Females may fight for social access to males and a male may intervene. Male gorillas have weak social bonds, particularly in multiple-male groups with apparent dominance hierarchies and strong competition for mates. Males in all-male groups, though, tend to have friendly interactions and socialise through play, grooming, and staying together, and occasionally they even engage in homosexual interactions. Severe aggression is rare in stable groups, but when two mountain gorilla groups meet the two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries.