Bison are herbivores, and their diet is almost entirely grass, though they will eat low-lying shrubs, and when winter snows are deep enough to prevent them from grazing they can subsist temporarily on the twigs and branches of bushes and shrubs.  They travel in large herds, with the exception of non-dominant males, who tend to form smaller groups.  These groups come together during the summer mating season, and males compete for access to the females.  Though the males charge and butt one another during these contests, serious injuries are rare. Bison do not fight to the death, the goal being not to kill the rival, but simply to establish dominance in order to gain access to mates.