Bioacoustics of Megaptera novaeangliae

Breeding. Reproductive biology of the Humpback Whale is probably the best known and most complex of any species of rorqual. Females and males reach sexual maturity at 4-5 years of age, although this figure may be closer to c.8 years in some areas. Female Humpback Whales in the Southern Hemisphere reach sexual maturity at an average body length of 1160-1250 cm and males at 1110-1200 cm. Comparable measurements for the Northern Hemisphere are 1000-1200 cm and 1000-1150 cm, respectively. Reproductive cycle of the Humpback Whale, like most rorquals,is strongly seasonal and in tune with annual migration to low-latitude breeding and birthing grounds. Females come into estrus in winter, and males follow suit by undergoing spermatogenesis and increased testosterone production. Conception primarily occurs during respective austral and boreal winters, with a peak around September in the Southern Hemisphere and February in the Northern Hemisphere. Whaling data, however, suggest that ovulation and successful mating also may occur at other times of the year. Gestation typically lasts 11-11-5 months, with a single young born in the subsequent austral or boreal winter. Although twin fetuses have been reported from butchered pregnant females, live twin births have not been documented. Whaling data also suggest that embryos grow at an average rate of 17-35 cm/month. Young are ¢.390-450 cm in length at birth and weigh 900 kg. Nursing young consume a minimum of 43 kg of milk/day. Young are weaned at 6-10 months on summer feeding grounds when they are ¢.990 cm in length. This suggests an average neonatal growth rate of ¢.3 cm/day. Young often remain with their mothers for up to a year. Females reportedly give birth every 2-3 years, although several examples are known of individual females giving birth nearly annually. Postpartum estrus may occur on a regular basis in Humpback Whales, which may explain male escorts in close company with mother—offspring pairs late in the breeding season. If females were not in estrus, it is unlikely that males would be showing such interest in them. Sex ratio of neonates is essentially 1:1, which probably holds for much of the year except for periods when differential segregation of sexes or sexual classes occurs on or at the ends of the seasonal migrations. On breeding grounds, operational sex ratio is male dominated because males typically remain longer in the area and relative number of fertilizable females is reduced by presence of immature and postpartum females. Both sexes apparently are promiscuous to some extent because paternity tests of young from individually identifiable females show that over successive seasons a given female may mate with several different males. This level of promiscuity calls into question the characterization of the Humpback Whale’s breeding strategy as solely polygynous. Details of the complexity of Humpback Whale’s breeding behavior have emerged from sustained research conducted since the late 1970s in warm waters around the main Hawaiian Islands. Humpback Whales begin arriving by December (some arrive as early as September), with peak abundance in February-March. The last Humpback Whales have usually departed by May, although some individuals remain as late as June in some years. Research suggests that the Humpback Whale practices a mainly polygynous mating system that grossly resembles a lek (males gathering to attract females). Although this mating system does not involve establishment of defensible territories, male Humpback Whales do exhibit aggressive competitive behaviors that include bubble screens, where a whale will forcefully blow bubbles from his blowholes; rear-body thrashing, where a whale throws his tail and peduncle out of the water before smashing it down sideways on the surface; horizontal tail slashing, where whales slash their tails horizontally through the water; and lobtailing, where a whale raises his flukes above the water before bringing them down to forcefully slap the surface. There are times when these aggressive behaviors result in actual physical contact that can lead to injury and,in rare instances, death. Males are also known to form temporary “coalitions” to drive away males in other competitive groups. Besides these male-male competition behaviors, Humpback Whales are also known to perform courtship behaviors that include rubbing or touching one another with their pectoral flippers; “flippering,” where whales raise their pectoral flippers out of the water and slap the surface; head-up displays, where whales horizontally elevate their heads above the surface and then slowly sink without swimming forward; vertical tail extensions, where whales vertically lift their tails out of the water; and escorting, where adult males join with a solitary or, more frequently, a female and her offspring. Vocalization, involving singing males, also plays an important role on the breeding grounds. Individual songs may last for 10-20 minutes, and some solo “performances” that repeat the song over and over may last for several hours or even days. All males in a given area basically sing the same, highly structured song, which consists of several tonal themes or “verses” that are sung in a generally consistent order. The performance appears to be highly ritualized and singers typically assume a motionless posture with head down, tail up, and flukes 7-15 m below the water’s surface. Over the course of the breeding season, the song in a given area evolves as new “verses” are added here and there. Within the context of a modified lek mating system, the song (vocal display) of singing males has been viewed as driven by sexual selection to attract females or dissuade competing males. This hypothesis suggests that the song provides some indication of the fitness of the singer. Although only adult male Humpback Whales sing, males and females produce a variety of different types of non-song vocalizations on breeding (and feeding) grounds and during migration. Other than mother—offspring pairs, the most stable social group among Humpback Whales on breeding grounds is a mother—offspring pair with an adult male escort. This group may remain together for hours or even days, and the escorting male typically remains within a body length of the female and appears to synchronize his breathing and dive patterns with hers. There are also tactile interactions where the male will touch the female with his pectoral flippers. Studies in the Hawaiian breeding grounds of the Humpback Whale suggest that as many as 86% of mother—offspring pairs in a given season have been accompanied by an adult male escort. These interactions generally occur later in the breeding season after some of the postpartum females have come into estrus. Studies of growth layers formed in wax plugs taken from external ear canals of dead Humpback Whales suggest that they may live up to 48 years.

Food and Feeding. The diet of the Humpback Whale has been compared with that of the Fin Whale because both species are known to feed on marine invertebrates and vertebrates. This is especially true for populations of Humpback Whales in the Northern Hemisphere where studies of stomach contents of butchered individuals found distinct dietary patterns. Stomach contents often contained a single species of schooling fish, either herring (Chupeaspp.), capelin (Mallotus villosus), sand lance ( Ammodytes spp. ), or pollock (Pollachius spp.), but others had combinations of various vertebrate prey. Invertebrate prey species found in stomachs of Humpback Whales in the Northern Hemisphere included monotypic or polytypic aggregations of planktonic crustaceans (euphausids, mysids, or copepods) and benthonic species (amphipods). The apparent benthonic feeding strategy exhibited by some foraging Humpback Whales is rare for a species of rorqual and results in some individuals ingesting quantities of sand and gravel—also reported in stomach contents of butchered individuals. More recent studies using stable isotope analysis of biopsy dart samples, in combination with ship-based observation of feeding Humpback Whales in the Bering Sea, suggest a link between prey preference and habitat, with individuals tending to feed on fish in neritic areas and on plankton in deep oceanic waters. Unlike northern populations, Humpback Whales in the Southern Hemisphere appear to feed almost exclusively on pelagic planktonic crustaceans, primarily krill (Euphausia superba). Like all species of rorquals, the Humpback Whale is a lunge-feeder but also has evolved an impressive repertoire of specialized feeding strategies. Perhaps the most unique of these is the practice of bubble-net feeding in which a single individual or, more often, a cooperative grouping of several individuals use bubbles to concentrate schooling fish. A typical cooperative “bubble-netting” event begins with individuals diving to a level below schooling fish, where one individual begins to swim in an ever-narrowing circle while blowing a ring of bubbles. As the curtain of bubbles rises toward the surface, reflections and noise from bubbles scare fish into a tighter grouping, which is made even more concentrated and compact by loud feeding calls made by another individual in the group. When the resulting bait ball is near the water’s surface, the whales ascend through the bubble net and lunge-feed, often breaking the surface in what some observers refer to as a single collective gulp. Such cooperative feeding groups of Humpback Whales are fluid in terms of membership and may number as many as 15 individuals. A variation of bubble netting, called lobtail feeding, has been documented in Humpback Whales feeding in the North Atlantic Ocean and involves an individual slapping the water's surface above a school of fish before diving to blow the bubble screen. Both behaviors are thought to help concentrate prey, and a study that tracked the spread of this unique foraging innovation over 27years concluded that lobtail feeding was being “culturally” transmitted through the population. Humpback Whales also herd prey by using their flukes, pectoral flippers, or both to disturb the water column. Humpback Whales often herd prey toward the water’s surface, which functions as an escape barrier to concentrate prey. There is also some evidence of other forms of cooperative hunting with individual groups of Humpback Whales feeding in formation or using acoustic calls to synchronize feeding lunges.

Movements, Home range and Social organization. Migratory patterns, breeding and feeding areas, and population structure of the Humpback Whale are probably better known than for any other species of rorquals. A number of factors combine to provide this increased level of understanding including its general near-shore migratory routes; its preference for breeding areas near islands, banks, and reefs; and its preference for feeding areas near shelf-slope breaks, regional marine embayments, or channels. Slower swimming speed of Humpback Whales and its affinity for exuberant aquabatics make it easier to study directly. The Humpback Whale holds the record for the longest known migration of any mammal. In support of this claim is the report of a group of seven whales (including a young) that traveled from the tropical North Pacific breeding and birthing area off Costa Rica and Panama to feeding grounds around Antarctica, a distance of more than 8300 km. Another example of long distance migration involves the well-defined migration route of Humpback Whales between summer feeding grounds in Alaskan waters and winter breeding and birthing grounds in warm waters around Hawaii, a distance of ¢.4830 km. There are several accounts of photo-identified individuals completing this crossing in as few as 36 days, which translates to an average swimming speed of 134 km/day or an average of 5:6 km/h. In the North Pacific Ocean, Humpback Whales feed during summer in waters from the Chukchi Sea south to Honshu in the west and southern California in the east. Their winter migration takes them to three primary breeding and birthing grounds: western subtropical to tropical waters off southern China, Taiwan, and the Philippines east to the Marshall Islands; waters around the main oceanic islands of the Hawaiian archipelago; and warm waters off the southern tip of Baja California, the Revillagigedo Islands, and adjacent mainland Mexico. In the North Atlantic Ocean, Humpback Whales feed during boreal summer in waters as far north as the Labrador Sea, Greenland Sea, and Barents Sea, and as far south as the Gulf of Maine in the west and Ireland in the east. Primary boreal winter migration routes in the North Atlantic Ocean take Humpback Whales to breeding and birthing grounds in the West Indies around Hispaniola (19° N). A small number of Humpback Whales also visit breeding grounds around the Cape Verde Islands (15°-17° N). The likelihood of at least a third, currently unknown, breeding and birthing area in the North Atlantic Ocean is suggested by genetic evidence. The population of Humpback Whales in the northern Indian Ocean is blocked from migrating northward during boreal winter by the Eurasian landmass and consequently remains year-round in warm waters of this region from the Gulf of Aden through the Arabian Sea and into the Bay of Bengal. Various populations of Humpback Whales in the Southern Hemisphere spend austral summer feeding in highly productive waters south of the Antarctic Convergence to edge of the pack ice around Antarctica. The International Whaling Commission (IWC) recognizessix latitudinally arranged feeding areas in the Southern Ocean that are visited by southward migrating Humpback Whales beginning in November, reaching a peak in January, and departing entirely by May. During austral winter migration, groups of Humpback Whales move north out of the Southern Ocean to seven primary breeding and birthing grounds that include waters of the western South Atlantic Ocean (coastal waters of Brazil from Natal to Cabo Frio and the Abrolhos Banks), the eastern South Atlantic Ocean (coastal waters from the Gulf of Guinea south to Cape Town), the south-western Indian Ocean (coastal waters of Mozambique and Madagascar), the south-eastern Indian Ocean (coastal waters of north-western Australia), the western South Pacific Ocean (the Coral Sea and tropical waters around Vanuatu, Fiji, and Tonga), the central South Pacific Ocean (French Polynesia and the Cook Islands), and the eastern South Pacific Ocean (Gulf of Panama south to the Gulf of Guyaquil and the Galapagos Islands). Interestingly, breeding grounds in the eastern South Atlantic Ocean and eastern South Pacific Ocean extend into tropical waters north of the Equator—a pattern that is probably related to the strong eastern boundary currents in these areas. Unlike other species of rorquals, Humpback Whales have been the focus of rather intensive fieldwork to determine population structure on feeding and breeding grounds. From this work,it appears that populations of Humpback Whale in the Northern Hemisphere typically come together on common breeding grounds but disperse into separate loosely affiliated groups as they leave for their summer feeding grounds. These migrating subpopulations show a high degree of genetic structure that suggests matrilineal fidelity to particular feeding grounds. During migration, social organization of individual groupsis quite fluid in terms of membership, with individualsjoining and leaving generally small groups on a somewhat regular basis. Population structure on the more communal feeding grounds, however, is probably diffuse, a possible result of male-mediated gene flow among subpopulations arriving from different feeding grounds. Feeding associations composed of multiple individuals tend to be temporary and dependent on constantly varying prey-patch size. Nevertheless, certain feeding associations (e.g. Humpback Whales in the Gulf of Maine) have been reported to be more stable and to last over most of the feeding season. Vocalization in Humpback Whales is a well-known social aspect of their behavior, especially relatively complex songs sung by solitary males on winter breeding grounds. Other forms of vocalization are known, and both males and females have been reported to make calls on summer feeding grounds or during annual migrations. As a form of communication among Humpback Whales, these calls or vocal sounds appear to have specific functions and, in some cases, are restricted to particular social groups. For example, vocal sounds termed “wops” seem to represent a contact call between a mother and her offspring, while “thwops” are thought to be a more general contact call among individuals of no particular gender or age group. A series of grunts or grunt trains has been associated with comingling of groups. Other vocal sounds produced by Humpback Whales include groans, snorts, “crys,” barks, and underwater blows or bubble bursts. Another form of social sounds used by Humpback Whales in communication are surface generated sounds such as those produced by breaching, which appears to communicate location and body size when an individual is leaving or joining a group, or pectoral slapping, which may be a signal from females to nearby males. These surface-generated sounds also increase in frequency when whales are excited and,at times, may represent no more than simple play.