The skin on a female blue shark is nearly twice as thick as that of males. It is believed that this is because of the viciousness of mating. Males will often bite females during face to face copulation leaving them with wounds. Without the extra thickness of the skin females could be severely injured. Mating behaviour in sharks can be very complex and evidence suggests that there is some degree of mate selection within certain species.
Sexes often segregate, and only come together to mate.
Occasionally congregations form in mating behaviour. For instance, basking sharks have been seen forming mating circles like the one seen in this picture. The exact purpose of this behaviour is not known however it is clear that it related to mating. There are three modes of reproduction within the sharks. In general most sharks bear live young, however there are some sharks who lay eggs.