Mule deer are very vocal under the right conditions. If you hear them, you will never forget the noise. I was hunting with some freinds a couple of years ago and grunted while they were hunting, I can't tell you for sure if they would have been killed without the call being used, but it didn't hurt any as we killed a 145 and a 170 buck. I have had them show up during the late season archery while I am grunting. I did some reasearch and found very little on the topic. I do know for sure that during the rut, I have heard a wide variety of vocalizations coming from the bucks.