The Nature of Humpback Whale (Megaptera novaeangliae) Song

The nature of humpback whale (Megaptera novaeangliae) song has long interested both scientists and the general public. Research strongly indicates that humpback whale song is an important component in the social behaviour of breeding humpback whales, with proposals that the song has an intraand/or inter-sexual selection component. Some scientists, however, have alternate hypotheses, such as song being a means of sonar for detecting females. Song is not the only factor involved in humpback whale breeding behaviour, as groups of males follow and may physically compete for females. Hence, the exact nature of humpback whale song and its relation to their breeding behaviour is unclear. Why does whale song continuously change throughout the breeding season, and why do new songs spread so quickly throughout a population? In many respects, the nature of humpback whale song may resemble and parallel bird song. For example, many bird species that display innovation in the male’s song also have increased reproductive fitness, and a similar situation may occur in male humpback whales. To explain why such innovation may be selected for in humpback whales, this paper postulates that sexual selection in humpback whales may have both a physical and cognitive fitness component. —————————————————————————————————— “No one knew why humpbacks sang. Nate had been listening to them, observing them, photographing them, and poking them with sticks for twenty-five years, and still he had no idea why, exactly, they sang”. From Christopher Moore. Fluke. Or I know why the winged whale sings. Harper Collins. 2004. Introduction Why do male humpback whales (Megaptera novaeangliae) sing? The purpose and nature of the captivating songs of the humpbacks have been an enigma for modern zoologists ever since they were first discovered by scientists (79). Opinions as to the purpose of humpback song varies widely, from the calls being no different to those of herding animals to the idea that they may be a way of transmitting highly complex information, perhaps even being equivalent to language. There is debate as to whether the song is a mating call (vocalisation that contains information about the breeding intentions of the caller), a way of defending an area such as a territory (an area occupied and defended by one or more animals against conspecifics through overt defence or advertisement), a means of co-ordinating cooperation between males, a combination of these functions, or an as yet undetermined function. This theoretical paper seeks to review information on the nature of humpback song and the hypotheses as to its function, and to suggest some new hypotheses. Humpback whale behaviour Humpback whales (humpbacks) typically undertake extensive seasonal migration between high latitude summer feeding grounds and low latitude, tropical winter reproductive areas (c.f., Arabian Sea population; 63). On the high latitude feeding grounds, several of them will associate and co-ordinate with each other to produce “bubble nets” that aid in trapping and herding shoaling fish (98). Humpbacks appear to have traditional feeding and breeding grounds as they return to particular local habitats and regional feeding areas, which seems to be a result of early experience and maternal influence. Craig and Herman (26) and Weinrich (101) documented these results through individual return rates and population genetics in the southern Gulf of Maine and on the breeding grounds off the Hawaiian Islands. Research also shows that individuals from various feeding grounds may use one breeding area, presumably to increase their mating opportunities, although humpbacks from one breeding area may also visit several feeding grounds (12,90). Craig and Herman (26) suggested that not all females completed or even began the migration to Hawaii each year, as they may have become pregnant prior to migrating and returned to, or remained on, their feeding grounds. Some females along eastern Australia also remained in feeding grounds during winter (11). As a result, a biased sex ratio of 2.4 males to 1 female was found in both northand southbound migrations off eastern Australia by Brown et al. (11). This consequently limits the number of females on the winter breeding grounds. As males outnumber females, males have to compete physically for proximity to females (27). Furthermore, sexual segregation has been observed during the migration from the North Atlantic feeding grounds to the breeding grounds in the West Indies (90). Male humpbacks migrating off eastern Australia, associating together frequently, were found in larger groups than females (10). Males from all feeding grounds also arrive earlier at the breeding area in the West Indies than do females (90). Finally, comparing females with and without calves during the progression of the breeding season, Craig et al. (27) found that males in Hawaii associate preferentially with, and competed more vigorously for, females without a calf (i.e., females with high reproductive potential). This criterion appeared to be less important as the breeding season progressed, presumably as the number of births significantly reduced the availability of calf-less females. These various findings set the scene for very active male competition for breeding females. Not all humpback populations migrate, however. Mikhalev (63) argued that there is at least one population in the Arabian Sea that remains in these subtropical-tropical waters yearround and noted that this was unusual for the species. As the Arabian Sea is the site of a major oceanic upwelling, productivReceived July 2007; Accepted December 2008. Correspondance: E.C.M. Parsons, Department of Environmental Science and Policy, George Mason University, Fairfax, VA 22030-4444, [email protected] Journal of Marine Animals and Their Ecology Copyright © 2008 Oceanographic Environmental Research Society Vol 1, No1, 2008 Printed in Canada