<p class="first" id="d1417455e54">Humpback whales (Megaptera novaeangliae) are seasonal
breeders, annually migrating
from high-latitude summer feeding grounds to low-latitude winter breeding grounds.
The social matrix on the winter grounds is a loose network of interacting individuals
and groups and notably includes lone males that produce long bouts of complex song
that collectively yield an asynchronous chorus. Occasionally, a male will sing while
accompanying other whales. Despite a wealth of knowledge about the social matrix,
the full characterization of the mating system remains unresolved, without any firm
consensus, as does the function of song within that system. Here, I consider and critically
analyse three proposed functions of song that have received the most attention in
the literature: female attraction to individual singers, determining or facilitating
male-male interactions, and attracting females to a male aggregation within the context
of a lekking system. Female attraction suggests that humpback song is an advertisement
and invitation to females, but field observations and song playback studies reveal
that female visits to individual singers are virtually absent. Other observations
suggest instead that females might convey their presence to singers (or to other males)
through the percussive sounds of flipper or tail slapping or possibly through vocalizations.
There is some evidence for male-male interactions, both dominance and affiliative:
visits to singers are almost always other lone males not singing at that time. The
joiner may be seeking a coalition with the singer to engage cooperatively in attempts
to obtain females, or may be seeking to disrupt the song or to affirm his dominance.
Some observations support one or the other intent. However, other observations, in
part based on the brevity of most pairings, suggest that the joiner is prospecting,
seeking to determine whether the singer is accompanying a female, and if not soon
departs. In the lekking hypothesis, the aggregation of vocalizing males on a winter
ground and the visits there by non-maternal females apparently for mating meet the
fundamental definition of a lekking system and its role though communal display in
attracting females to the aggregation, although not to an individual singer. Communal
singing is viewed as a form of by-product mutualism in which individuals benefit one
another as incidental consequences of their own selfish actions. Possibly, communal
singing may also act to stimulate female receptivity. Thus, there are both limitations
and merit in all three proposals. Full consideration of song as serving multiple functions
is therefore necessary to understand its role in the mating system and the forces
acting on the evolution of song. I suggest that song may be the prime vector recruiting
colonists to new winter grounds pioneered by vagrant males as population pressures
increase or as former winter grounds become unavailable or undesirable, with such
instances documented relatively recently. Speculatively, song may have evolved historically
as an aggregating call during the dynamic ocean conditions and resulting habitat uncertainties
in the late Miocene-early Pliocene epochs when Megaptera began to proliferate. Early
song may have been comprised of simpler precursor sounds that through natural selection
and ritualization evolved into complex song.
</p>