Pectoral slapping, informally known as pec-slapping, is when a cetacean turns on its side, exposes one or both pectoral fins into the air, and then slaps them against the surface of the water. It is a form of non-vocal communication commonly observed in a variety of whale and dolphin species as well as seals. The motion is slow and controlled, and the behaviour can occur repeatedly by one individual over a few minutes. The humpback whale's pectoral fin is the largest appendage of any mammal and humpbacks are known for their extremely acrobatic behaviour. Pec-slapping varies between groups of different social structure, such as not occurring in lone males but being common in mother calf pairs and also when they are accompanied by an escort. The reasons for pec-slapping therefore can vary depending on age and sex of individual humpback whales. During the breeding season adult males pec-slap before they disassociate with a group of males that are vying for a female, whereas adult females pec-slap to attract potential mates and indicate that she is sexually receptive. Its function between mother calf pairs is less well known but is likely to be a form of play and communication that is taught to the calf by the mother for use when it is sexually mature. Pectoral slapping has also been observed in the right whale, but due to its smaller size, the sound produced will be quieter and therefore used for communication over smaller distances unlike the humpback. Exposure of the pectoral fin and consequent slapping has also been infrequently observed in blue whales, where it is most often a by-product of lunge feeding followed by rolling on to its side.
