They likely didn't metamorph at all - it would've started as "since you're just going to moving as fast as you can so you can find a mate, burning through fat s

They likely didn't metamorph at all - it would've started as "since you're just going to moving as fast as you can so you can find a mate, burning through fat stores faster is good...also thinking about it, you're not going to miss that mouth you're not using anymore either?" There would've been some sort of weight/reproductive efficiency trade off going on (and it would've been heavily environment dependent).