How did Tyrannosaurus rex move on a leisurely foray through his realm? According to a study, a person could even have accompanied him in the junk aisle. According to the biomechanical modeling, T. rex was only walking at around 4.6 kilometers per hour. This was due to the vibrational behavior of the predatory dinosaur’s long tail, explain the scientists: An energy-saving resonance was only created at the low speed. This result rounds off the rather leisurely picture of T. rex, because earlier studies have already confirmed that it has rather modest top speeds.
In the film Jurassic Park, a fast-paced T. rex created tension: In an impressive scene, he chases a car with the fleeing protagonists. In order to escape the snarling sprinter, you have to accelerate properly. But probably a bike would have been enough for the escape. Because according to biomechanical calculations, the maximum speed of the predatory dinosaur, which is up to 13 meters long and weighs seven tons, was probably only around 30 kilometers per hour. Because the following applies: Above a certain size, the inertia consumes the possible top speeds. But how was T. rex usually out and about – in the everyday strolling? This is the question that the researchers working with Pasha van Bijlert from the Free University of Amsterdam pursued.
Resonance in energy saving mode
As they explain, different living beings have a typical speed in comfortable gait, which is, as it were, the energy-saving mode of locomotion. This results in movement sequences which, based on the physical characteristics, guarantee the lowest possible energy consumption. An important aspect here is that the body should literally be well coordinated when out and about: The step frequency of the person is therefore in harmony with the pendulum movements of the arms. The scientists explain that walking is only less strenuous and therefore saves energy when there is a resonance in the body. That was certainly also the case for the Tyrannosaurus rex.
Similar to us, these predatory dinosaurs walked on two legs – but an aspect shaped their biomechanics that does not exist in ours: The long and heavy tail played an important role in movement behavior. “There have already been studies examining the walking speeds of dinosaurs, but the focus was on the legs and the tail was largely ignored, although it was what distinguishes these animals,” says Van Bijlert. He and his colleagues are convinced that the vibration behavior of the tail played a central role in the typical movement sequence of T. rex when walking. Because, as they explain, the long appendage bobbed up and down with every step. At a step frequency that would have been in dissonance with the natural frequency of the tail, an unfavorable energy consumption would have arisen, according to the researchers.
In tune with the tail vibration
In order to be able to estimate the natural frequency, Van Bijlert and his colleagues have now modeled the tail of “Trix”, a T. rex fossil that is on display in the Naturalis Biodiversity Center in Leiden. They added muscles and tendons to the bone structure on the computer to provide a basis for their biomechanical analysis. The model calculations finally resulted in a value of 0.66 Hertz for the typical vibration behavior of the T. rex tail. The researchers were then able to use this value as a basis for assessing the animal’s step frequency, which had to be coordinated with the tail oscillation in order to ensure optimal energy consumption.
In combination with the typical stride lengths of T. rex, which are known from fossil footprints, the researchers finally came up with an estimate of the typical speed. Accordingly, the kings of the predatory dinosaurs were only about 4.6 kilometers per hour on a walk. “We found that walking speeds were significantly lower than if we only took the characteristics of the legs into account,” says Van Bijlert. The value is even at the lower end of the range of typical walking speeds in humans. This may seem surprising for such a large animal, but given the values of today’s large animals, it seems plausible. For example, elephants or giraffes travel at similarly low speeds when they are walking comfortably, say the researchers.
Van Bijlert and his colleagues now want to stay on the ball: They are planning to apply their approach, known as the “Natural Frequency Method”, to investigating the movement patterns of other types of dinosaurs. “And the tail will certainly play an important role again”, concludes Van Bijlert.
Source: Naturalis Biodiversity Center, specialist article: Royal Society Open Science, doi: 10.1098 / rsos.201441
Video: Animation of T. rex “Trix” walking with tail resonance. (Credit: Rick Stikkelorum, Arthur Ulmann & Pasha van Bijlert)