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Humpback Whale: Natural History and Ecology

The following sections provide some of the latest information on humpback whales, based primarily on research findings. Although humpback whales are found in all of the world's oceans, special emphasis is given to the stock of North Pacific humpbacks, as this has been the focus of study of the Kewalo Basin Marine Mammal Laboratory and The Dolphin Institute. Much of the material reviewed below has appeared in dissertations by former KBMML students, including C. Scott Baker, Gordon Bauer, Alison Craig, and Adam Frankel. We appreciate their contributions greatly.

Taxonomy

The order Cetacea includes all species commonly referred to as whales, dolphins, and porpoises. Cetaceans are divided into two suborders: the odontocetes, or toothed whales, and the mysticetes, or baleen whales. The
mysticetes, Latin for "mustached", have no teeth. Instead, they have evolved a specialized filtering system consisting of hundreds of stiff keratinaceous plates known as baleen. The length and number of plates differ from species to species but essentially consist of a double series of elongated horny plates, which descend, at approximately right angles, from the roof of the mouth (Gaskin 1982). The inner side of the baleen plates is frayed, forming a bristle resembling coarse hair. The fringes of one baleen plate overlap those of the next forming a fibrous mate that efficiently filters zooplankton and small fish from seawater.

Recent work (Rice 1998) divides the mysticetes into four families: the Balaenidae or right whales (2 species), the Neobalaenidae or pygmy right whale (1 species), the Eschrichtiidae or gray whale (1 species), and the Balaenopteridae or rorqual whales (8 species). The humpback whale is the sole member of the genus Megaptera of the family Balaenopteridae. Common names of other member of the family Balaenopteridae include the blue, fin, and sei whales, as well as the Bryde's whale and the Minke whale.

The scientific name for humpbacks (Megaptera novaeangliae) means "big-winged New Englander." The name pays tribute to the exceptionally long pectoral fins (flippers) on this whale (the longest flippers of any whale species), and to the historic, prime feeding area for the North Atlantic stock in the waters off the New England coast. Humpback whales are also found in the South Atlantic, North Pacific, South Pacific and Indian oceans.

Population Abundance

The original, unexploited North Pacific population of humpback whales has been estimated as approximately 15,000 animals (Allen, 1980; Rice, 1978). Between the years of 1905 and 1965 vast numbers of these whales were killed by the whaling industry until the population was reduced below the level at which exploitation was economically feasible. Beginning in 1966, the member nations of the International Whaling Commission agreed to ban further whaling of humpbacks in the North Pacific, thereby effecting the worldwide protection of this species. It was estimated that by 1966 the North Pacific population had been reduced to approximately 1,000 animals, roughly 7% of the estimated original population (Rice, 1978; Wada, 1972; Wolman, 1978). In United States waters, the humpback whale is further safeguarded by the Marine Mammal Protection Act of 1972 and the Endangered Species Act of 1973. Based on photographic identification records for the years 1991 through 1993, it was estimated that the North Pacific population had increased to about 5,000 - 6,000 animals (Calambokidis et al., 2000). Although currently safe from commercial whaling, the humpback whale faces threats and disturbances from net or fishing gear entanglement, boat strikes, competition with commercial fisheries for food resources, habitat degradation through, offshore runoff and pollution, increased undersea noise from boat traffic, Navy sonar, and commercial undersea seismic exploration, and even unregulated whale-watching. Some of these threats can be controlled or ameliorated through public education and government regulations and policies.

Whale watching has proven to be a great economic asset to many locales, as well as providing experiences and education for the public. Public viewing of humpback whales form vessels has been especially well developed in Hawaii, Alaska, and the western North Atlantic. However, smaller enterprises in such places as the Ryukyuan Islands of Japan and along the coasts of Australia are increasing in popularity, and illustrates that benign, nonconsumptive approaches to whales can have an economic impact far greater than whaling.

Summer and winter grounds of humpbacks

Humpback whales migrate each year from summer coastal feeding grounds in high-latitude waters to breeding and calving grounds near islands or shallow banks in low-latitude waters.

Summer Feeding grounds

The North Pacific population of humpback whales feeds in the summer and fall along the upper rim of the North Pacific Ocean. Known summering grounds include sites along the coast of British Columbia, the Alexander Archipelago (southeastern Alaska), the Gulf of Alaska, and the Aleutian Islands (Andrews, 1916; Nemoto 1957; Pike and MacAskie, 1969; Jurasz and Jurasz, 1979; Rice and Wolman, 1982; Baker et al., 1985). Humpback whales also feed in the southern Bering Sea, the waters near the Kamchatka Peninsula, and the Farallon Islands of central California (Omura, 1955; Tomilin, 1957; Nasu, 1963; Dohl, 1983).

Nemoto (1959) categorized baleen whales as "skimmers" or "gulpers" based on their predominate feeding technique and prey size. Skimmers feed by filtering water, more or less continuously, as they swim slowly through the water. Their primary prey is the smaller zooplankton, particularly copepods. Nemoto considered the sei (Balaenoptera borealis), right, bowhead, and the gray whales to be skimmers. Gulpers, or more accurately, "lungers", feed by engulfing a vast volume of water and filtering the prey as they push the water back out through their baleen (Pivorunas 1979). The greater speed of the lungers allows them to feed on both macroplankton and small fish. Nemoto (1959) considered the humpback, minke (Balaenoptera acutorostrata), and fin whales to be lungers.

Humpback whales are well known for two characteristic surface-feeding strategies - lunge feeding and bubble netting (Ingebrigsten 1929; Jurasz and Jurasz, 1979; Watkins and Schevill, 1979; Hain et al., 1982). Surface-lunge feeding is associated primarily with feeding on near-surface patches of euphausiids of schools of small fish. The whale's trajectory may be the result of its attempt to trap prey against the surface (Jurasz and Jurasz, 1979). During bubblenet feeding, a humpback whale forms a spiral-shaped curtain of bubbles, apparently to concentrate of contain patches of euphausiids of schooling fish. Bubblenets a few meters in diameter are generally associated with feeding on euphausiids. Bubblenets as large as 25 meters are associated with feeding on larger schooling fish such as herring (Jurasz and Jurasz, 1979).

Winter reproductive grounds

There are three main wintering areas for humpback whales in the North Pacific. In the eastern Pacific, humpbacks winter in the waters of the Revillagigedo Archipelago, the northern Gulf of California, the southern coast of Baja California, and the mainland coast of Mexico, including the area around the Islas Tres Marias and the Isla Isabel (Urban & Aguayo, 1987). In the central Pacific, humpbacks aggregate in the waters surrounding the main Hawaiian Islands, with the most dense concentrations occurring in the Penguin Bank area (a relatively shallow bank that extends southwesterly from Laau and Ilio Points on Molokai) and in the "four-island region" between Maui, Lanai, Molokai and Kahoolawe (Baker & Herman, 1981; Herman & Antinoja, 1977; Herman, Forestell, & Antinoja, 1980; Mobley, Bauer, & Herman, 1999). Finally, in the western Pacific, humpback whales winter primarily in the waters surrounding the Ryukyuan and Bonin Islands (Nishiwaki, 1966). The Hawaiian Assembly is the numerically largest of the three (Calambokidis et al., 1997).

Certain features are common to the major wintering areas are situated at a latitude of approximately 20 in both northern and southern hemisphere (Clapham & Mead, 1999), and have a sea-surface temperature of between 24 C and 28 C in the winter (Dawbin, 1966; Herman, 1979, Whitehead & Moore, 1982). The main wintering areas are also characterized be extensive regions of relatively shallow water. Humpback whales are often described as a coastal species, but the high densities of these whales on offshore, shallow banks (e.g., Penguin Bank in Hawaii, and Silver and Navidad Banks in the West Indies) suggest that shallow water is more important than coastal areas per se. The majority of humpbacks in Hawaii occur within the 183 m (100 fathom) isobath (Herman & Antinoja, 1977; Herman et al., 1980; Mobley, Forestall, & Grotefendt, 1997). More specifically, the areas supporting the most dense aggregations of humpbacks in Hawaii are of limited depth: the waters of Penguin Bank are, on average, about 60 m deep, and the waters of the four-island region range in depth from 30 m to about 80 m (NOAA, 1997). Winn, Edel, and Taruski (1975) reported that 99% of all sightings of humpbacks in the West Indies were at depths between 10 and 100 fathoms. Whitehead and Moore (1982) were even more specific, and reported that the majority of humpbacks in the West Indies are found in waters less than 60 m deep. Thus, overall, it appears that bathymetry is an important influence on habitat selection by humpback whales on the winter grounds.

Reproductive Biology

Reproduction is strongly seasonal in humpback whales. Most of what is know about reproductive biology comes from biologists that examined carcasses during the intensive whaling era in the approximate first 65 years of the 20th century. These biologists found that the testes underwent considerable increase in mass during the winter breeding season (Chittleborough, 1955; Symons & Weston, 1958), and that spermatogenesis (in humpbacks of the Southern Hemisphere was found to peak during the austral winter in late July and August. Northern Hemisphere populations of humpback whale presumably show similar peaks of spermatogenesis, albeit six months later than southern humpbacks. The timing of ovulation in females coincides with spermatogenesis in males. In the Southern Hemisphere, most ovulations occurred in late July, although there were also considerable numbers of ovulations in August and September (Chittleborough, 1954). Data collected from the North Pacific revealed high rates of ovulations in February, although ovulation occurred throughout the sampling period of January through April (Nishiwaki, 1959). Some females are seasonally polyestrous (Chittleborough, 1954; Matthews, 1937; Robins, 1960). Chittleborough (1954) reported that most females (approximately 62 - 84%) ovulate only once during the winter, but some (approximately 16 - 28%) ovulate twice, and a few (up to 8%) ovulate three times. Thus, an estrous cycle that does not result in conception may be followed by another cycle. This suggestion is supported by the findings of Robins (1960), who also examined the ovaries of whales that had been caught commercially. He noted that as the reproductive season progressed, there was an increase in the mean number of same-season ovulations per female. This trend was evident in each of the three years during which data were collected.

Sexual maturity in both males and females occurs at approximately five years of age (Chittleborough, 1965; Clapham, 1992, Nishiwaki, 1959). Females conceive during the winter season on or enroute to the winter grounds, and return to give birth to a single calf the following winter after a gestation period of 11-12 months (Chittleborough, 1958; 1965; Matthews, 1937; Tomilin, 1967). The mean length of lactation reported by Chittleborough (1958) was 10.5 months, and most mothers and calves have separated permanently by the time the calf is one year old (Clapham & Mayo, 1987; 1990). However, separation does not occur consistently at the same stage in the migratory cycle. Some separations of mothers and calves occur on the summer feeding grounds (Baraff & Weinrich, 1993), whereas others are delayed until after the mother and yearling have arrived on the winter grounds (Baker, Perry, & Herman, 1987; Chittleborough, 1965; Glockner-Ferrari & Ferrari, 1984).

Crude birth rates, (the ratio of the number of calves to the total number of whales, including calves), calculated from long-term data collected in the summer feeding grounds have ranged from 0.075 to 0.079 (Clapham & Mayo, 1987; 1990) (i.e., calves constituted The most common inter-birth interval for female humpbacks is two years, although one-year inter-birth intervals occur on occasion (Baker et al., 1987; Barlow & Clapham, 1997; Chittleborough, 1958; Clapham & Mayo, 1987; 1990; Glockner-Ferrari & Ferrari, 1990; Matthews, 1937; Straley, Gabriele, & Baker, 1994). An inter-birth interval of one year means that female humpbacks may enter estrous and conceive shortly after parturition. Indeed, whaling biologists observed that some females were simultaneously pregnant and lactating (Chittleborough, 1958; Matthews, 1937). The fact that ovulation may occur in females with young calves has important implications for an understanding of the social dynamics of humpback whales in winter grounds.

Characteristics of migrating population

The long-distance migration of humpback whales between summer and winter grounds has important influences on many aspects of humpback whale behavior and ecology. Data collected from commercial whaling operations suggested that migrations to and from the winter grounds were loosely segregated based on age and sex (Chittleborough, 1965; Dawbin, 1966; 1997; Nishiwaki, 1959; 1966). Dawbin (1997) summarized data collected from 65,600 humpbacks caught in the southern hemisphere at locations including Cook Strait, Tonga, Western Australia, Durban, and the Congo. The data from this large sample confirmed the order of migration he had reported previously (Dawbin, 1966). On average, the first humpbacks to migrate to the winter grounds of the southern hemisphere were females in late lactation, together with their yearling offspring. These females were then followed by immature animals of both sexes, mature males, "resting" females (whose ovaries and mammary glands showed no evidence of recent activity), and, finally, late-pregnant females. The first to begin the return migration to the summer feeding grounds were the newly pregnant females and resting females, followed by immature animals of both sexes, then mature males and, finally, females in early lactation along with their calves. This "migratory parade" of whales of different ages, sexes, and reproductive states suggests that in the winter grounds, the relative proportions of different classes of whale may be in a state of flux throughout the season. Consequently, the migratory parade may have profound effects upon the activities and social groupings of humpback whales on the winter grounds as the breeding season progresses.

Migration to the winter grounds represents a considerable energetic investment. Although migratory swimming itself arguably incurs no more energetic demands than does swimming within a more restricted locale (Corkeron & Connor, 1999), the absence of an exploitable food resource in the winter grounds and the rarity of feeding during most of the migration mean that humpbacks must subsist on stored fat reserves for extended periods of time (Chittleborough, 1965; Dawbin, 1966; Nishiwaki, 1959). Thus, the differing energy requirements of different classes of whale are likely to exert considerable influence over migratory habits. For example, late-pregnant females are the last class of whale to migrate to the winter grounds presumably because of the pressure to maximize food intake before commencing the energetically expensive process of lactation. The high energy costs of lactation in humpback whales were revealed by commercial whaling data, which showed that pregnant female humpbacks yielded around twice as much oil as was obtained from equivalent-sized females in late lactation (Dawbin, 1966). Apparently, not all females migrate annually to the winter grounds of Australia (Brown, Corkeron, Hale, Schults, & Bryden, 1995), Hawaii (Craig & Herman, 1997), and the North Atlantic (Smith et al., 1999). This is most likely another response to the energetic demands of migration and residence in the winter grounds.

Social Organization and Behavior on the Winter Grounds

The absence of feeding behavior in the winter grounds means that social groupings are not influenced by foraging strategies. In addition, the absence of significant predation pressure on humpbacks in low latitudes releases them from the need to group together for the purpose of communal defense or increased vigilance (Clapham, 1996). Consequently, group sizes tend to be relatively small, with modal pod size of one or two reported for humpbacks in the winter grounds of the North Pacific, North Atlantic, and Southern Hemisphere (Chaloupka & Osmond, 1999; Helweg & Herman, 1994; Herman & Antinoja, 1977; Herman et al., 1980; Mattila & Clapham, 1989; Mattila, Clapham, Vasquez, & Bowman, 1994; Mobley & Herman, 1985). For example, Herman et al. (1980) reported the following group sizes from aerial surveys of the Hawaiian Islands conducted in 1977: one whale (37.5%), two whales (31.5%), three whales (21.5%), four whales (6.0%), five whales (3.0%), and eight whales (0.5%).

The role of the escort whale

Females (with or without a calf) are often accompanied by one or more adults, termed "escorts" (Herman & Antinoja, 1977). Underwater observations of the genital region and DNA studies have indicated that escorts are male (Clapham, Palsboll, Mattila & Vasquez, 1992; Glockner, 1983; Glockner-Ferrari & Ferrari, 1985). Escorts are assumed to be prospecting for receptive females, or perhaps mate-guarding after copulation (Brown & Corkeron, 1995; Clapham, 1996; Mobley & Herman, 1985). A related phenomenon observed on the winter grounds is the "competitive group." This grouping, often characterized by rapid swimming, much near-surface and at-surface activity, and vigorous and aggressive interactions among group members, consists of multiple escorts competing for proximity to the lone female in the group. Tyack & Whitehead (1983) called the animal defending the position closest to the female the "primary escort" and the others "secondary escorts." Recent work at our laboratory has shown that primary escorts are, on average, significantly larger than secondary escorts (Spitz, 1999; Spitz et al., 2000). For the female, the competitive group may function as a mechanism for mate choice (L. Herman, personal observations) as the males sort out through threat and aggression who will occupy the role of principal escort. The female may indirectly encourage prolonged competition and avoid mating until later by continued rapid swimming, and by behaviors such as inverted postures and pectoral fin waving. Because actual mating has never been observed, the mechanisms, such as the one postulated, remain speculative.

In general, females are seen simultaneously and serially with multiple males, and males are seen serially with different females (Baker & Herman, 1984). The affiliation between an individual escort and the female generally lasts for a few hours only (Tyack & Whitehead, 1983; Mobley & Herman, 1985). As would be predicted from the fluidity of social groups observed in the winter grounds, recent DNA analysis revealed that calves born to the same females in different years had been fathered by different males (Clapham & Palsboll, 1997).

The role of "song"

Another important aspect of behavior on the winter grounds is a complex vocalization known as "song" (Payne & McVay, 1971). All singing whales that have been sexed by either observation of the genital region or molecular analysis have been male (Glockner, 1983; Glockner-Ferrari & Ferrari, 1985; Lambertsen, Baker, Duffield & Chamberlim-Lea, 1988; Medrano et al., 1994; Winn, Bischoff & Taruski, 1973). Song has occasionally been reported from the feeding grounds, but occurs primarily on the winter grounds and during migration and (Clapham & Mattila, 1990; Mattila, Guinee, & Mayo, 1987; McSweeney, Chu, Dolphin, & Guinee, 1989; Winn & Winn, 1978). Various functions have been proposed for song, including sexual advertisement (Payne & McVay, 1971; Winn & Winn, 1978), the maintenance of spacing between males (Winn & Winn, 1978; Tyack, 1981; Frankel, Clark, Herman, & Gabriele, 1995), the synchronization of ovulation in females (Baker & Herman, 1984), and a means for male-male assessment of each other (Darling & Berube, 2001). The roles of singer and escort may be adopted by the same male at different times or even at the same time (Baker & Herman, 1984; Darling et al., 1983; Tyack & Whitehead, 1983). The concentration of singing humpback whales exhibit many features typical of a lek mating system. Because individual males do not appear to be territorial, as would be the case on a classic lek, Clapham (1996) suggested that the humpback whale mating system is best classified as a "floating lek" (p. 35).

One of the intriguing aspects of song is its dynamic character. In a given geographical winter region, such as Hawaii, the song undergoes change throughout the season, but all whales in the region adopt the new changes, apparently through a process of vocal mimicry (e.g., see review in Helweg, Frankel, Mobley & Herman, 1991). In their seminal work, Payne and McVay (1971) described the structure of humpback whale song as a succession of four to seven themes. Each theme consisted of two or more phrases, and each phrase was composed of individual sound units, defined as the smallest acoustic segment that could be identified as a whole unit by the human ear. Singers will transition in a fixed sequence from one theme to another during a song session, which may last for hours. After the last theme is reached, the whale returns to the beginning. Singers typically are found alone and stationary at depths of about perhaps 15-30 m, although, as was noted, occasionally they may sing while moving or while accompanying a mother-calf pair. Playback studies have revealed that females rarely approach singers (Mobley, Herman & Frankel, 1988). Males more frequently approach singers, but remain typically for only brief time (darling & Berube, 2001). Hence, the functions of song continue to be elusive.

Click here for an example of whale song!

The operational sex ratio

Because the average two-year reproductive cycle of females effectively removes half of them from the annual reproductive pool (Mackintosh, 1972), it has been suggested that the operational sex ratio in the winter grounds is probably about one female to two males (Herman & Tavolga, 1980). However, given the recent evidence that not all females migrate annually to the winter grounds (Brown et al., 1995; Craig & Herman, 1997; Smith et al., 1999), the operational sex ratio is likely to be even more skewed. This helps to explain the intensity of competition between males (see Baker & Herman, 1984). It also helps to explain the frequency with which males escort females and their calves, despite the fact that these females are less likely to come into estrus than are females without calves. Because newly pregnant females are the first class of whale to leave the winter grounds (Chittleborough, 1965; Dawbin, 1966, 1997), the operational sex ratio will become increasingly skewed as the season progresses until mature males begin to depart for the feeding grounds.

These changes in operational sex ratio and the temporally staggered migrations from which they result, together with seasonal trends in reproductive physiology, may be expected to bring about seasonal trends in pod size and levels of aggression. Indeed, Baker and Herman (1984) reported that in Hawaii, the percentage of pods engaged in aggression peaked at the same time as did mean pod size and overall abundance. All peaks were observed to occur during the first two weeks of March. Mobley and Herman (1985) found that the peak rate of pod membership change occurred during the first two weeks of March. Herman et al. (1980) and Mobley and Herman (1985) reported that pods containing a calf in Hawaii tended to increase in size as the season progressed. Thus, the behaviors and social groupings of humpback whales are not uniform throughout the reproductive season, but may be influenced by various factors including density and the relative proportion of different classes of whale.

Future research

Much remains to be learned about humpback whale society and behavior. Active research is continuing by many different research groups in the feeding areas of the whales in the western North Atlantic and the eastern and central North Pacific. Similarly, many research groups continue to study the whales in their winter grounds, particularly in the North Pacific. There is also increasing research activity in the south Pacific, particularly along the migration route of the whales past the eastern coast of Australia. There is therefore reason to be optimistic that many of the mysteries of the humpback will begin to reveal them selves in the next decade or so.

 

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