Humpback Whale (marine mammals)

 

The humpback whale (Fig. 1) is one of the best known and easily recognizable of the large whales. It is known for its frequent acrobatic behavior and its occasional tendency to approach vessels. In recent years, thousands of humpback whales have been identified individually from natural markings (notably the pattern on the ventral surface of the tail flukes), and as a result, much has been learned about the biology and behavior of this species.

I. Characters and Taxonomic Relationships

At close range, humpback whales are easily distinguished from any other large whale by their remarkably long flippers, which are approximately one-third the length of the body. The flippers are ventrally white and can be either white or black dorsally depending on the population and the individual; flippers of North Atlantic humpbacks tend to be white, whereas those in the North Pacific are usually black (Fig. 1). The body color is black dorsally, with variable pigmentation on the underside (black, white, or mottled). The head and jaws have numerous knobs called tubercles, which are also diagnostic of the species. The dorsal fin is small but highly variable in shape, ranging from low (almost absent) to high and falcate. Like all rorquals, humpbacks have a series of ventral pleats running back from the tip of the lower jaw, in this species to the umbilicus. The tail is usually raised during a dive: the underside exhibits a pattern that is unique to each individual, which ranges from all white to all black. The presence of white on the ventral surface, and the prominent serration of the trailing edge, distinguishes humpbacks from other whales that “fluke” while diving, such as right, bowhead, blue, gray, and sperm whales.

Adult female humpback whales are typically 1 to 1.5 m longer than males. Maximum reliably recorded adult lengths are in the 16- to 17-m range, although 14-15 m is more typical (Clapham and Mead, 1999). Calves are 3.96 to 4.57 m at birth and approximately 8-10 ni at independence (Clapham et al. 1999), which occurs at the end of the calfs natal year. There are no easily observable differences between male and female humpbacks. Females possess a grapefruit-sized lobe at the rear of the genital slit: this lobe is absent in males (Glockner-Fer-rari and Ferrari, 1990). In addition, the spacing between the genital slit and the anus is considerably greater in males.

The skull of the humpback whale is easily distinguished from that of other baleen whales by the narrowness of the rostrum relative to the zygomatic width. The humpback has between 270 and 400 baleen plates on each side of the mouth. The plates are usually black, although those close to the tip of the jaw are sometimes white or partly white.

The genus Megaptera is monotypic and is one of two genera in the family Balaenopteridae (the “rorquals”). No subspecies are recognized. The binomial Megaptera novaeangliae derives from the Greek for “big wing” (mega + pteron) and the Latin for “New England,” which was the origin of the specimen used by Borowski in his description of the species in 1781.

II. Distribution and Ecology

Humpback whales are found in all oceans of the world (Fig. 2). They are a highly migratory species, spending spring through fall on feeding grounds in mid- or high-latitude waters, and wintering on calving grounds in the tropics, where they do not eat (Dawbin, 1966). Humpback whales are typically found in coastal or shelf waters in summer and close to islands or reef systems in winter. Some documented migratory movements of this species represent the longest known migration of any mammal, being almost 5000 miles one way (Palsb0ll et al., 1997). Not all humpbacks migrate ever)’ year, although the sex/age class of nonmigratory animals remains unclear. Remarkably, the purpose of migration remains unknown; it mav reflect a need to maximize energetic gain by exploiting pulses of productivity in high latitudes in summer and then gaining thermodynamic advantages by overwintering in warm water in winter. The only nonmigratory population is that residing in the Arabian Sea, where monsoon-driven productivity in summer permits the whales to remain in tropical waters year-round (Mikhalev, 1997).

The long flippers of humpback whales are white, whereas the dorsum of the bodij is usually black. The displaij of the flipper is a small part of a spectacular behavioral repertoire that includes tail slashing, breaching, and other behaviors.

Figure 1 The long flippers of humpback whales are white, whereas the dorsum of the bodij is usually black. The displaij of the flipper is a small part of a spectacular behavioral repertoire that includes tail slashing, breaching, and other behaviors.

In the North Atlantic, humpbacks return each spring to specific feeding grounds in the Gulf of Maine. Gulf of St. Lawrence, Newfoundland, Labrador, Greenland, Iceland, and Norway. Fidelity to these areas is strong and is determined by where a calf was taken by its mother in the formers natal year. Recent genetic analysis has indicated that this fidelity is maintained on an evolutionary time scale in at least Iceland and Norway (Palsb0ll et al, 1995; Larsen et al, 1996). Despite this fidelity, whales from all feeding grounds migrate to the a common breeding area in the West Indies, where they mate and calve (Katona and Beard, 1990). Historically important breeding areas in the Cape Verde Islands and the southeastern Caribbean appear to be utilized by relatively few whales today.

In the North Pacific, there are at least four separate breeding grounds in Hawaii, coastal Mexico, offshore Mexico (Revil-lagigedos Islands), and Japan (Calambokidis et al, 1997). Whales from these wintering areas migrate primarily to Alaska, California, possibly the Bering Sea, and the western North Pacific, respectively. However, crossover is not unknown and some transoceanic movements have been recorded (e.g., British Columbia to Japan and back).

The distribution of the humpback whale (Megaptera novaeangliae) illustrates a common pattern that, for the most part, excludes tropical warm tvaters in summer (Arabian humpbacks are an exception). The humpback is especially well known as a migratory species, tending to feed and mate at higher latitudes but calving at lower latitudes.

Figure 2 The distribution of the humpback whale (Megaptera novaeangliae) illustrates a common pattern that, for the most part, excludes tropical warm tvaters in summer (Arabian humpbacks are an exception). The humpback is especially well known as a migratory species, tending to feed and mate at higher latitudes but calving at lower latitudes.

In the Southern Hemisphere, humpbacks feed in circumpolar waters around the Antarctic and migrate to relatively discrete breeding grounds in tropical waters to the north. Six populations or “management areas’ are recognized by the International Whaling Commission in the Southern Hemisphere, but some movement between these areas is very likely.

The humpback whale has a generalist diet, feeding on eu-phausiids and various species of small schooling fish. The latter include herring (Chipea spp.), capelin (Mallotus vilhsiis), sand lance (Ammodytes spp.), and mackerel (Scomber scombrtis). Humpbacks appear to be unique among large whales in their use of bubbles to corral or trap schooling fish. Whales blow nets, clouds, or curtains of bubbles around or below schools offish and then lunge with mouths open into the center of the bubble structure (Jurasz and Jurasz, 1979; Hain et al, 1982). As with other balaenopterids, the ventral pleats expand when a humpback is feeding, allowing the animal to increase the capacity of its mouth greatly.

Rake-mark scars from teeth attest to the fact that humpbacks are commonly attacked by killer whales (Orchitis orca). However, it seems likely that fatal attacks are largely confined to very young calves, and predation does not appear to be a significant effector in the social organization of the humpback (Clapham, 1996).

III. Life History and Behavior

Breeding in humpback whales is strongly seasonal. Females come into estrus in winter, at which time testosterone production and spermatogenesis also peak in males (Chittleborough, 1965). The gestation period is about 11 months, with the great majority of calves born in midwinter. Calves probably begin to feed independently at about 6 months of age, but nursing likely continues in many animals until shortly before independence at about a year of age. Sexual maturity is reached in both sexes on average at 5 years. Interbirth intervals in females are most commonly 2 years, although annual calving is not unknown (Clapham and Mayo, 1990; Glockner-Ferrari and Ferrari, 1990). Although multiple fetuses have been recorded in dead pregnant females, living twins or multiplets are unknown.

The social organization of the humpback is characterized by small unstable groups, and individuals typically associate with many companions on both feeding and breeding grounds (Clapham, 1996). Longer-term associations (those lasting days or weeks) are occasionally recorded, but their basis is unclear. There appears to be no territoriality in this species.

In winter, male humpback whales sing long, complex songs, the primary function of which is presumably to attract females. All whales in a given population sing essentially the same song, and although the form and content of all songs change over time, the whales somehow coordinate these changes. Males also compete very aggressively for access to females (Tyack and Whitehead, 1983), and the resulting “competitive groups” can last for hours and involve tail slashing, ramming, or head butting. Males may also form coalitions, but further research is required to assess the significance and composition of such alliances.

In part because of the prominent male display aspect (i.e., singing behavior), the mating system has been compared to a lek (Mobley and Herman, 1985), although it does not possess the rigid territoriality common to such systems. Males almost certainly remain in breeding areas longer than females and attempt to obtain repeated matings, whereas newly pregnant females return quickly to higher latitudes (Dawbin, 1966) where they will feed for many months in order to prepare for the considerable energetic cost of lactation.

Humpbacks are well known for their often spectacular aerial behaviors. These include breaching, lobtailing, and flip-pering. Such behaviors occur at all times of year and in a variety of contexts, and it is clear that they perform a range of functions. These may include play, communication, parasite removal, and expression of excitement or annoyance.

IV. Conservation Status

The humpback whale was heavily exploited by the whaling industry for several centuries. Because of its coastal distribution, it was often the first species to be hunted in a newly discovered area. This century, some 200,000 humpbacks were slaughtered in the Southern Hemisphere alone; of these, more than 48,000 were taken illegally by the Soviet Union (Yablokov et al, 1998). It is quite likely that more than 90% of the animals in some populations were killed during the most intensive periods of exploitation. As a result, the humpback is considered an endangered species. Despite this, most studied populations appear to be making a strong recovery. The North Atlantic population has been estimated at 10,400 animals (Smith et al, 1999) and the North Pacific at 6000-8000 (Calambokidis et al., 1997). Strong population growth rates have been reported for many areas, ranging from 6.5% in the Gulf of Maine to more than 10% in some Southern Hemisphere populations (IWC, 1999). Commercial whaling for humpbacks officially ended worldwide in 1966, although the Soviets continued to hunt this species for some years afterward. Small aboriginal hunts for humpbacks still occur in a couple of locations, and many more whales die from entanglement in fishing gear or collisions with ships. However, none of these impacts appears to be significant at the population level, and the outlook for this once overexploited species appears good in most areas.

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