Mating and breeding take place during the winter months, which is when females reach estrus and males reach peak testosterone and sperm levels. Humpback whales are promiscuous, with both sexes having multiple partners. Males will frequently trail both lone females and cow–calf pairs. These are known as "escorts", and the male that is closest to the female is known as the "principal escort", who fights off the other suitors known as "challengers". Other males, called "secondary escorts", trail further behind and are not directly involved in the conflict. Agonistic behavior between males consists of tail slashing, ramming, and head-butting.
