Any Monster Gecko needed to eat the fruits hanging from the bone trees for survival. On top of that, it also seemed that it was their prime mode of nourishment. Krune hadn't seen them hunt anything else from the waters above the Primary Landmass.
Well, even if they did, they didn't do it anywhere near him, so he hadn't witnessed such actions. In any case, it seemed the roots of the bone trees either absorbed the water from the ocean or the blood mixed into the ocean as the fishes within killed one another.
Either way, it accumulated a lot of Blood Essence, condensing them into the fruits. So, any monster that consumed such fruits would absorb all the Blood Essence in it, developing accordingly. Throughout its life, as it consumed more and more of such fruits, its body would continue evolving until it became a mutant.