Where Have All the Giants Gone? How Animals Deal with the Problem of Size

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Abstract

The survival of both the hunter and the hunted often comes down to speed. Yet how fast an animal can run is intricately linked to its size, such that the fastest animals are not the biggest nor the smallest. The ability to maintain high speeds is dependent on the body’s capacity to withstand the high stresses involved with locomotion. Yet even when standing still, scaling principles would suggest that the mechanical stress an animal feels will increase in greater demand than its body can support. So if big animals want to be fast, they must find solutions to overcome these high stresses. This article explores the ways in which extant animals mitigate size-related increases in musculoskeletal stress in an effort to help understand where all the giants have gone.

Figures

  • Fig 1. Scaling of length, area, and volume dimensions with body mass. Body masses for all mammals and lizards (data from [10–12]) are shown with predictions based on geometric similarity for length, area, and volume. Regression lines are shown for geometric scaling (isometry) for length, area, and volume (grey lines). Scaling relationships are also shown for muscle area (physiological cross-sectional area) in the iliofibularis of varanids (dashed blue line), which scales with positive allometry (original data reported in [12]) and femur length in mammals (dashed yellow line), which scales with negative allometry (original data reported in [10,11]).
  • Fig 2. Effect of body size and posture on limb EMA. (A) Hindlimb EMA scaling for mammals (data reported in [17, 31]) shows that EMA scales with M0.26. (B) Effect of hindlimb posture on limb EMA (ratio of extensor muscle moment arm (r) to the resultant threedimensional ground reaction force moment arm (R) shows that crouched animals have a lower limb EMA than upright animals. Dashed black arrows represent the ground reaction force vector.
  • Fig 3. Postural changes with body size in felids and varanids. Joint angles for felids (ankle: red points, dashed line; knee: orange points, solid line) and varanids (ankle: dark blue points, dashed line; knee: light blue points, solid line) display that posture remains relatively unchanged across body masses in both groups and that varanids adopt a more crouched posture compared to felids. Joint angles for felids are represented in a two-dimensional sagittal plane, whereas for varanids, angles are given as the three-dimensional angle between the femur and the tibia for the knee and the angle between the tibia and the foot for the ankle. Original data reported in [35,38].
  • Fig 4. (A) Top running speeds are reached at different body sizes in (nonfelid) mammals (n = 142), felids (n = 8), and varanids (n = 19). The curvilinear relationship between body mass (kg) and running speed (km h-1) for mammals (yellow), felids (orange), and varanids (blue). Optimum body mass for speed is denoted by the peak of each curvilinear relationship. Original data reported in [40,46–51]. (B) Frequency distribution for the body masses of extant running mammals [52] (n = 2,919; yellow bars) and extinct dinosaur species [1] (n = 406; red bars) shown with the relationship of optimum mass for speed in nonfelid mammals.

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CITATION STYLE

APA

Dick, T. J. M., & Clemente, C. J. (2017). Where Have All the Giants Gone? How Animals Deal with the Problem of Size. PLoS Biology, 15(1). https://doi.org/10.1371/journal.pbio.2000473

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