August 08, 2017

Why Aren't Elephants Faster Than Cheetahs?

A mathematical model provides a long-sought answer for why the largest animals are not the fastest.
By JoAnna Pendergrass, DVM
A mathematical model developed by a research team answers a long-standing conundrum in movement ecology: Why aren’t the largest animals also the fastest? This model “unravels a fundamental constraint on the upper limit of animal movement, thus enabling a better understanding of realized movement patterns in nature and their multifold ecological consequences,” the researchers wrote in a recent Nature Ecology & Evolution paper.

For animals, movement is necessary to reach resources, escape predators, and move between habitats. An animal’s speed constrains its movement and has long been thought to relate to body mass, following the "power-law" concept: a change in one parameter produces a relative and proportional change in the other.

However, this concept does not match reality, given that large animals are not necessarily faster than smaller animals. In a recent press release, Dr. Myriam Hirt, the lead author, remarked that if using the power-law concept, an elephant’s maximum speed would be 373 miles per hour.

Previous studies have sought to address this phenomenon with biomechanical models. However, these models have not been generalizable across taxonomies and ecosystems. The current study aimed to create a general and widely applicable scaling model for body mass and speed.

Model Development
Researchers first developed a body mass-dependent maximum speed model following power-law scaling. This model illustrated a saturation curve—an animal’s speed becomes saturated during acceleration toward a theoretical maximum speed. This saturation occurs because acceleration uses finite anaerobic capacity, which depends on short-term ATP storage within fast twitch muscle fibers.

Researchers also created a time-dependent model, which considered the time at which acceleration energy becomes depleted. At this time—the critical time—an animal attains its realized maximum speed, which could fall short of its theoretical maximum speed. Compared with smaller animals, larger animals accelerate more slowly, deplete their energy more quickly, and thus fall short of their theoretical maximum speed.

This time-dependent model, the researchers wrote, “predicts a hump-shaped relationship between realized maximum speed and body mass.”

Model Testing
Researchers performed a literature search to collect data on maximum speeds for 474 species of running, flying, or swimming animals. Body masses ranged from a few micrograms to over 100 tons. Statistical analysis was used to compare the data with the models.

Testing of the power-law model revealed several notable findings:
  • Swimming animals were slower than flying and running animals
  • Large swimming animals were about as fast as comparably-sized running animals, indicating that large body mass aids acceleration in water
The time-dependent maximum speed model was deemed most adequate, with nearly 90% accuracy with the speed data. To determine what accounted for the 10% unexplained variation, the researchers evaluated the effects of taxonomy, diet type (eg, herbivore), thermoregulation (ectotherm, endotherm), and locomotion mode (running, flying, swimming) on maximum speed.

Locomotion mode and thermoregulation had significant effects: (1) Endotherms were faster than ectotherms among running and flying animals, and (2) ectotherms were faster than endotherms among swimming animals. Given that ambient or body temperatures affect many life processes, researchers were not surprised that thermoregulation also affected speed and acceleration.

Model Benefits and Applications
According to the researchers, the time-dependent model provides several benefits:
  • Accurate speed predictions for living and extinct species
  • Inferences about evolutionary pressures on maximum speed
  • Mechanistic understanding on what constrains animal movement
For the future, the researchers noted that the model could be used to analyze whether the hump-shaped relationship between body mass and speed also applies to long-distance travel, such as migration.

Dr. JoAnna Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.

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