 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 . 
