When it comes to social behaviour,mammals are in a league of their own.Some birds may form pairs,or even cooperate to hunt,but the complexity   of their relationships can hardly compare to those within a school of dolphins,a herd of elephants or a group of humans.What makes mammalian social groups different from a flock of starlings or a shoal of fish,is that in many cases the individuals could recognize each other.Although we may think that  all elephants look pretty much the same,we can easily tell individuals of our own species apart and it has become clear through studies that the same is true of other species.Dolphins have their own signature whistles that act like "names",and elephants can recognize and greet other individuals they have known but not seen for many years.This is something that only a few species of birds appear to be able to do. Mammals in complex social groups not only recognize each other as individuals,but also they remember a lot of information about that individual.Social groups often rely on this memory--such as knowing who is dominant to whom,who is related to whom,and who has done what to whom in the past.It's like remembering who you have lent money to or done a favour for,and who hasn't repaid that money or has talked about you behind your back.You have to learn who to trust,who your friends are,and who to watch out for. All this remembering goes on in a particular part of the brain called the neocortex  .If you compare the size of a mammal's social group with the size of this part of the brain,you will find they are remarkably closely related.This area seems to take a long time to develop fully and animals in which it is very large take a very  long time to grow up to adulthood.During this time,the youngster has to learn all the rules of social behaviour in their group and to piece together all  the relationships between the group members:Knowledge that will be needed to avoid getting into trouble. What is special about a mammalian social group according to the passage?
All the individuals cooperate to hunt. The individuals can communicate with each other. The individuals can recognize each other. The individuals form closer relationship.