A Deer's Diet

A deer, and a moose are eating forage from underneath ice on a snow-capped mountains, with this simple form of food, it grows larger, stronger and fattens really fast, the bears and wolves then come in trying to prey on the deer's young, or if possible the deer itself. Why does it have to wait for the dear to finish up the grass before it swallows them whole? Why can't it just chew the fodder directly? When lion king misses or fails to catch his meal he simply eats grass like a buffalo, he then grows stronger and gets to live longer.