How do crocodiles maintain their explosive power despite being motionless for long periods, while if I am sedentary, my muscles weaken?
Stephen Johnson Eugene, Oregon, US
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Reptiles, such as crocodiles, and mammals, such as humans, have two basic types of muscle fibre. Slow-twitch muscle fibres support continual exercise and respond to endurance training. Fast-twitch muscle fibres support explosive power and respond to short, intense activity.
When humans are sedentary, their slow-twitch fibres will, over time, convert to become fast-twitch. As an extreme example, a person who is paralysed as a result of spinal cord injury will eventually convert their paralysed muscles into almost 100 per cent fast-twitch fibres.
Crocodiles only need fast-twitch muscles to capture their prey. As a consequence, their lifestyle of mostly lying in wait for prey works to their advantage.
In addition, they possess a form of super-fast-twitch muscles in their jaw to aid in applying a very large, almost instant force to anything they catch.
Finally, their legs and hips have evolved to aid them in jumping forward quickly.
What the author of this question experiences when they are sedentary is a loss of function in their slow-twitch muscles, those which support a continuous effort.
Mike Follows Sutton Coldfield, West Midlands, UK
The secret of the explosive power of crocodiles is their fast-twitch muscles. Sprinters generally have more fast-twitch muscle than distance runners. The downside is that sprinters 鈥� and crocodiles 鈥� can鈥檛 sustain their high-power output for long.
Ectotherms (cold-blooded animals, which includes crocodiles) generally feed during the day when the higher temperature enables their muscles to function better. In fact, crocodiles can have body temperatures in excess of 30掳C, which is only a little lower than ours (37掳C) and they change location to achieve this, for example by basking in the sun.
They also don鈥檛 need to feed as often because they don鈥檛 need extra 鈥渇uel鈥� to run an internal heating system to keep their bodies warm. Consequently, they can sit and wait to ambush prey.
They are well-adapted for this, but, size-for-size, the explosive power of the crocodile is nothing compared with a mantis shrimp. This marine creature can punch its prey with a force of more than 700 newtons, equivalent to over 1000 times its own weight.
Hinged clubs at the front of its body, called dactyls, can accelerate an astonishing 10,000 times faster than an object in free fall to deliver a blow at speeds close to 100 kilometres per second. This is accomplished when the shrimp transfers chemical potential energy from its food into elastic potential energy, using its muscles to change the shape of part of its exoskeleton. A latch releases the stored energy. Humans achieve the same effect with a crossbow.
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