They fall off because that’s an area that a predator commonly grabs and the gecko can detach it without dying (no major organs in there). That way the predator is left with a mouth full of tail but the gecko gets away to live another day and regrow the tail. Very clever
It is a means of escaping predators! In some species the tails even continue to twitch once the ‘body’ of the animal has run away, which is designed to distract the predator while they get to safety! Pretty cool huh!
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