The social and reproductive biology of Humpback Whales: an ecological perspective

Abstract

Existing knowledge of the social organization, mating system and reproduction of Humpback Whales (Megaptera novaeangliae) is reviewed to assess how our current understanding of this wide‐ranging marine mammal fits into the predictive framework developed from ecological studies of more accessible terrestrial taxa. The small unstable groups characteristic of this species on its summer feeding grounds appear to be a function of absence of predation and of the patchy, mobile nature of most prey; the absence of territoriality and the minimal importance of kinship in associations are also predictable consequences of the latter. The mating system is similar to both leks and to male dominance polygyny, in which males display (sing) or directly compete (perhaps sometimes in coalitions) for access to females. However, the rigid spatial structure characteristic of classic leks is absent. The mating system of this species is sufficiently different to merit a novel category, and ‘floating lek’ is proposed. The widespread distribution of females resulting from absence of both predation and resources during the breeding season preclude simultaneous monopolization by males of more than one potential mate. Furthermore, these factors, together with a male‐biased operational sex ratio, minimize the possibility of competition among females. The intensity of intrasexual competition among males conforms to predictions derived from information on testis size and from expectation of future reproductive success. Female choice and, to a lesser extent, differential allocation of competitive effort by males, appears likely. Lack of interpopulation variation in social and mating behaviour, and in general reproductive biology, is likely a response to similarity of marine environmental conditions. Year‐to‐year variation in reproductive rates may be linked to variations in the abundance of prey. The invariably uniparous nature of female Humpback Whales is assumed to be related to the energetic demands of lactation, and the lower ratio of available energy partitioned to reproduction that is characteristic of larger mammals. The reversed sexual size dimorphism of this species may reflect different selective pressures on males and females. Finally, there is now evidence that, as in some other taxa, offspring sex ratio is related to maternal condition.

Geographic variation in killer whale attacks on humpback whales in the North Pacific: implications for predation pressure

“…90 yr, estimated from Chittleborough 1959), the proportion of time that a whale is under threat of attack over its lifetime may be small. Because it appears that most attacks appear to occur when whales are calves (Clapham 1996, Naessig & Lanyon 2004, Mehta et al 2007), the high prevalence of rake mark scarring in some areas indicates that predation could be a significant source of calf mortality, particularly in some regions.…”

A quarter of a world away: female humpback whale moves 10 000 km between breeding areas

“…The mating system of humpback whales has been described as a 'floating lek', with high concentrations of individuals at relatively discrete mating and calving sites [13]. Substantial movement has been documented within these sites, but although instances of movement between two breeding sites that are separated by approximately 6000 km have been reported [14,15], long-distance movement of individuals between discrete breeding grounds is rare [16].…”

Acoustic monitoring on a humpback whale ( Megaptera novaeangliae ) feeding ground shows continual singing into late spring

“…Two major hypotheses have been proposed for the function of song in humpback whales: (i) that it serves primarily to attract females (Winn & Winn 1978;Tyack 1981); and (ii) that it establishes dominance rankings among males (Darling & Bérubé 2001). Song is clearly an important element of the humpback's mating system, which has been described as polygynous/promiscuous with some similarity to a lek (Mobley & Herman 1985;Clapham 1996Clapham , 2000. Humpback whales migrate from summer feeding grounds in temperate or polar waters to winter breeding and calving areas in the tropics (Kellogg 1929).…”