understand “how posture and bone shape and movement relate.” And, he adds, “it was nice to see I wasn’t the only lunatic who wanted to do this kind of thing.”
Both Carrano and Grossi used chickens as their “dinosaur” of choice. José Iriarte-Díaz is a functional morphologist now at the University of Illinois in Chicago and coauthor of the paper. He explains that when it comes to birds, you can’t beat a chicken for research convenience. But he acknowledges that some chickens have been bred for certain traits (like those large chicken breasts that fill grocery store cases) that may affect how the birds react to a tail. Michael Habib, a paleontologist at the University of Southern California, notes that other bird models might help to understand “if this is what happens when you put a tail on a chicken, or if it’s all birds with tails on them.”
Habib calls the experiment “deceptively simple and clever.” It not only says something about how dinosaurs may have moved, but also helps scientists understand “what contributes to the weird stance of modern birds,” he says. But of course, there are limitations. The chickens got wooden sticks with clay bases, not real tails with muscle and bone. So while the scientists could add to the mass of the tail, Habib explains, “you just can’t get the mechanical advantage of the tail. The only way you could do that is with a virtual model.” But models are only as good as the data that you put into them. To get that data, sometimes you have to put some wood tails on some chickens. It’s all the name of science.