A couple of decades back, I was on my way back down from Half Dome and there was a bit of a traffic jam in the trees of jittery hikers and even more nervous pack horses--a black bear was about 50 yards ahead. Without warning, the bear made a dash for a deer not seen (by humans) until startled.
The bear had made a considered cost/benefit analysis that it could catch a deer. Still gives me the willies.