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T. Rex Was a Slacker

A natural wonder of the big theropod was how it conserved energy.

In our evolving understanding of dinosaurs, Tyrannosaurus rex has acquired a new persona in recent decades. It’s always been the…By Mark MacNamara

In our evolving understanding of dinosaurs, Tyrannosaurus rex has acquired a new persona in recent decades. It’s always been the imperious, curiously agile two-ton gargantua, personified as a gaunt, grizzle-faced gunfighter at the dark end of the bar, a soulless creature that lived to 30 and possessed a keen sense of smell, good eyesight, surprisingly pliable skin and banana-sized teeth. Add a taste for Triceratops, which it ate by pulling off the head then using its smaller front teeth to nip the soft flesh behind the frill.

Its newer persona suggests a softer side, a demon looking creature but finally not a psychopath, merely a misunderstood everyman with feathers. Literally feathering on different parts of its body, mainly on its back, like a mane perhaps, wherever it was, an “extravagance” in evolutionary terms. Although still somewhat speculative for T. rex, feathers are definitively found in close relatives, including the 30-foot-long Zhuchengtyrannus, which lived in China. Paleontologists have found intact downy feathers of different colors, and speculate that while feathers obviously didn’t portend flight, or a show of intimidation to predators since there weren’t any for T. rex, the feathering may have been a display, a peacock affect, as though to say, “Hey, how sexy a gal or guy am I?” Incidentally, some of the biggest T. rexes ever found were female.

Chasing down prey is a dangerous business. It’s better to have a body that allows you to hunt like a casual forager.

And now, to add to this updated portrait, there is new insight into the physique of theropods like T. rex, distinguished by their hollow bones, three-toed limbs, and long hind legs. For small theropods, like the chicken-sized Comsognathus, from the Greek for “dainty,” lanky limbs helped it run like a demon to find brunch and escape being brunch. But it turns out that feature doesn’t hold for fellow theropods at the other end of the size scale. To be sure, long legs helped T. rex sprint up to maybe 30 mph for 50 to 100 yards—not nearly at the NASCAR speeds suggested in the film, Jurassic Park. But a new study, “The Fast and the Frugal,”1 undercuts the image of T. Rex on a rampage to devour Laura Dern. T. rex evolved long legs to amble through its habitat with a chill gait that conserved its “energy budget”—a giant body requires a massive amount of fuel—and helped the big theropod, “at once efficient and elegant,” rule the land for more than 2 million years. Until an unfortunate asteroid arrived.

To Alex Dececchi, lead author of “The Fast and the Frugal,” generous with his time and insights in a recent interview, T. rex’s efficient gait bears a larger message. “We don’t have infinite resources; we’re trying to conserve the best we can,” Dececchi says. “That’s what nature is really good at. It’s really good at conserving and minimizing excess costs to a goal. We should look back and say, ‘OK, how did nature do it?’ ”


Dececchi, a paleontologist, is an assistant professor of biology at Mount Marty College in South Dakota. It’s a Benedictine school with a convent from which a dwindling number of nuns still appear on the faculty list. Dececchi is part of a small natural sciences department. The evolution of T. rex, he says, of its huge body and elongated limbs, is all about hunting. But not in the way you might think. Chasing down prey is a dangerous business. It’s better to have a body that allows you to hunt like a casual forager.

“Most hunts end in failure—only about 25 percent succeed,” Dececchi says. “That’s a lot of energy to put out that you don’t get back. If you can save, just by normal hunting and searching, that little bit of energy, then you don’t have to hunt as much. When you’re actually bringing something down—and remember it’s often a dangerous process to take down a prey—you risk that injury a little bit less. The result is you can last through hard times a bit longer and risk yourself less.”

Conserving energy, Dececchi adds, is a valuable survival trait, “especially if you’re going through either a hard time, or as you’re getting older. You still have to hunt, and you can save energy on hunts, or if you can just save energy, period, that energy can be put to other uses. Maybe it makes you a bit stronger during the mating season or fighting off a rival. You have that extra little reserve you can call upon.”

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What’s interesting is this positive correlation between size and efficiency. In another study, Dececchi is working on a comparison of jerboas and kangaroo rats, which have similar skeletal structures. “Their hopping nature probably came about because they’re so small, and have to get away really fast if something comes at them. When you’re really small, speed becomes something that you’re usually selected for. The overall costs of survival are more important than saving a little bit of energy. For them, for really small things, speed is probably a stronger selection pressure than efficiency.”

It’s understandable to assume long legs were selected for their speed. But evolution doesn’t run predictable courses. “We might look at animal growth and say, ‘Well, they have longer legs than this guy, so they must have been faster,’ ” Dececchi says. “But no, they’re too big for speed to be a problem. There must be another solution for this.” That solution is energy efficiency. The wolf, for instance, has no reason to pursue the moose; he could do it but there’s plenty of prey easier to catch. The same is true for T. rex. Its steady gait allowed it to range widely in a day, where it preyed on smaller dinosaurs like ceratopsians and hadrosaurs, and, contrary to its fierce reputation, sometimes scavenged on carrion. T. rex also hunted in packs, allowing it to corner and kill some of the smaller and faster dinosaurs. The big theropod consumed over 400 pounds of meat per day. As a forager, it was able to gain several more tons of meat per year than it would have as a sprinting hunter.

This new understanding fleshes out T. rex as a real organism living in the day-to-day, not just bones in a drawer.

Of course, it helps to be kings and queens of the land. “You don’t want to use human emotions like confidence, but T. rex probably wasn’t afraid of anything else that was around,” Dececchio says. “When the adults and sub-adults were walking into an area, they were just looking to see what’s around, they weren’t trying to watch their back. They were trying to gauge the environment, figure out what’s a good place to set up a hunt, a good place to sleep. They wouldn’t be worrying about anything else.”

However, Dececchio points out, natural selection doesn’t always prize efficiency. Sexual selection can sometimes rule. Birds come to mind, especially in the case when the male bird is bright and colorful and the female is drab. “You can spot that male bird from a mile away,” Dececchi says. “How is it not eaten by predators? A lot of them are, but the cost of actually getting to mate versus not is worth it. In those instances, efficiency gets pushed aside. For a lot of mundane, functional, daily tasks, efficiency tends to be what you maximize. Once you get above the minimum threshold, like you need to be able to walk, or you need to be able to run, efficiency takes over. It’s a daily grind thing.”


The energy-saving habits of giant theropods is a revelation to paleontologists, at least to Dececchi. It’s helped pull back the curtain on the mystery of T. rex’s daily life.  “We know from other studies that these dinosaurs had acute senses,” Dececchi says. “They could detect prey from miles away. If you know their energetics, you can also figure out how many prey can be in an area. You can say, ‘OK, were they following herds? Were they staying put and letting herds come to them?’ So, for all these things, we could start to understand based on their energetics. It fleshes them out as real organisms living in the day-to-day, not just bones in a drawer.”

Dececchi’s work with dinosaurs has led him to see evolution similarly as a matter of nuance, a process of sculpting, the slow revealing, to him reminiscent of the Michelangelo statues in the Galleria dell’Accademia in Florence. “In many ways, that’s what evolution is, chipping away the outside until you have a core, which has the traits. Like sculptors, you don’t take a block of marble then add another block halfway through. You cut away from what you have. Think of one generation of T. rexes coming out. Obviously, not all of those babies will survive. That generation is going to be chipped away at, things taken away; sometimes through just pure happenstance and accidents, like earthquakes, floods, a volcano. Some traits will break off.” Some will be favored, others not.

There’s a story for humans in T. rex’s evolution and conservation of energy. We should be discussing the virtues of small changes, Dececchi says, “through refinements in long-term utility over short-term extreme behaviors; that is, make it last rather than make it fast. This is not a new concept; heck, it’s how things used to be when things were homemade or when we fixed things instead of bought new. I think it needs to make a comeback. As we start seeing more and more environmental impacts of our resource usage, we need to start thinking more on the years-to-decades-to-century scale rather than the hours-to-days-to-weeks scale. Perhaps engineers can start looking to nature not just to build a fast flier or a running robot dog, but how to make products that work day-in day-out for years to come.”


Mark MacNamara is an Asheville, North Carolina-based writer. His articles for Nautilus include “We Need to Talk About Peat” and “The Artist of the Unbreakable Code.”


References

1. Dececchi, T.A., Mloszewska, A.M., Holtz, Jr., T.R., Habib, M.B., & Larsson, H.C.E. The fast and the frugal: Divergent locomotory strategies drive limb lengthening in theropod dinosaurs. PLoS One 15, e0223698 (2020).


Lead image: metha1819 / Shutterstock

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