Social networks can still affect and cause evolution on their own by impending fitness differences on individuals. According to a 2012 study, male calves had a lower survival rate if they had stronger bonds with juvenile males. However, when other age and sex classes were tested, their survival rate did not significantly change. This suggests that juvenile males impose a social stress on their younger counterparts. In fact, it has been documented that juvenile males commonly perform acts of aggression, dominance, and intimidation against the male calves. According to a 2010 study, certain populations of Shark Bay dolphins had varying levels of fitness and calf success. This is either due to social learning (whether or not the mother passed on her knowledge of reproductive ability to the calves), or due to the strong association between mother dolphins in the population; by sticking in a group, an individual mother does not need to be as vigilant all the time for predators.