Why do tails of some lizards break so easily?
It's a self-defense mechanism, the tail falls off during threatening situations, to which the reptile responds by excessively contracting the tail muscles. This process is known as "Autotomy", or "Self-Amputation". Other animals have got "stronger tails" simply because such self-defense mechanism hasn't been developed.
https://science.sciencemag.org/content/20/500/149.2
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