- Email: [email protected]
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VI. Other Specialized Fats
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In addition to blubber, several other unusual and specialized fat bodies exist that are unique to a single group of cetaceans, the odontocetes or toothed whales. These fat bodies occur in the forehead tissue (melon) and in and around the mandibles of the lower jaw (mandibular fats) and play important roles in hearing and echolocation. They are composed of a unique array of lipid classes and fatty acids that are likely synthesized with these head tissues (Koopman et al., 2003, 2006). These unusual fats are believed to facilitate sound reception by acting in the melon to focus high frequency sound produced in the nasal passages, while in the mandibular fats, they are organized to form a channel to transmit received sounds to the ear. In all odontocetes examined, short- and branched-chain fatty acids appear to be concentrated in the center of the inner mandibular fat body and immediately adjacent to the earbones. Because sound travels more slowly through these types of fatty acids, this should cause sound entering an odontocete head to bend inwards and be directed to the ears (Koopman et al., 2006). The unique arrangement of lipids within these fat bodies and their direct effect on sound transmission is an important are of current research. In conclusion, blubber and other fats play a number of major roles in the lives of marine mammals. These fats can also be a powerful tool in trying to understand adaptive solutions of species living in marine environments as wells as insights into their ecology and behavior.
See Also the Following Articles Skeletal Anatomy ■ Swimming ■ Pinniped physiology
References Beck, C. A., Bowen, W. D., and Iverson, S. J. (2000). Seasonal changes in buoyancy and diving behaviour of adult grey seals. J. Exp. Biol. 203, 2323–2330. Bowen, W. D., Oftedal, O. T., and Boness, D. J. (1992). Mass and energy transfer during lactation in a small phocid, the harbor seal (Phoca vitulina). Physiol. Zool. 65, 844–866. Heath, M. E., and Ridgeway, S. H. (1999). How dolphins use their blubber to avoid heat stress during encounters with warm water. Am. J. Physiol. 276, R1188–R1194. Iverson, S. J. (1993). Milk secretion in marine mammals in relation to foraging: Can milk fatty acids predict diet? Symp. Zool. Soc. Lond. 66, 263–291. Iverson, S. J., Bowen, W. D., Boness, DJ., and Oftedal, O. T. (1993). The effect of maternal size and milk output on pup growth in grey seals (Halichoerus grypus). Physiol. Zool. 66, 61–88. Iverson, S. J., Oftedal, O. T., Bowen, W. D., Boness, DJ., and Sampugna, J. (1995). Prenatal and postnatal transfer of fatty acids from mother to pup in the hooded seal. J. Comp. Physiol. 165, 1–12. Iverson, S. J., Field, C., Bowen, W. D., and Blanchard, W. (2004). Quantitative fatty acid signature analysis: A new method of estimating predator diets. Ecol. Monogr. 74, 211–235. Iverson, S. J., Stirling, I., and Lang, S. L. C. (2006). Spatial and temporal variation in the diets of polar bears across the Canadian arctic: Indicators of changes in prey populations and environment. Symp. Zool. Soc. Lond.: Conservation Biology Series 12, 98–117. Kirsch, P. E., Iverson, S. J., and Bowen, W. D. (2000). Effect of diet on body composition and blubber fatty acids in captive harp seals (Phoca groenlandica). Physiol. Biochem. Zool. 73, 45–59. Koopman, H. N. (1998). Topographical distribution of the blubber of harbor porpoises (Phocoena phocoena). J. Mammal. 79, 260–270. Koopman, H. N. (2007). Phylogenetic, ecological, and ontogenetic factors influencing the biochemical structure of the blubber of odontocetes. Mar. Biol. 151, 277–291.
Koopman, H. N., Iverson, S. J., and Read, A. J. (2003). High concentrations of isovaleric acid in the fats of odontocetes: Variation and patterns of accumulation in blubber vs. stability in the melon. J. Comp. Physiol. 173, 247–261. Koopman, H. N., Budge, S. M., Ketten, D. R., and Iverson, S. J. (2006). The topographical distribution of lipids inside the mandibular fat bodies of odontocetes: Remarkable complexity and consistency. IEEE J. Oceanic Eng. 31, 95–106. Lockyer, C. (1987). Evaluation of the role of fat reserves in relation to the ecology of North Atlantic fin and sei whales. In “Approaches to Marine Mammal Energetics” (A. C. Huntley, D. P. Costa, G. A. J. Worthy, and M. A. Castellini, eds), pp. 184–203. Society for Marine Mammalogy Special Publication No. 1., Allen Press, Lawrence, KS. Mellish, J. E., Iverson, S. J., and Bowen, W. D. (1999). Individual variation in maternal energy allocation and milk production in grey seals and consequences for pup growth and weaning characteristics. Physiol. Biochem. Zool. 67, 677–690. Pabst, D. A. (1996). Springs in swimming animals. Am. Zool. 36, 723–735. Pabst, D. A., Rommel, S. A., and McLellan, W. A. (1999). The functional morphology of marine mammals. In “Biology of Marine Mammals” (J. E. Reynolds, and S. A. Rommel, eds), pp. 15–72. Smithsonian Institution Press, Washington, DC. Pond, C. M., Mattacks, C. A., Colby, R. H., and Ramsay, M. A. (1992). The anatomy, chemical composition, and metabolism of adipose tissue in wild polar bears (Ursus maritimus). Can. J. Zool. 70, 326–341. Thiemann, G. W., Iverson, S. J., and Stirling, I. (2006). Seasonal, sexual, and anatomical variability in the adipose tissue composition of polar bears (Ursus maritimus). J. Zool. (Lond.) 269, 65–76. Webb, P. M., Crocker, D. E., Blackwell, S. B., Costa, D. P., and Le Boeuf, B. J. (1998). Effects of buoyancy on the diving behavior of northern elephant seals. J. Exp. Biol. 201, 2349–2358. Worthy, G. A. J., and Lavigne, D. M. (1983). Changes in energy stores during postnatal development of the harp seal, Phoca groenlandica. J. Mammal. 64, 89–96.
Blue Whale Balaenoptera musculus RICHARD SEARS AND WILLIAM F. PERRIN
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he blue whale is a baleen whale belonging to the family Balaenopteridae, which includes the group of cetaceans known as rorquals (Fig. 1). Common names are blue whale, sulfur-bottom, Sibbald’s rorqual, great blue whale, and great northern rorqual. The largest animal known to have existed on Earth, it is found worldwide, ranging into all oceans (Yochem and Leatherwood, 1985).
I. Characteristics and Taxonomy On average, Southern Hemisphere blue whales are larger than those in the Northern Hemisphere. The largest recorded were caught off the South Shetlands and South Georgia and were 31.7–32.6 m (104–107 ft) long. The largest recorded for the Northern Hemisphere was a 28.1-m (92-foot) female reported in whaling statistics from catches in Davis Strait. In the North Pacific females of 26.8 m (88 ft) and 27.1 m (89 ft) have been recorded. A 190-ton female was reported taken off South Georgia in 1947; however, body weights of adults generally range from 50 to 150 tons. For maximum size descriptions, female measurements are used because female baleen whales are larger than males.
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B Figure 1 Blue whale showing the characteristic mottled pigmentation of the species. Drawing by Daniel Grenier. Blue whales project a tall (up to 10–12 m) spout, denser and broader than that of the fin whale, B. physalus, which in calm conditions can help distinguish between the two species. When surfacing, the blue whale raises its massive shoulder and blowhole region out of the water more than other rorquals. The prominent fleshy ridge just forward of the blowhole, known as the “splash guard,” is strikingly large in this species. When seen from above, blue whales have a tapered elongated shape, with a huge broad, relatively flat, U-shaped head, adorned by a prominent ridge from the splash guard to the tip of the upper jaw or rostrum and massive mandibles. The baleen is black, half as broad as its maximum 1-m length, and 270–395 plates can be found on each side of the upper jaw. There are 60–88 throat grooves or ventral pleats running longitudinally parallel from the tip of the lower jaw to the navel, which enable the throat or ventral pouch to distend when feeding. The dorsal fin is proportionally smaller than in other balaenopterids and varied in shape, ranging from a small nubbin to triangular and falcate and is positioned far back on the body. The flippers are long and bluntly pointed, slate gray, with a thin white border dorsally and white ventrally; they reach up to 15% of the body length. In the field, particularly on bright days, blue whales generally appear much paler in coloration than all species of large whale except for the gray whale, Eschrichtius robustus, with which it should not be confused due to a great difference in size. Above water, the characteristic mottled pigmentation is a blend of light and dark shades of gray displayed in patches of varying sizes and densities. The underwater color is slate blue on overcast days to silvery or turquoise blue on bright sunny days depending on the clarity of the water. The mottling is found along the body dorsoventrally, occasionally on the flippers, but not on the head and tail flukes. Two prominent pigmentation configurations are found in blue whales, one where a darker, dominant background is mottled with sparser pale patches of pigmentation, while in the other there is a predominantly pale background mottled with sparser dark patches. Blue whale pigmentation can vary, however, from very sparse mottling, where the individual appears uniformly pale or dark, to densely mottled individuals, where the pigmentation is a highly contrasted variegation of spots unique to each whale, which is used in studies involving individual identification. Distinct chevrons curving down and angled back from the apex on both sides of the back behind the blowholes and either pale or dark in tone can be found on some individuals. The tail flukes, sometimes striated ventrally, are predominantly gray above and below; however, some individuals do have white patches of pigmentation on the ventral surface that are used for individual identification (Calambokidis et al., 1990; Sears et al., 1990). The trailing margin of the tail is either straight or curves very slightly from each tip to the median notch. A yellow-green to brown cast, caused by the presence of a diatom (Cocconeis ceticola) film, can be seen covering all or part of the body of blue whales found in cold
waters. The yellowish, diatom-induced tint is the reason the “sulfurbottom” moniker was once used for blue whales. Three subspecies have been designated: what has been considered the largest, B. musculus intermedia, found in Antarctic waters; B. musculus musculus in the Northern Hemisphere; and B. musculus brevicauda, from the subantarctic zone of the southern Indian Ocean and south western Pacific Ocean, also colloquially known as the “pygmy” blue whale. Although the latter designation is now generally accepted, its validity remains in question. Our knowledge of the phylogeny of the baleen whales is still in flux. In recent molecular studies, the blue whale has been variously suggested to be the sister taxon of a clade including the Bryde’s (B. edeni) and sei (B. borealis) whales, in combination with them a sister clade of the fin and humpback (Megaptera novaeangliae) whales, with gray whales (Eschrichtius robustus) the next up the tree (Rychel et al., 2004); in the same arrangement, but with the minke whales (B. acutorostrata and B. bonaerensis) coming in before the gray whale (Nikaido et al., 2005; Sasaki et al., 2005); again in the same arrangement but with the gray whale not included in the analysis, the balaenids being a sister clade to all the balaenopterids (Nishida et al., 2007); and sister taxon to a clade containing all the other baleen whales except the balaenids (Hatch et al., 2006). In a morphological cladistic analyses, it grouped with the common minke whale in a clade sister to the humpback (Steeman, 2007). Further work is obviously needed.
II. Distribution and Abundance Despite having being reduced greatly due to whaling, the blue whale remains a cosmopolitan species separated into populations from the North Atlantic, North Pacific, and Southern Hemisphere (Fig. 2). In the North Atlantic, eastern and western subdivisions are recognized. Photo-identification work from eastern Canadian waters indicates that blue whales from the St. Lawrence, Newfoundland, Nova Scotia, New England, and Greenland all belong to the same population, whereas blue whales photo-identified off Iceland and the Azores appear to be part of a separate population. The best known population in the North Atlantic is that found in the St. Lawrence from April to January, where 435 individuals have been catalogued photographically (Sears et al., 1990). Apart from the Icelandic and Azores sightings, few blue whales have been reported from eastern North Atlantic waters recently. North Atlantic blue whale abundance probably ranges from 600 to 1500 at this time, although more extensive photo-identification and shipboard surveys are needed for more reliable estimates. In the North Atlantic, blue whales reach as far north as Davis Strait and Baffin Bay in the west, whereas to the east they travel as far north as Jan Mayen Island and Spitzbergen during summer months. Whales sighted recently in winter and spring off the Azores and Canary Islands could be migrating north along the mid-Atlantic ridge to Iceland, where they are seen from May to September.
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Guadalupe
Galapagos
Juan Fernandez South American
Cape
Subantarctic New Zealand
Australian
Falkland Island Antarctic New Zealand
Figure 2 Global distribution of blue whales. Darker gray indicates higher densities. Others probably migrate along the European coast, far offshore and out around Ireland north to either Iceland or Norway. It is not clear where the whales winter in the North Atlantic. Some have been observed in the St. Lawrence as late as February; however, acoustic studies have revealed that they are spread out across the North Atlantic basin, south as far as Bermuda and Florida, with concentrations south of Iceland, off Newfoundland and Nova Scotia. The southernmost observations on the eastern side of the North Atlantic are in the waters between the coast of Africa and the Cape Verde Islands. In the North Pacific, where as many as five subpopulations were thought to exist, acoustic analysis of blue whale vocalizations now indicates there are no more than two. The best known is that from the eastern North Pacific where blue whales can be found as far north as Alaska but are regularly observed from California in summer, south to Mexican and the Costa Rica Dome waters in winter. Abundance estimates of approximately 3000 animals (CV 0.14) by line-transect methods and 2000 by capture–recapture (photo-identification) have been determined for this population, which has been studied extensively over a good portion of its range (Calambokidis and Barlow, 2004). From late fall to spring, blue whales can be found in the Gulf of California, Mexico, and south to offshore waters of Central America. By April and May they migrate north along the West coast of North America, where a large proportion is found in California waters. From there some reach Canadian waters, and other groups may disperse north to the Gulf of Alaska or west toward the Aleutian Islands. Few blue whales have been reported recently from the western North Pacific, including the Aleutian Islands, Kamchatka, Kurils, and Japan. They are thought to migrate to Kamchatka or the Kuril Islands and probably farther northeast. Blue whales are also found in the northern Indian Ocean; however, it is not clear whether these form a distinct population. In the Southern Ocean, where the blue whale was historically most abundant, it is very rare today, with the abundance estimate at 1700 (95% confidence interval 860–2900) (Branch et al., 2007). A population of 424 (CV 0.42) has been estimated to frequent the Madagascar Plateau in the austral summer (Best et al., 2003). Although the general population structure in the Southern Ocean is not well understood, evidence shows discrete feeding stocks. A feeding and nursing ground was recently discovered in southern Chile (Hucke-Gaete et al., 2004). Consistent with these feeding
areas, the International Whaling Commission has assigned six stock areas for blue whales in the Southern Hemisphere.
III. Ecology Food availability probably dictates blue whale distribution for most of the year. Although they can be found in coastal waters of the St. Lawrence, Gulf of California, Mexico, and California, they are found predominantly offshore. They appear to feed almost exclusively on euphausiids (krill) worldwide in areas of cold current upwellings (e.g., in the Southern Hemisphere—Branch et al., 2007). When they locate suitably high concentrations of euphausiids, they feed by lunging with mouth wide open and gulping large mouthfuls of prey and water. The mouth is then almost completely closed and the water is expelled by muscular action of the distended ventral pouch and tongue through the still exposed baleen plates. Once the water is expelled, the prey is swallowed. When they feed just a few meters below the surface, they often surface slowly, belly first, exposing the throat grooves of the ventral pouch, roll to breathe, and evacuate the water before diving to take their next mouthful. If the prey is close to the surface, blue whales lunge vigorously on their sides or lunge up vertically by projecting their cavernous lower jaws 4–6 m up through the surface. Although surface feeding has often been observed during the day, it is more usual for blue whales to dive to at least 100 m into layers of euphausiid concentrations during daylight hours and rise to feed near the surface in the evening, following the ascent of their prey in the water column. In the North Atlantic, blue whales feed on the krill species Meganyctiphanes norvegica, Thysanoessa raschii, T. inermis, and T. longicaudata; in the North Pacific, Euphausia pacifica, T. inermis, T. longipes, T. spinifera, and Nyctiphanes symplex. In Antarctic waters they prey on E. superba, E. crystallorophias, and E. vallentini. Documentation of natural mortality is rare. The principal predator is the killer whale, Orcinus orca, but there is little evidence of attacks on blue whales in the North Atlantic or Southern Hemisphere. However, in the Gulf of California, Mexico, 25% of the blue whales photo-identified carry rake-like killer whale teeth scars on their tails, indicating that attacks occur with some regularity but are probably rarely successful. In the St. Lawrence, ice entrapment, where animals have been crushed, stranded, or suffocated by current and winddriven ice floes in the late winter-early spring, has been reported.
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IV. Behavior and Physiology
VI. Interactions with Humans
Blue whales are observed most commonly alone or in pairs; however, concentrations of 50 or more can be found spread out in areas of high productivity. Although not noted for raising their flukes when diving, approximately 18% of blue whales observed in the western North Atlantic and Northeast Pacific do so. This is an individual characteristic, and if the individual is relaxed it will generally raise its flukes high up in the air on each sounding dive. When disturbed, blue whales that raise their flukes when diving will often not raise their tails as high out of the water or not at all and dive more quickly from the surface. When foraging or feeding at depth, blue whales will generally dive for 8–15 min; 20-min dives are not uncommon. The longest dive recorded was of 36 min; however, dives of more than 30 min are rare. They generally swim at 3–6 km/h when feeding. When traveling, they can attain speeds of 5–30 km/h and when chased by boats, predators, or interacting with other blue whales, they can reach upward of 35 km/h. Blue whales vocalize regularly throughout the year with peaks from midsummer into winter months. The majority of vocalizations are low frequency or infrasonic sounds of 17–20 Hz, lower than humans can detect. Their sounds, at 188 decibels (re: 1 μPa at 1 m) are one of the loudest and lowest made by any animal. The calls can be heard easily for hundreds of kilometers, thousands of kilometers under optimal oceanographic conditions, and may cover whole ocean basins. The low frequencies are ideal for communication between individuals of a widely dispersed and nomadic species through water without much loss of information. Geographic variation, seasonality, and diel variation in the sounds have been studied intensively in recent years (Stafford, 2003; Širovíc et al., 2004; Stafford et al., 2004, 2005; Wiggins et al., 2005); the sounds may prove to be useful in delineating populations (McDonald et al., 2006). Little is known of mating behavior in the species. However, female–male pairings have been noted with regularity in the St. Lawrence from summer into fall, some lasting for as long as 5 weeks. When a female–male pair is approached by a third blue whale, or even a fin whale, vigorous surface displays ensue, where all three animals can be seen racing high out of the water, porpoising forward, causing an explosive bow wave splash, and even at times breaching. Such interactions usually last for 7–25 min.
Because of its great size and the commercial value of the products it yielded, the blue whale was hunted relentlessly beginning in the late 1800s. The greatest number of blue whales was taken from the early 1900s until the late 1930s, with the peak being in the 1930–1931 season when nearly 30,000 were killed. The height of blue whale whaling coincided with the advent of explosive harpoons, steam power vessels, and the construction of factory ships, which could process whale carcasses at sea. The blue whale was severely depleted by whaling, particularly in the Southern Hemisphere, where during the first half of the twentieth century 325,000–360,000 were killed in Antarctic waters alone. A further 11,000 were taken in the North Atlantic, primarily in Icelandic waters, and 9500 in the North Pacific. This unbridled hunt for blue whales, which lasted until its worldwide protection in 1966, brought the blue whale to the brink of extinction and it is still an endangered species today. However, there is evidence for population increase in the Antarctic (Branch, 2004). Although reports of blue whales approaching vessels are rare, at least 25% of the blue whales photo-identified in the St. Lawrence carry scars that can be attributed to collisions with ships, including whale-watching vessels. This type of scarring has been reported for a few Northeast Pacific blue whales as well. Ship strikes in heavy shipping areas, such as the St. Lawrence and California coast, may have an impact at populations, but data are not available at this point. Though 12% of blue whales found in eastern Canadian waters carry marks related to contacts with fishing gear, few lethal entanglements have been reported. The size and power of this whale probably enables it to tear through fishing gear relatively unscathed. Persistent contaminants accumulated over time, such as PCBs commonly found in blue whales from eastern Canadian waters, may have an impact on reproduction and limit the recovery of certain populations. It has been shown that blue whales react strongly to approaching vessels. The degree of reaction depends on the whale’s behavior, as well as the distance, speed, and direction of the vessel at the time of approach. The increasing anthropogenic noise may have an impact on blue whales and their habitat and could also limit recovery of this species.
V. Life History Blue whales reach sexual maturity at 5–15 years of age; however, 8–10 years appear to be more usual for both sexes. Length at sexual maturation in females from the Northern Hemisphere is 21–23 m and is 23–24 m in the Southern Hemisphere. Males reach sexual maturity at 20–21 m in the Northern Hemisphere and at 22 m in the Southern Hemisphere. Mating takes place starting in late fall and continues throughout the winter. Females give birth every 2–3 years in winter after a 10- to 12-month gestation period. The calves, which weight 2–3 tons and measure 6–7 m at birth, are weaned when approximately 16 m long at 6–8 months. No specific breeding ground has been discovered for blue whales in any ocean, although mothers and calves are sighted regularly in the Gulf of California, Mexico, in late winter and spring. A portion of the Northeast Pacific Ocean blue whale population could be using this region as a breeding ground. Longevity is thought to be at least 80–90 years and probably longer. What is certain, however, after extensive photo-identification fieldwork on known individuals in the St. Lawrence and northeast Pacific, is that they live for at least 40 years.
See Also the Following Articles Baleen Whales Cetacean Life History Fluking Noise ■ Effects of Pollution and Marine Mammals
References Best, P. B., et al. (2003). The abundance of blue whales on the Madagascar Plateau, December 1996. J. Cetacean Res. Manage. 5, 253–260. Branch, T. A. (2004). Summary of evidence for increase in Antarctic (true) blue whales. J. Cetacean Res. Manage. (Suppl) 6, 256–258. Branch, T. A., et al. (2007). Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean. Mam. Rev. 7, 116–175. Calambokidis, J., and Barlow, J. (2004). Abundance of blue and humpback whales in the eastern North Pacific estimated by capturerecapture and line-transect methods. Mar. Mamm. Sci. 20, 63–85. Calambokidis, J., et al. (1990). Sightings and movements of blue whales off central California 1986–88 from photo-identification of individuals. Rep. Int. Whal. Commn 12, 343–348.
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Clark, C. W., and Charif, R. A. (1998). Acoustic monitoring of large whales to the West of Britain and Ireland using bottom-mounted hydrophone arrays. October 1996–September 1997. JNCC Rep. 281. Hatch, L. T., Dopman, E. B., and Harrison, R. G. (2006). Phylogenetic relationships among the baleen whales based on maternally and paternally inherited characters. Mol. Phylogenet. Evol. 41, 12–27. Hucke-Gaete, R., Osman, L. P., Moreno, C. A., Findlay, K. P., and Llunjblad, D. K. (2004). Discovery of a blue whale feeding and nursing ground in southern Chile. Proc. R. Soc. Lond. B 271(Suppl.), S170–S173. McDonald, M. A., Mesnick, S. L., and Hildebrand, J. A. (2006). Biogeographic characterization of blue whale song worldwide: Using song to identify populations. J. Cetacean Res. Manage. 8, 55–65. Nikaido, M., et al. (8 authors). The baleen whale phylogeny and a past extensive radiation event revealed by SINE insertion analysis. Mol. Biol. Evol. 23, 866–873. Nishida, S., Goto, M., Pastene, L. A., Kanda, N., and Koike, H. (2007). Phylogenetic relationships among cetaceans revealed by Y-chromosome sequences. Zool. Sci. 24, 723–732. Rychel, A. L., Reeder, T. W., and Berta, A. (2004). Phylogeny of mysticete whales based on mitochondrial and nuclear data. Mol. Phylogenet. Evol. 32, 892–901. Sasaki, T. M., et al. (2005). Mitochondrial phylogenetics and evolution of mysticete whales. Syst. Biol. 54, 77–90. Sears, R., Williamson, J. M., Wenzel, F., Bérubé, M., Gendron, D., and Jones, P. W. (1990). The photographic identification of the blue whale (Balaenoptera musculus) in the Gulf of St. Lawrence, Canada. Rep. Int. Whal. Commn (Spec. Iss.) 12, 335–342. Širovíc, A., Hildebrand, J. A., Wiggins, S. M., McDonald, M. A., Moore, S. E., and Thiele, D. (2004). Seasonality of blue and fin whale calls and the influence of sea ice in the Western Arctic Peninsula. Deep-Sea Res. II 51, 2327–2344. Stafford, K. M. (2003). Two types of blue whale calls recorded in the Gulf of Alaska. Mar. Mamm. Sci. 19, 682–693. Stafford, K. M., Bohnenstiehl, D. R., Tolstoy, M., Chapp, E., Mellinger, D. K., and Moore, S. E. (2004). Antarctic-type blue whale calls recorded at low latitudes in the Indian and eastern Pacific Oceans. Deep-Sea Res. I 51, 1337–1346. Stafford, K. M., Moore, S. E., and Fox, C. G. (2005). Diel variation in blue whale calls recorded in the eastern tropical Pacific. Anim. Behav. 69, 951–958. Steeman, M. E. (2007). Cladistic analysis and a revised classification of fossil and recent mysticetes. Zool. J. Linn. Soc. 150, 875–894. Wiggins, S. M., Oleson, E. M., McDonald, M. A., and Hildebrand, J. A. (2005). Blue whale (Balaenoptera musculus) diel call patterns offshore of southern California. Aquat. Mamm. 31, 161–168. Yochem, P. K., and Leatherwood, S. (1985). Blue whale (Balaenoptera musculus Linnaeus, 1758). In “Handbook of Marine Mammals” (S. H. Ridgway, and R. Harrison, eds), Vol. 3, pp. 193–240. Academic Press, London.
Bones and Teeth, Histology of MARY C. MAAS
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he bones and teeth of marine mammals, like those of other vertebrates, consist of both organic and mineral components. Because the mineral component (mostly calcium phosphate) predominates, the constituents of bones (bone and calcified cartilage) and teeth (cementum, dentine, and enamel) are referred as “hard tissues.” Each of these hard tissues is distinguished both by its composition and by its microscopic structure. Many of the histological
features of marine mammal teeth and bones are typical for mammals, and vertebrates, in general, but others are unique or unusual. Some of these may have evolved in conjunction with their shifts to marine habitats.
I. Bone A. Bone Structure and Composition Bone consists of highly calcified, intercellular bone matrix, and three types of cells—osteocytes, osteoblasts, and osteoclasts. The outer surface of bone is covered by periosteum, which is bound to bone by bundles of collagen fibers known as Sharpey’s fibers, and the inner bone surface is lined with endosteum (Fig. 1). Periosteum is thicker than endosteum, but both consist of fibrous connective tissue lined with osteoprogenitor cells, from which osteoblasts are derived. Osteoblasts are the cells that synthesize bone matrix proteins and are active in bone matrix mineralization. Bone matrix (also known as osteoid) consists of about 33% organic matter (mostly Type I collagen) and 67% inorganic matter (calcium phosphate, mostly hydroxyapatite crystals). The osteoblasts occur as simple, epithelial-like layer at the developing bone surface. As the bone matrix mineralizes, some osteoblasts become trapped in small spaces within the matrix (lacunae). These trapped osteoblasts become osteocytes, the cells responsible for maintenance of the bony matrix. Each lacuna holds only a single osteocyte but is connected with adjacent lacunae by microscopic canaliculi, which house cytoplasmic processes of the osteocytes. Osteoclasts are large, multi-nucleated cells that occur in shallow erosional depressions (Howship’s lacunae) on the resorbing bone surface and secrete enzymes that promote local digestion of collagen and dissolution of mineral crystals. Bone is commonly classified according to its gross appearance as cancellous bone (bone with numerous, macroscopic interconnecting cavities, or trabeculae, also known as spongy or trabecular bone) or compact bone (dense lamellar bone without trabeculae), but both types have the same basic histological structure. In a typical mammalian long bone the diaphysis (shaft) is composed predominantly of compact bone, with cancellous bone confined to the inner surface around a central, medullary cavity (Fig. 1A), while the epiphyses (articular ends) consist mostly of cancellous bone overlain by a thin, smooth layer of compact bone. In short bones a core of cancellous bone is completely surrounded by compact bone, and in the flat bones of the skull, inner and outer plates of compact bone are separated by the diploë, a layer of cancellous bone. Bone also can be classified histologically, as woven (primary) bone and lamellar (secondary) bone. Woven bone, or primary bone has an irregular structure and is usually replaced in adults by the more highly mineralized lamellar bone. Lamellar bone is organized into thin layers (lamellae), usually 3–7μm thick, which contain parallel collagen fiber bundles. Lacunae containing osteocytes are located between lamellae. There are three types of lamellae: concentric, interstitial, and circumferential (Fig. 1B). Concentric lamellae are arranged in circular layers around a long axis, the haversian canal, which is a vascular channel containing blood vessels, nerves, and connective tissue. Adjacent vertical channels are connected by more horizontally oriented vascular channels (Volkmann’s canals). The entire complex consisting of several layers of concentric lamellae around a vascular channel is known as an osteon or haversian system. Interstitial lamellae, which appear as irregularly shaped areas between adjacent osteons, consist of lamellae that are remnants of osteons destroyed during bone remodeling. Circumferential lamellae are arranged parallel to each other and comprise the outer circumferential lamellae laid down next to the periosteum and the inner circumferential lamellae laid down next to the endosteum.
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VI. Other Specialized Fats
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In addition to blubber, several other unusual and specialized fat bodies exist that are unique to a single group of cetaceans, the odontocetes or toothed whales. These fat bodies occur in the forehead tissue (melon) and in and around the mandibles of the lower jaw (mandibular fats) and play important roles in hearing and echolocation. They are composed of a unique array of lipid classes and fatty acids that are likely synthesized with these head tissues (Koopman et al., 2003, 2006). These unusual fats are believed to facilitate sound reception by acting in the melon to focus high frequency sound produced in the nasal passages, while in the mandibular fats, they are organized to form a channel to transmit received sounds to the ear. In all odontocetes examined, short- and branched-chain fatty acids appear to be concentrated in the center of the inner mandibular fat body and immediately adjacent to the earbones. Because sound travels more slowly through these types of fatty acids, this should cause sound entering an odontocete head to bend inwards and be directed to the ears (Koopman et al., 2006). The unique arrangement of lipids within these fat bodies and their direct effect on sound transmission is an important are of current research. In conclusion, blubber and other fats play a number of major roles in the lives of marine mammals. These fats can also be a powerful tool in trying to understand adaptive solutions of species living in marine environments as wells as insights into their ecology and behavior.
See Also the Following Articles Skeletal Anatomy ■ Swimming ■ Pinniped physiology
References Beck, C. A., Bowen, W. D., and Iverson, S. J. (2000). Seasonal changes in buoyancy and diving behaviour of adult grey seals. J. Exp. Biol. 203, 2323–2330. Bowen, W. D., Oftedal, O. T., and Boness, D. J. (1992). Mass and energy transfer during lactation in a small phocid, the harbor seal (Phoca vitulina). Physiol. Zool. 65, 844–866. Heath, M. E., and Ridgeway, S. H. (1999). How dolphins use their blubber to avoid heat stress during encounters with warm water. Am. J. Physiol. 276, R1188–R1194. Iverson, S. J. (1993). Milk secretion in marine mammals in relation to foraging: Can milk fatty acids predict diet? Symp. Zool. Soc. Lond. 66, 263–291. Iverson, S. J., Bowen, W. D., Boness, DJ., and Oftedal, O. T. (1993). The effect of maternal size and milk output on pup growth in grey seals (Halichoerus grypus). Physiol. Zool. 66, 61–88. Iverson, S. J., Oftedal, O. T., Bowen, W. D., Boness, DJ., and Sampugna, J. (1995). Prenatal and postnatal transfer of fatty acids from mother to pup in the hooded seal. J. Comp. Physiol. 165, 1–12. Iverson, S. J., Field, C., Bowen, W. D., and Blanchard, W. (2004). Quantitative fatty acid signature analysis: A new method of estimating predator diets. Ecol. Monogr. 74, 211–235. Iverson, S. J., Stirling, I., and Lang, S. L. C. (2006). Spatial and temporal variation in the diets of polar bears across the Canadian arctic: Indicators of changes in prey populations and environment. Symp. Zool. Soc. Lond.: Conservation Biology Series 12, 98–117. Kirsch, P. E., Iverson, S. J., and Bowen, W. D. (2000). Effect of diet on body composition and blubber fatty acids in captive harp seals (Phoca groenlandica). Physiol. Biochem. Zool. 73, 45–59. Koopman, H. N. (1998). Topographical distribution of the blubber of harbor porpoises (Phocoena phocoena). J. Mammal. 79, 260–270. Koopman, H. N. (2007). Phylogenetic, ecological, and ontogenetic factors influencing the biochemical structure of the blubber of odontocetes. Mar. Biol. 151, 277–291.
Koopman, H. N., Iverson, S. J., and Read, A. J. (2003). High concentrations of isovaleric acid in the fats of odontocetes: Variation and patterns of accumulation in blubber vs. stability in the melon. J. Comp. Physiol. 173, 247–261. Koopman, H. N., Budge, S. M., Ketten, D. R., and Iverson, S. J. (2006). The topographical distribution of lipids inside the mandibular fat bodies of odontocetes: Remarkable complexity and consistency. IEEE J. Oceanic Eng. 31, 95–106. Lockyer, C. (1987). Evaluation of the role of fat reserves in relation to the ecology of North Atlantic fin and sei whales. In “Approaches to Marine Mammal Energetics” (A. C. Huntley, D. P. Costa, G. A. J. Worthy, and M. A. Castellini, eds), pp. 184–203. Society for Marine Mammalogy Special Publication No. 1., Allen Press, Lawrence, KS. Mellish, J. E., Iverson, S. J., and Bowen, W. D. (1999). Individual variation in maternal energy allocation and milk production in grey seals and consequences for pup growth and weaning characteristics. Physiol. Biochem. Zool. 67, 677–690. Pabst, D. A. (1996). Springs in swimming animals. Am. Zool. 36, 723–735. Pabst, D. A., Rommel, S. A., and McLellan, W. A. (1999). The functional morphology of marine mammals. In “Biology of Marine Mammals” (J. E. Reynolds, and S. A. Rommel, eds), pp. 15–72. Smithsonian Institution Press, Washington, DC. Pond, C. M., Mattacks, C. A., Colby, R. H., and Ramsay, M. A. (1992). The anatomy, chemical composition, and metabolism of adipose tissue in wild polar bears (Ursus maritimus). Can. J. Zool. 70, 326–341. Thiemann, G. W., Iverson, S. J., and Stirling, I. (2006). Seasonal, sexual, and anatomical variability in the adipose tissue composition of polar bears (Ursus maritimus). J. Zool. (Lond.) 269, 65–76. Webb, P. M., Crocker, D. E., Blackwell, S. B., Costa, D. P., and Le Boeuf, B. J. (1998). Effects of buoyancy on the diving behavior of northern elephant seals. J. Exp. Biol. 201, 2349–2358. Worthy, G. A. J., and Lavigne, D. M. (1983). Changes in energy stores during postnatal development of the harp seal, Phoca groenlandica. J. Mammal. 64, 89–96.
Blue Whale Balaenoptera musculus RICHARD SEARS AND WILLIAM F. PERRIN
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he blue whale is a baleen whale belonging to the family Balaenopteridae, which includes the group of cetaceans known as rorquals (Fig. 1). Common names are blue whale, sulfur-bottom, Sibbald’s rorqual, great blue whale, and great northern rorqual. The largest animal known to have existed on Earth, it is found worldwide, ranging into all oceans (Yochem and Leatherwood, 1985).
I. Characteristics and Taxonomy On average, Southern Hemisphere blue whales are larger than those in the Northern Hemisphere. The largest recorded were caught off the South Shetlands and South Georgia and were 31.7–32.6 m (104–107 ft) long. The largest recorded for the Northern Hemisphere was a 28.1-m (92-foot) female reported in whaling statistics from catches in Davis Strait. In the North Pacific females of 26.8 m (88 ft) and 27.1 m (89 ft) have been recorded. A 190-ton female was reported taken off South Georgia in 1947; however, body weights of adults generally range from 50 to 150 tons. For maximum size descriptions, female measurements are used because female baleen whales are larger than males.
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B Figure 1 Blue whale showing the characteristic mottled pigmentation of the species. Drawing by Daniel Grenier. Blue whales project a tall (up to 10–12 m) spout, denser and broader than that of the fin whale, B. physalus, which in calm conditions can help distinguish between the two species. When surfacing, the blue whale raises its massive shoulder and blowhole region out of the water more than other rorquals. The prominent fleshy ridge just forward of the blowhole, known as the “splash guard,” is strikingly large in this species. When seen from above, blue whales have a tapered elongated shape, with a huge broad, relatively flat, U-shaped head, adorned by a prominent ridge from the splash guard to the tip of the upper jaw or rostrum and massive mandibles. The baleen is black, half as broad as its maximum 1-m length, and 270–395 plates can be found on each side of the upper jaw. There are 60–88 throat grooves or ventral pleats running longitudinally parallel from the tip of the lower jaw to the navel, which enable the throat or ventral pouch to distend when feeding. The dorsal fin is proportionally smaller than in other balaenopterids and varied in shape, ranging from a small nubbin to triangular and falcate and is positioned far back on the body. The flippers are long and bluntly pointed, slate gray, with a thin white border dorsally and white ventrally; they reach up to 15% of the body length. In the field, particularly on bright days, blue whales generally appear much paler in coloration than all species of large whale except for the gray whale, Eschrichtius robustus, with which it should not be confused due to a great difference in size. Above water, the characteristic mottled pigmentation is a blend of light and dark shades of gray displayed in patches of varying sizes and densities. The underwater color is slate blue on overcast days to silvery or turquoise blue on bright sunny days depending on the clarity of the water. The mottling is found along the body dorsoventrally, occasionally on the flippers, but not on the head and tail flukes. Two prominent pigmentation configurations are found in blue whales, one where a darker, dominant background is mottled with sparser pale patches of pigmentation, while in the other there is a predominantly pale background mottled with sparser dark patches. Blue whale pigmentation can vary, however, from very sparse mottling, where the individual appears uniformly pale or dark, to densely mottled individuals, where the pigmentation is a highly contrasted variegation of spots unique to each whale, which is used in studies involving individual identification. Distinct chevrons curving down and angled back from the apex on both sides of the back behind the blowholes and either pale or dark in tone can be found on some individuals. The tail flukes, sometimes striated ventrally, are predominantly gray above and below; however, some individuals do have white patches of pigmentation on the ventral surface that are used for individual identification (Calambokidis et al., 1990; Sears et al., 1990). The trailing margin of the tail is either straight or curves very slightly from each tip to the median notch. A yellow-green to brown cast, caused by the presence of a diatom (Cocconeis ceticola) film, can be seen covering all or part of the body of blue whales found in cold
waters. The yellowish, diatom-induced tint is the reason the “sulfurbottom” moniker was once used for blue whales. Three subspecies have been designated: what has been considered the largest, B. musculus intermedia, found in Antarctic waters; B. musculus musculus in the Northern Hemisphere; and B. musculus brevicauda, from the subantarctic zone of the southern Indian Ocean and south western Pacific Ocean, also colloquially known as the “pygmy” blue whale. Although the latter designation is now generally accepted, its validity remains in question. Our knowledge of the phylogeny of the baleen whales is still in flux. In recent molecular studies, the blue whale has been variously suggested to be the sister taxon of a clade including the Bryde’s (B. edeni) and sei (B. borealis) whales, in combination with them a sister clade of the fin and humpback (Megaptera novaeangliae) whales, with gray whales (Eschrichtius robustus) the next up the tree (Rychel et al., 2004); in the same arrangement, but with the minke whales (B. acutorostrata and B. bonaerensis) coming in before the gray whale (Nikaido et al., 2005; Sasaki et al., 2005); again in the same arrangement but with the gray whale not included in the analysis, the balaenids being a sister clade to all the balaenopterids (Nishida et al., 2007); and sister taxon to a clade containing all the other baleen whales except the balaenids (Hatch et al., 2006). In a morphological cladistic analyses, it grouped with the common minke whale in a clade sister to the humpback (Steeman, 2007). Further work is obviously needed.
II. Distribution and Abundance Despite having being reduced greatly due to whaling, the blue whale remains a cosmopolitan species separated into populations from the North Atlantic, North Pacific, and Southern Hemisphere (Fig. 2). In the North Atlantic, eastern and western subdivisions are recognized. Photo-identification work from eastern Canadian waters indicates that blue whales from the St. Lawrence, Newfoundland, Nova Scotia, New England, and Greenland all belong to the same population, whereas blue whales photo-identified off Iceland and the Azores appear to be part of a separate population. The best known population in the North Atlantic is that found in the St. Lawrence from April to January, where 435 individuals have been catalogued photographically (Sears et al., 1990). Apart from the Icelandic and Azores sightings, few blue whales have been reported from eastern North Atlantic waters recently. North Atlantic blue whale abundance probably ranges from 600 to 1500 at this time, although more extensive photo-identification and shipboard surveys are needed for more reliable estimates. In the North Atlantic, blue whales reach as far north as Davis Strait and Baffin Bay in the west, whereas to the east they travel as far north as Jan Mayen Island and Spitzbergen during summer months. Whales sighted recently in winter and spring off the Azores and Canary Islands could be migrating north along the mid-Atlantic ridge to Iceland, where they are seen from May to September.
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Blue Whale
Guadalupe
Galapagos
Juan Fernandez South American
Cape
Subantarctic New Zealand
Australian
Falkland Island Antarctic New Zealand
Figure 2 Global distribution of blue whales. Darker gray indicates higher densities. Others probably migrate along the European coast, far offshore and out around Ireland north to either Iceland or Norway. It is not clear where the whales winter in the North Atlantic. Some have been observed in the St. Lawrence as late as February; however, acoustic studies have revealed that they are spread out across the North Atlantic basin, south as far as Bermuda and Florida, with concentrations south of Iceland, off Newfoundland and Nova Scotia. The southernmost observations on the eastern side of the North Atlantic are in the waters between the coast of Africa and the Cape Verde Islands. In the North Pacific, where as many as five subpopulations were thought to exist, acoustic analysis of blue whale vocalizations now indicates there are no more than two. The best known is that from the eastern North Pacific where blue whales can be found as far north as Alaska but are regularly observed from California in summer, south to Mexican and the Costa Rica Dome waters in winter. Abundance estimates of approximately 3000 animals (CV 0.14) by line-transect methods and 2000 by capture–recapture (photo-identification) have been determined for this population, which has been studied extensively over a good portion of its range (Calambokidis and Barlow, 2004). From late fall to spring, blue whales can be found in the Gulf of California, Mexico, and south to offshore waters of Central America. By April and May they migrate north along the West coast of North America, where a large proportion is found in California waters. From there some reach Canadian waters, and other groups may disperse north to the Gulf of Alaska or west toward the Aleutian Islands. Few blue whales have been reported recently from the western North Pacific, including the Aleutian Islands, Kamchatka, Kurils, and Japan. They are thought to migrate to Kamchatka or the Kuril Islands and probably farther northeast. Blue whales are also found in the northern Indian Ocean; however, it is not clear whether these form a distinct population. In the Southern Ocean, where the blue whale was historically most abundant, it is very rare today, with the abundance estimate at 1700 (95% confidence interval 860–2900) (Branch et al., 2007). A population of 424 (CV 0.42) has been estimated to frequent the Madagascar Plateau in the austral summer (Best et al., 2003). Although the general population structure in the Southern Ocean is not well understood, evidence shows discrete feeding stocks. A feeding and nursing ground was recently discovered in southern Chile (Hucke-Gaete et al., 2004). Consistent with these feeding
areas, the International Whaling Commission has assigned six stock areas for blue whales in the Southern Hemisphere.
III. Ecology Food availability probably dictates blue whale distribution for most of the year. Although they can be found in coastal waters of the St. Lawrence, Gulf of California, Mexico, and California, they are found predominantly offshore. They appear to feed almost exclusively on euphausiids (krill) worldwide in areas of cold current upwellings (e.g., in the Southern Hemisphere—Branch et al., 2007). When they locate suitably high concentrations of euphausiids, they feed by lunging with mouth wide open and gulping large mouthfuls of prey and water. The mouth is then almost completely closed and the water is expelled by muscular action of the distended ventral pouch and tongue through the still exposed baleen plates. Once the water is expelled, the prey is swallowed. When they feed just a few meters below the surface, they often surface slowly, belly first, exposing the throat grooves of the ventral pouch, roll to breathe, and evacuate the water before diving to take their next mouthful. If the prey is close to the surface, blue whales lunge vigorously on their sides or lunge up vertically by projecting their cavernous lower jaws 4–6 m up through the surface. Although surface feeding has often been observed during the day, it is more usual for blue whales to dive to at least 100 m into layers of euphausiid concentrations during daylight hours and rise to feed near the surface in the evening, following the ascent of their prey in the water column. In the North Atlantic, blue whales feed on the krill species Meganyctiphanes norvegica, Thysanoessa raschii, T. inermis, and T. longicaudata; in the North Pacific, Euphausia pacifica, T. inermis, T. longipes, T. spinifera, and Nyctiphanes symplex. In Antarctic waters they prey on E. superba, E. crystallorophias, and E. vallentini. Documentation of natural mortality is rare. The principal predator is the killer whale, Orcinus orca, but there is little evidence of attacks on blue whales in the North Atlantic or Southern Hemisphere. However, in the Gulf of California, Mexico, 25% of the blue whales photo-identified carry rake-like killer whale teeth scars on their tails, indicating that attacks occur with some regularity but are probably rarely successful. In the St. Lawrence, ice entrapment, where animals have been crushed, stranded, or suffocated by current and winddriven ice floes in the late winter-early spring, has been reported.
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IV. Behavior and Physiology
VI. Interactions with Humans
Blue whales are observed most commonly alone or in pairs; however, concentrations of 50 or more can be found spread out in areas of high productivity. Although not noted for raising their flukes when diving, approximately 18% of blue whales observed in the western North Atlantic and Northeast Pacific do so. This is an individual characteristic, and if the individual is relaxed it will generally raise its flukes high up in the air on each sounding dive. When disturbed, blue whales that raise their flukes when diving will often not raise their tails as high out of the water or not at all and dive more quickly from the surface. When foraging or feeding at depth, blue whales will generally dive for 8–15 min; 20-min dives are not uncommon. The longest dive recorded was of 36 min; however, dives of more than 30 min are rare. They generally swim at 3–6 km/h when feeding. When traveling, they can attain speeds of 5–30 km/h and when chased by boats, predators, or interacting with other blue whales, they can reach upward of 35 km/h. Blue whales vocalize regularly throughout the year with peaks from midsummer into winter months. The majority of vocalizations are low frequency or infrasonic sounds of 17–20 Hz, lower than humans can detect. Their sounds, at 188 decibels (re: 1 μPa at 1 m) are one of the loudest and lowest made by any animal. The calls can be heard easily for hundreds of kilometers, thousands of kilometers under optimal oceanographic conditions, and may cover whole ocean basins. The low frequencies are ideal for communication between individuals of a widely dispersed and nomadic species through water without much loss of information. Geographic variation, seasonality, and diel variation in the sounds have been studied intensively in recent years (Stafford, 2003; Širovíc et al., 2004; Stafford et al., 2004, 2005; Wiggins et al., 2005); the sounds may prove to be useful in delineating populations (McDonald et al., 2006). Little is known of mating behavior in the species. However, female–male pairings have been noted with regularity in the St. Lawrence from summer into fall, some lasting for as long as 5 weeks. When a female–male pair is approached by a third blue whale, or even a fin whale, vigorous surface displays ensue, where all three animals can be seen racing high out of the water, porpoising forward, causing an explosive bow wave splash, and even at times breaching. Such interactions usually last for 7–25 min.
Because of its great size and the commercial value of the products it yielded, the blue whale was hunted relentlessly beginning in the late 1800s. The greatest number of blue whales was taken from the early 1900s until the late 1930s, with the peak being in the 1930–1931 season when nearly 30,000 were killed. The height of blue whale whaling coincided with the advent of explosive harpoons, steam power vessels, and the construction of factory ships, which could process whale carcasses at sea. The blue whale was severely depleted by whaling, particularly in the Southern Hemisphere, where during the first half of the twentieth century 325,000–360,000 were killed in Antarctic waters alone. A further 11,000 were taken in the North Atlantic, primarily in Icelandic waters, and 9500 in the North Pacific. This unbridled hunt for blue whales, which lasted until its worldwide protection in 1966, brought the blue whale to the brink of extinction and it is still an endangered species today. However, there is evidence for population increase in the Antarctic (Branch, 2004). Although reports of blue whales approaching vessels are rare, at least 25% of the blue whales photo-identified in the St. Lawrence carry scars that can be attributed to collisions with ships, including whale-watching vessels. This type of scarring has been reported for a few Northeast Pacific blue whales as well. Ship strikes in heavy shipping areas, such as the St. Lawrence and California coast, may have an impact at populations, but data are not available at this point. Though 12% of blue whales found in eastern Canadian waters carry marks related to contacts with fishing gear, few lethal entanglements have been reported. The size and power of this whale probably enables it to tear through fishing gear relatively unscathed. Persistent contaminants accumulated over time, such as PCBs commonly found in blue whales from eastern Canadian waters, may have an impact on reproduction and limit the recovery of certain populations. It has been shown that blue whales react strongly to approaching vessels. The degree of reaction depends on the whale’s behavior, as well as the distance, speed, and direction of the vessel at the time of approach. The increasing anthropogenic noise may have an impact on blue whales and their habitat and could also limit recovery of this species.
V. Life History Blue whales reach sexual maturity at 5–15 years of age; however, 8–10 years appear to be more usual for both sexes. Length at sexual maturation in females from the Northern Hemisphere is 21–23 m and is 23–24 m in the Southern Hemisphere. Males reach sexual maturity at 20–21 m in the Northern Hemisphere and at 22 m in the Southern Hemisphere. Mating takes place starting in late fall and continues throughout the winter. Females give birth every 2–3 years in winter after a 10- to 12-month gestation period. The calves, which weight 2–3 tons and measure 6–7 m at birth, are weaned when approximately 16 m long at 6–8 months. No specific breeding ground has been discovered for blue whales in any ocean, although mothers and calves are sighted regularly in the Gulf of California, Mexico, in late winter and spring. A portion of the Northeast Pacific Ocean blue whale population could be using this region as a breeding ground. Longevity is thought to be at least 80–90 years and probably longer. What is certain, however, after extensive photo-identification fieldwork on known individuals in the St. Lawrence and northeast Pacific, is that they live for at least 40 years.
See Also the Following Articles Baleen Whales Cetacean Life History Fluking Noise ■ Effects of Pollution and Marine Mammals
References Best, P. B., et al. (2003). The abundance of blue whales on the Madagascar Plateau, December 1996. J. Cetacean Res. Manage. 5, 253–260. Branch, T. A. (2004). Summary of evidence for increase in Antarctic (true) blue whales. J. Cetacean Res. Manage. (Suppl) 6, 256–258. Branch, T. A., et al. (2007). Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean. Mam. Rev. 7, 116–175. Calambokidis, J., and Barlow, J. (2004). Abundance of blue and humpback whales in the eastern North Pacific estimated by capturerecapture and line-transect methods. Mar. Mamm. Sci. 20, 63–85. Calambokidis, J., et al. (1990). Sightings and movements of blue whales off central California 1986–88 from photo-identification of individuals. Rep. Int. Whal. Commn 12, 343–348.
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Clark, C. W., and Charif, R. A. (1998). Acoustic monitoring of large whales to the West of Britain and Ireland using bottom-mounted hydrophone arrays. October 1996–September 1997. JNCC Rep. 281. Hatch, L. T., Dopman, E. B., and Harrison, R. G. (2006). Phylogenetic relationships among the baleen whales based on maternally and paternally inherited characters. Mol. Phylogenet. Evol. 41, 12–27. Hucke-Gaete, R., Osman, L. P., Moreno, C. A., Findlay, K. P., and Llunjblad, D. K. (2004). Discovery of a blue whale feeding and nursing ground in southern Chile. Proc. R. Soc. Lond. B 271(Suppl.), S170–S173. McDonald, M. A., Mesnick, S. L., and Hildebrand, J. A. (2006). Biogeographic characterization of blue whale song worldwide: Using song to identify populations. J. Cetacean Res. Manage. 8, 55–65. Nikaido, M., et al. (8 authors). The baleen whale phylogeny and a past extensive radiation event revealed by SINE insertion analysis. Mol. Biol. Evol. 23, 866–873. Nishida, S., Goto, M., Pastene, L. A., Kanda, N., and Koike, H. (2007). Phylogenetic relationships among cetaceans revealed by Y-chromosome sequences. Zool. Sci. 24, 723–732. Rychel, A. L., Reeder, T. W., and Berta, A. (2004). Phylogeny of mysticete whales based on mitochondrial and nuclear data. Mol. Phylogenet. Evol. 32, 892–901. Sasaki, T. M., et al. (2005). Mitochondrial phylogenetics and evolution of mysticete whales. Syst. Biol. 54, 77–90. Sears, R., Williamson, J. M., Wenzel, F., Bérubé, M., Gendron, D., and Jones, P. W. (1990). The photographic identification of the blue whale (Balaenoptera musculus) in the Gulf of St. Lawrence, Canada. Rep. Int. Whal. Commn (Spec. Iss.) 12, 335–342. Širovíc, A., Hildebrand, J. A., Wiggins, S. M., McDonald, M. A., Moore, S. E., and Thiele, D. (2004). Seasonality of blue and fin whale calls and the influence of sea ice in the Western Arctic Peninsula. Deep-Sea Res. II 51, 2327–2344. Stafford, K. M. (2003). Two types of blue whale calls recorded in the Gulf of Alaska. Mar. Mamm. Sci. 19, 682–693. Stafford, K. M., Bohnenstiehl, D. R., Tolstoy, M., Chapp, E., Mellinger, D. K., and Moore, S. E. (2004). Antarctic-type blue whale calls recorded at low latitudes in the Indian and eastern Pacific Oceans. Deep-Sea Res. I 51, 1337–1346. Stafford, K. M., Moore, S. E., and Fox, C. G. (2005). Diel variation in blue whale calls recorded in the eastern tropical Pacific. Anim. Behav. 69, 951–958. Steeman, M. E. (2007). Cladistic analysis and a revised classification of fossil and recent mysticetes. Zool. J. Linn. Soc. 150, 875–894. Wiggins, S. M., Oleson, E. M., McDonald, M. A., and Hildebrand, J. A. (2005). Blue whale (Balaenoptera musculus) diel call patterns offshore of southern California. Aquat. Mamm. 31, 161–168. Yochem, P. K., and Leatherwood, S. (1985). Blue whale (Balaenoptera musculus Linnaeus, 1758). In “Handbook of Marine Mammals” (S. H. Ridgway, and R. Harrison, eds), Vol. 3, pp. 193–240. Academic Press, London.
Bones and Teeth, Histology of MARY C. MAAS
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he bones and teeth of marine mammals, like those of other vertebrates, consist of both organic and mineral components. Because the mineral component (mostly calcium phosphate) predominates, the constituents of bones (bone and calcified cartilage) and teeth (cementum, dentine, and enamel) are referred as “hard tissues.” Each of these hard tissues is distinguished both by its composition and by its microscopic structure. Many of the histological
features of marine mammal teeth and bones are typical for mammals, and vertebrates, in general, but others are unique or unusual. Some of these may have evolved in conjunction with their shifts to marine habitats.
I. Bone A. Bone Structure and Composition Bone consists of highly calcified, intercellular bone matrix, and three types of cells—osteocytes, osteoblasts, and osteoclasts. The outer surface of bone is covered by periosteum, which is bound to bone by bundles of collagen fibers known as Sharpey’s fibers, and the inner bone surface is lined with endosteum (Fig. 1). Periosteum is thicker than endosteum, but both consist of fibrous connective tissue lined with osteoprogenitor cells, from which osteoblasts are derived. Osteoblasts are the cells that synthesize bone matrix proteins and are active in bone matrix mineralization. Bone matrix (also known as osteoid) consists of about 33% organic matter (mostly Type I collagen) and 67% inorganic matter (calcium phosphate, mostly hydroxyapatite crystals). The osteoblasts occur as simple, epithelial-like layer at the developing bone surface. As the bone matrix mineralizes, some osteoblasts become trapped in small spaces within the matrix (lacunae). These trapped osteoblasts become osteocytes, the cells responsible for maintenance of the bony matrix. Each lacuna holds only a single osteocyte but is connected with adjacent lacunae by microscopic canaliculi, which house cytoplasmic processes of the osteocytes. Osteoclasts are large, multi-nucleated cells that occur in shallow erosional depressions (Howship’s lacunae) on the resorbing bone surface and secrete enzymes that promote local digestion of collagen and dissolution of mineral crystals. Bone is commonly classified according to its gross appearance as cancellous bone (bone with numerous, macroscopic interconnecting cavities, or trabeculae, also known as spongy or trabecular bone) or compact bone (dense lamellar bone without trabeculae), but both types have the same basic histological structure. In a typical mammalian long bone the diaphysis (shaft) is composed predominantly of compact bone, with cancellous bone confined to the inner surface around a central, medullary cavity (Fig. 1A), while the epiphyses (articular ends) consist mostly of cancellous bone overlain by a thin, smooth layer of compact bone. In short bones a core of cancellous bone is completely surrounded by compact bone, and in the flat bones of the skull, inner and outer plates of compact bone are separated by the diploë, a layer of cancellous bone. Bone also can be classified histologically, as woven (primary) bone and lamellar (secondary) bone. Woven bone, or primary bone has an irregular structure and is usually replaced in adults by the more highly mineralized lamellar bone. Lamellar bone is organized into thin layers (lamellae), usually 3–7μm thick, which contain parallel collagen fiber bundles. Lacunae containing osteocytes are located between lamellae. There are three types of lamellae: concentric, interstitial, and circumferential (Fig. 1B). Concentric lamellae are arranged in circular layers around a long axis, the haversian canal, which is a vascular channel containing blood vessels, nerves, and connective tissue. Adjacent vertical channels are connected by more horizontally oriented vascular channels (Volkmann’s canals). The entire complex consisting of several layers of concentric lamellae around a vascular channel is known as an osteon or haversian system. Interstitial lamellae, which appear as irregularly shaped areas between adjacent osteons, consist of lamellae that are remnants of osteons destroyed during bone remodeling. Circumferential lamellae are arranged parallel to each other and comprise the outer circumferential lamellae laid down next to the periosteum and the inner circumferential lamellae laid down next to the endosteum.