The "risky" business of singing: tactical use of song during joining by male humpback whales

Balancing the costs and benefits of emitting a mating signal is complex. There are direct costs, such as time taken away from essential activities like feeding, and indirect costs, such as attracting unintended receivers, who may gain advantageous information from eavesdropping. As a consequence, the signaler may reduce his chances of mating if the costs outweigh the benefits. Male humpback whales (Megaptera noveangliae) sing. Although it is still unclear whether this signal is aimed at males, females, or both, it is generally accepted this is a breeding signal emitted in the presence of females. This study tested the hypothesis that the cost of singing in humpback whales is attracting other competitors to the group and therefore reducing the chances of the singer successfully mating. Males should therefore "choose" to sing only if the social conditions are not "costly." We found that males were less likely to sing when joining another group if there were two or more adults already in the group (other competitive males), if there was another singing male in the area (a known competitor), and in higher group densities (resulting in an increased likelihood of having another group, and therefore other competitive males close by). This result was confirmed in later years, where the density of whales had increased substantially but the probability of a male singing while joining other animals had reduced. These groups were more likely to be joined by additional animals in higher density, "riskier" conditions, especially if they included a whale that had joined the group previously while singing. It seems, therefore, that male humpback whales will join other animals while singing when the 'audience' comprises of fewer competitive males to reduce the potential cost of attracting eavesdroppers to the group. Significance statement The likelihood of a male joining a group while singing was tested against within-group dynamics (number of additional competitive males, presence of a calf), the presence of a nearby singing whale, the nearest neighbor, and the number of groups in the area (group density). Whales were more likely to join groups while singing if there were no other competitive males in the group and no calf, if there were no other singing whales in the area, and if group density was low. In later years, group density substantially increased, resulting in a decrease in the likelihood of a whale joining a group while singing. Results suggest male humpback whales are monitoring their audience composition (surrounding conspecifics) and using this information to make "vocal decisions" when joining a group containing a female.