sc98, You are assuming the collar still has battery life... Once weak, the receiver has to be very close to pick up the signal... So at a certain point in the study, the official researchers are no longer able to compile data...
Took me a while to find it. Some are solar powered and so continue to operate virtually indefinitely, at least relative to the life of the game.
Dropoff mechanism can be installed so that the collar drops off, ideally while still sending data and so can be recovered fairly readily, assuming the critter isn't crossing a swift stream when it drops off.