Tyrannosaurus terrorized the Earth — at least in the Hollywood version of history. But an estimate of the muscle volume in its hind legs suggests that the mighty giant could only walk, not run.
Over the course of history, vertebrates have evolved an enormous range of sizes, spanning well over six orders of magnitude in body mass. The largest and most captivating terrestrial giants were the dinosaurs, and Tyrannosaurus — although not the largest at around 6,000 kg — is perhaps the most famous and terrifying representative of this group (Fig. 1). Some workers1,2 have argued that bipedal tyrannosaurs and other huge dinosaurs could not move fast because their size would have imposed severe constraints on physiological and mechanical functions. But others claim that these creatures were much more athletic3,4.
An obvious difficulty in resolving this argument is that dinosaurs have been extinct for a long time, so reconstructing how they moved is a challenge. But on page 1018 of this issue5, Hutchinson and Garcia introduce a new biomechanical approach to the problem, applying an analysis of living animals to their ancient dinosaur relative. They show that Tyrannosaurus simply did not have large enough leg muscles to produce the forces required for an animal of such size to run.
The skeletal muscles in all animals are made of the same contractile proteins, so their intrinsic capacity for generating force is very nearly the same. The force that can be produced depends on the cross-sectional area of a muscle's fibres. But as body size increases, the geometrical effects of scale mean that muscle capability does not increase proportionately. The force that a muscle can generate increases less rapidly than body weight, so, despite their greater volume, the muscles of larger animals generate less force per unit weight.
In addition, the ability of an animal's skeleton to support mechanical loads decreases with size because bone area does not increase nearly as fast as an animal's weight. Living terrestrial mammals can accommodate these problems of scale by altering their limb posture when they run: larger animals run on more erect limbs than much smaller animals, which gives their muscles greater mechanical advantage6 and allows them to maintain similar capacities of force generation and bone loading. But this only applies to animals as large as 300 kg or so. Above this weight, further changes in muscle mechanical advantage are probably limited7, and sustaining force capacity for movement at greater speeds becomes a problem.
So how fast might a 6,000-kg dinosaur have moved? Previous estimates of the speed and locomotive capacity of dinosaurs and other extinct animals have been purely qualitative. Some models are based on the limb motion deduced from the step length and stride frequency derived from fossilized tracks1,2,4,8. However, such estimates depend on assumptions about body mass distribution, limb posture and limb length, and about kinematic similarities between species. The data8 from fossilized tracks uncovered so far suggest that large bipedal dinosaurs moved at speeds of less than 5 m s−1. But it may be that tracks left by faster-moving dinosaurs just haven't been discovered yet.
In their analysis of Tyrannosaurus, Hutchinson and Garcia5 introduce an approach based on estimates of the minimum muscle mass needed for fast running. First they applied their analysis to alligators and chickens — two living relatives of bipedal dinosaurs. The results show that alligators have less than half the muscle mass that they would need to run fast (if, like bipedal dinosaurs, they used only their hind limbs), whereas chickens have nearly twice the necessary hind-limb muscle mass. This agrees with the observed fact that chickens and many other avian bipeds are good runners, but alligators must support themselves on four limbs and move at relatively modest speeds.
Hutchinson and Garcia then extended their analysis to estimate the limb muscle mass of extinct animals and quantify their locomotive performance. From fossil specimens of Tyrannosaurus, the authors estimated body and segment mass, worked out areas of muscle attachment, and deduced the forces and moments that the creature's leg muscles could have generated. Their analysis rests on assumptions about the limb posture and the magnitude of reaction forces exerted by the ground on the limbs of Tyrannosaurus, and about the kinematic similarity between dinosaurs and living birds and mammals9. But their results show that, even if the creature used all its hind-limb muscle mass, it could not have generated the forces necessary for running. They show that for a chicken scaled up to 6,000 kg to run, it would need muscles in each leg equivalent to 99% of its body mass — which is obviously impossible. The results for smaller bipeds, however, show they probably could run quickly, in agreement with estimates of their speeds from fossil tracks8.
A pleasing aspect of Hutchinson and Garcia's study is that they apply sensitivity analysis — allowing for a degree of parameter uncertainty — to evaluate the robustness of their results. For instance, they find that the estimated muscle mass is very sensitive to differences in limb posture, but is less sensitive to other parameters, such as muscle-fibre length. Collectively, these uncertainties contribute to up to a threefold variation in the estimated muscle mass for the various models of Tyrannosaurus, but the conclusion is unchanged. Palaeontological analysis of functional performance in fossil organisms will always be an uncertain science, dependent on the availability of fragmentary, long-dead material. So it is welcome when new analytical approaches such as this, and others (such as finite element analysis10), are brought to bear on such problems.
But what of the reputation that Tyrannosaurus has as a fearsome hunter? Hutchinson and Garcia's results suggest that the creature would have had little success chasing smaller, more fleet-footed prey; it may even have fed on carrion. But I suspect that it could still have moved fast enough to attack other large dinosaurs whose locomotive ability was also limited.
The dinosaurs are famous for being the largest creatures that ever inhabited our planet. But as a group they represent a broad range of size and diversity of form, with a similarly wide range of locomotive capacities and lifestyles11,12. It will be interesting to see what insights future investigations of dinosaur diversity yield as new analytical and computational approaches are explored.
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