Movement and Its Limits
Speed, Slowness, and the Convergent Evolution of Flight
1. The Spectrum of Animal Locomotion
Animal locomotion spans an extraordinary range — from the cheetah's explosive sprint at over 100 km/h to the sloth's deliberate crawl at less than 0.3 km/h. Each extreme carries trade-offs that natural selection has fine-tuned over millions of years. The key insight is that speed itself is not the goal; survival and reproduction are.
🐆 Speed Strategy
High metabolic cost, short bursts, specialised anatomy
Examples: Cheetah, Peregrine Falcon
🦥 Slowness Strategy
Minimal metabolic cost, energy conservation, crypsis
Examples: Sloth, Sea Cucumber
Comparative Speeds Across the Animal Kingdom
2. Speed as Strategy: The Cheetah 🐆
Cheetahs (Acinonyx jubatus) are the fastest land animals, reaching speeds of 96–112 km/h (60–70 mph). Their biomechanics are a masterclass in evolutionary engineering:
- Rotary gallop: A unique gait in which the flexible spine coils and uncoils with each stride, allowing stride lengths of up to 7 metres.
- Airborne time: During a sprint, cheetahs spend more time airborne than on the ground.
- Sprint duration: Top speed can only be maintained for approximately 20–30 seconds.
⚠️ Misconception Alert
Older textbooks claim cheetahs stop running because they overheat. Research by Hetem et al. (2013) published in Biology Letters found that cheetahs do not abandon hunts primarily due to overheating. The limiting factors appear to be muscular fatigue and oxygen depletion. Body temperature does rise during a chase, but this is a consequence, not the cause of stopping.
Cheetah Sprint Trade-offs
| Advantage | Cost |
|---|---|
| Extreme acceleration | Very short endurance |
| Flexible spine → long stride | Lightweight build → poor defence |
| Semi-retractable claws → grip | Cannot climb trees well |
| Large nasal passages → O₂ intake | Weaker jaw/bite force |
Every adaptation for speed comes at a cost — this is the principle of trade-offs.