Imagine you're a leech in one of the lush rainforests of Southeast Asia (or Madagascar or mainland Africa) that you call home. Perhaps you're clinging to the underbelly of a low-lying plant or burrowed just below the surface in a patch of damp soil.
Then, dozens of human tourists start marching through the terrain, providing countless opportunities for you and your companions to suction yourselves onto their boots, or drop down from the trees above.
Surely, for the leeches, large groups of humans trekking through their habitat is akin to stumbling upon a potluck in a forest. But whose blood do the segmented, parasitic worms suck when we’re not around?
And perhaps more importantly, why on Earth would an animal evolve to do something as complicated — and potentially dangerous — as sucking blood to survive?
