What is hidden in 3D image?

Two hunters went moose hunting every winter without success. Finally, they came up with a foolproof plan. They got a very authentic female moose costume and learned the mating call of a female moose. The plan was to hide in the costume, lure the bull, then come out of the costume and shoot the bull. They set themselves up on the edge of a clearing, donned their costume and began to give the moose love call. Before long, their call was answered as a bull came crashing out of the forest and into the clearing. When the bull was close enough, the guy in front said, "Okay, let's get out and get him." After a moment that seemed like an eternity, the guy in the back shouted, "The zipper is stuck! What are we going to do?" The guy in the front says, "Well, I'm going to start nibbling grass, but you'd better brace yourself."