From Tanyas evidence it would seem that behemoths suffer from disease and die rather rapidly. Their bodies clearly are not meant to sustain their massive strength and stature, so it breaks down. Or so it would seem.

The one Riley shot with an arrow seemed to stick around quite long. We don't know yet how capable these things are and if the dead ones they found are the norm for all of them. It is quite curious, though. Are the baddies dying out?