Scientists have been testing and calculating and debating some of the Traits of T. rex, which can be hard when all that's left is their bones. How do scientists figure out how an extinct creature's body worked, like its organs and muscles? Check out the findings, conversations, and debates about T. rex's "softer side."

Can we tell boys from girls? Some scientists say yes, some say maybe, some say no way. There are four basic pieces of evidence that lead the "yes" group of T. rex researchers to their conclusions.

1. One morphotype, the robust one, has a deeper pelvis, which would allow for the passage of eggs.
2. The robust morphotype also has a wider pelvis, which would allow for the T. rex to lay two eggs at a time - something evidenced in theropod nests.
3. The way the bones under the tail are arranged in the two morphotypes are exactly the same as in T. rex's closest "living ancestor," the crocodile. In crocodiles, one of the bones hooked to the bottom of the tail vertebrae, called a chevron, is the anchor for the male sex organ; females have a shorter, more angled bone (and no such anchor). T. rex has this same configuration, with the gracile form like the male croc.
4. In Sue, there is an injury to the top of the tail, where spurs of bone grew to support the vertebrae - where a male would mount for copulation. Sue is of the robust morphotype.

These four pieces of evidence suggest to many scientists that the robust, "biggest, baddest" T. rex were female. Scientists who do not agree say that not enough specimens have been collected to truly recognize robust and gracile morphotypes or any significant differences in the skeletons, and we probably never will know the answer.

While Velociraptor would have beaten T. rex in a game of chess - because its cerebrum, the brain's "thinking" part, was relatively larger - T. rex still was no dummy. It's entire brain was the largest by volume of any dinosaur's, and it had the capacity to rule the Cretaceous (with its tiny arms tied behind its back). Here are the facts:

Smell: A dog's "smelling part" of its brain takes up about one-fourth of the brain case, and it's our Excellent Smeller model. A T. rex's smelling part? More than half of its brain capacity! In Jurassic Park, remember when the kid was told to stand still so the can't-see-a-non-moving-boy T. rex wouldn't eat him? Even if that T. rex was blind, he would have smelled that boy, not to mention being able to identify what he'd had for dinner an hour earlier. One boy = one "smelly" appetizer.

Sight: T. rex had an eye about the size of a softball, one of the largest eyes ever developed in the animal kingdom - past or present. This would have included plenty of space for black-and-white and color receptors; since its ancestors (crocs) and its descendants (birds) see in color, scientists think T. rex did, too. Also, it's optic nerve was huge (2 cm), which meant that eyesight was keen - not to mention binocular. That is, since T. rex eyes faced forward, it had three-dimensional depth perception. Distance, size, shape, color, depth, and motion - a two-story-tall eagle.

Hearing: Although T. rex hearing hardware (including a long stapes) seems to closely resemble that of living birds and crocodiles, this sense is harder to pin down. Large internal chambers near the inner ear would have amplified low-frequency sounds...but not much more has been documented. With everything else going on in T. rex senses, even moderate hearing would have been enough for this hunter to ferret out its dinner!

The Mechanics: The presence of "honeycombs" within its vertebrae demonstrates that Tyrannosaurus rex breathed through a complex system of passages, pockets, and air sacs that acted like bellows - forcing air through the lungs in one direction. This one-way system constantly bathed the lungs in fresh oxygen, whether the bird was breathing in or out. The honeycomb was a result of the air sacs invading the bone, but the result was that T. rex may have weighed less than we first thought.

The Implication, part 1: Where else do we see this "honeycomb" in a skeleton? In only one living creature: birds. The close relationship between theropods and birds has been hinted at since Richard Owen first noticed the honeycombing in three Megalosaurus vertebrae in 1856. Today, in living birds, this air sac system constantly bathes the lungs in fresh air, making these creatures the most efficient breathers anywhere. In T. rex, it would have resulted in excellent dinosaur athletes who would have no trouble running the Boston Marathon!

The Implication, part 2: Modern birds also have a large four-chambered heart and circulatory system to go with its respiratory system. This efficient oxygen-delivery process is only necessary in an animal with a high metabolic rate. We think this means that T. rex was an active, quick and mobile endotherm. If ever an animal could be termed 'hot-blooded,' it was Tyrannosaurus rex.

It's hard to know what the soft parts of extinct animals looked like. For example, did T. rex's skin look like a lizard's, or maybe a rhino's? Was it brightly colored, or safely green, or gray? Is there a way to tell? So far, we've found imprints of skin of other dinosaurs - and its texture varies from pebbly to scaly. But nothing in the fossil record has saved an indication of pigment. If we think of male creatures whose skin color or feathers are especially bright - to attract mates - we may assume that dinosaurs did the same. Unless our gender ideas are correct. If so, then - as in today's birds of prey - males and females might have been the same, or similar colors. But what colors? How will we ever know?

For generations, we considered dinosaurs to look (and even act) like reptiles. However, in recent years, plenty of evidence has come to light that dinosaurs gave rise to birds - and we've even found fossil dinosaurs complete with feathers! We have found downy feathers, and flight feathers, but we have NOT found babies with down like a chick's - or large-bodied dinosaurs with preserved feathers. So far, it's small-ish creatures who wore feather coats, and it's very probable that T. rex also had feathers. Just imagine: a four-ton ostrich!

Arms

There has been much debate about the supposedly useless arms of Tyrannosaurus rex. That these arms were used for something is obvious from scars on the skeleton, reflecting their massive musculature. Analysis of these muscle scars shows that they could lift more than 200 kg (440 pounds) with each arm. Several upper arm bones (humeri) show healed injuries - injuries that wouldn't have been possible unless T. rex was using its limbs. Perhaps T. rex used its hands as meat hooks, grasping and immobilizing the prey as it sheared off chunks of flesh with its jaws.

But why so short? Scientists think that through evolution, the longer arms of ancestor-dinosaurs gradually shrunk because T. rex needed to compensate for the great mass of the skull in order to maintain balance. The heavier that jam-packed skull became, the more some other part of the body had to diminish.

Legs

The rear limbs of Tyrannosaurus rex are proportioned for speed. When we compare the legs of T. rex to those of Edmontosaurus, and Triceratops (its two main food groups), we find that T. rex legs have relatively longer lower limb bones (tibia and metatarsals) and that T. rex has much longer total leg length. Independent speed calculations suggest that a reasonable cruising speed for Tyrannosaurus rex was 50 km (30 miles) per hour. Even if you believe that T. rex had a top speed much lower than that, when you consider the limb proportions and total limb length, there is no question that T. rex was much faster than its most abundant source of protein.

Trackways can help us determine the speed that an animal actually ran. Scientists measure the stride length (the distance from one step to the next for the same foot) and then divide that distance by the length of time it took to make the stride. Since we can't watch the extinct animals walking or running, scientists use living animals as models to measure how long it takes them to step. Single T. rex tracks have been found, but trackways with more than one step are rare. So far, the best specimen shows that the particular T. rex who made the tracks was moving at about 10 miles per hour.

Using T. rex skeletons, along with "models" of living giants, like elephants, scientists have tried to estimate T. rex's weight. Estimates have ranged from six to eight tons. However, the estimates may have been too high. When scientists take into consideration T. rex's unique breathing style - and its related air sacs and honeycombed bones - the weight may have been as little as four tons for a full-grown adult. We have more work to do on this question!

What bone clues might tell us how long one creature lived? Some scientists have long hoped that the bones themselves would provide a map: perhaps we could count growth rings just like we do in trees. Early attempts at this, which relied upon each ring's representing one year, were problematic. Even being able to see growth rings in limb bones was difficult, since these bones were constantly remodeled - this would have obliterated the evidence, if there ever was any. New research using certain non-weight-bearing bones has been more successful and shows some promising results. We hope to see them published soon.

One method to guess a T. rex's age is to compare it with living animals. Large animals like elephants live relatively long lives, as much as 80 years. T. rex's closest relatives, crocodiles and some birds, can also live for 80 or more years, so it's not unreasonable to think that T. rex's maximum age might have been somewhere in that vicinity. However, one factor that might have cut short an otherwise long and happy T. rex life: being a carnivore is dangerous. Year after year of physical combat may easily have reduced even a strong, healthy animal's life span to a fraction of its potential.

And speaking of combat, there's a way to use a T. rex's injuries, or pathologies, to guess its age. Evidence of events in a dinosaur's life make it possible for us to imagine a timeline. For example, Sue had multiple broken ribs and gastralia, multiple puncture wounds on her face, pulled tendons and ligaments, crushed tail vertebrae, infections, and even a broken leg. She couldn't have received all of these injuries in one fight, or she would have died from them. Plus, most of these injuries had completely healed. We can compare her injuries to other large creatures who live a tough life...and we can use living models to imagine how long it would take to heal from each injury. From this kind of hypothesizing, we can guess that Sue lived a long and exciting life.