Anatomy

The cardiovascular system of a whale is one of the most impressive biological structures in the animal kingdom. At its center is a heart that, in the case of the blue whale, weighs approximately 400 pounds and measures roughly five feet in length. This enormous organ beats only about 8 to 10 times per minute at the surface, and its powerful contractions can be detected from approximately two miles away using specialized instruments. During deep dives, a whale's heart rate drops dramatically in a process called bradycardia. A blue whale's heart rate can slow to as few as 2 beats per minute during a dive, compared to its resting surface rate. This remarkable physiological adaptation helps conserve oxygen and extend the duration of dives. The heart rate then surges when the whale surfaces to breathe, rapidly redistributing oxygenated blood throughout the body. The blood vessels of large whales are proportionally enormous. The aorta of a blue whale is large enough for a human head to fit through, and the major blood vessels are wide enough for a small child to crawl through. Whales have a significantly higher blood volume relative to their body size compared to land mammals, and their blood has an enhanced capacity to carry oxygen, which is critical for their deep-diving lifestyle. A specialized network of blood vessels called the rete mirabile (Latin for 'wonderful net') is found in the flippers, flukes, and dorsal fin of whales. This counter-current heat exchange system ensures that warm arterial blood flowing to the extremities heats the cooler venous blood returning to the body core. This mechanism prevents excessive heat loss through the thin skin of the fins and flukes while allowing the whale to maintain a stable core body temperature even in frigid polar waters. Whales also possess a unique adaptation called the thoracic retia mirabilia, a dense network of blood vessels along the spine that acts as an oxygen reservoir during dives. This system, combined with high concentrations of myoglobin in their muscles (which gives whale meat its characteristically dark color), allows whales to store far more oxygen than similarly sized land mammals.

One of the most fundamental anatomical divisions among whales is between the two suborders: Mysticeti (baleen whales) and Odontoceti (toothed whales). This distinction reflects two radically different approaches to feeding that have shaped nearly every aspect of whale anatomy and behavior. Baleen whales, including blue whales, humpback whales, gray whales, and right whales, have plates of baleen instead of teeth. Baleen is made of keratin, the same protein found in human fingernails and hair. These plates hang from the upper jaw in rows and act as a filtration system, allowing whales to strain enormous volumes of water and trap small prey such as krill, copepods, and small schooling fish. The size, shape, and number of baleen plates vary significantly among species and reflect their dietary specializations. Right whales and bowhead whales have the longest baleen plates, up to 14 feet in bowheads, which they use for skim feeding: swimming slowly with their mouths open to continuously filter tiny zooplankton. Blue whales and fin whales have shorter, coarser baleen suited to lunge feeding, where they engulf massive mouthfuls of water and prey, then push the water out through the baleen using their tongues. Toothed whales, including sperm whales, killer whales, beluga whales, and narwhals, have teeth that they use to catch individual prey items. The number and type of teeth vary widely. Sperm whales have 20 to 26 pairs of conical teeth on their lower jaw only. Killer whales have 40 to 56 interlocking teeth designed for gripping slippery prey. Narwhals are unique in possessing a single spiraling tusk up to 10 feet long, which is actually an elongated left canine tooth. The evolution from teeth to baleen is one of the most fascinating chapters in whale evolutionary history. Fossil evidence shows that the earliest baleen whales had both teeth and baleen, and that teeth were gradually lost as baleen became the primary feeding structure. This transition enabled baleen whales to exploit vast populations of small prey in the ocean, ultimately allowing them to achieve the enormous body sizes seen in species like the blue whale.

Whales are air-breathing mammals that must regularly return to the surface to inhale, yet many species routinely dive to extraordinary depths and remain submerged for extended periods. The anatomical adaptations that make this possible represent some of the most remarkable feats of evolutionary engineering in the animal kingdom. The blowhole is the whale's nostril, which has migrated from the front of the skull to the top of the head over the course of evolution. Baleen whales have two blowholes arranged side by side, while toothed whales have a single blowhole. A muscular flap covers the blowhole and creates a watertight seal during dives. When a whale surfaces, this flap opens, and the whale forcefully exhales stale air at speeds up to 300 miles per hour, creating the characteristic spout or blow that can be seen from miles away. The shape and height of a whale's blow are species-specific and are used by whale watchers and researchers for identification. Blue whales produce a tall, columnar blow up to 30 feet high. Humpback whales produce a lower, bushy blow. Right whales produce a distinctive V-shaped blow because their two blowholes are widely spaced. Sperm whales produce a blow that angles forward and to the left, reflecting their asymmetrical skull anatomy. Whale lungs are proportionally smaller than those of land mammals of similar size, which might seem counterintuitive for an animal that holds its breath for long periods. However, this is actually an adaptation to prevent nitrogen narcosis (the bends) during deep dives. Whales store oxygen not in their lungs but primarily in their blood and muscles. Their blood contains high concentrations of hemoglobin, and their muscles are loaded with myoglobin, a protein that binds oxygen and gives whale muscle its distinctive dark red or near-black color. Sperm whales are the champion divers, routinely reaching depths of 1,000 to 2,000 meters and remaining submerged for up to 90 minutes. At these depths, the pressure is immense, and whale anatomy has several adaptations to cope. Their ribs are flexibly attached to the spine and can fold inward under pressure. Their lungs collapse at depth, pushing air into reinforced upper airways where gas exchange is minimal, preventing nitrogen from dissolving into the blood. Their heart rate slows dramatically, and blood is preferentially shunted to essential organs like the brain and heart.

Blubber is the thick layer of fat beneath a whale's skin that serves multiple critical functions: insulation, energy storage, buoyancy, and streamlining the body shape. The thickness of blubber varies by species, age, nutritional condition, and the water temperatures the whale inhabits. Bowhead whales, which spend their entire lives in frigid Arctic waters, have the thickest blubber of any whale species, measuring up to two feet thick. This extraordinary insulation allows them to thrive in waters near freezing. In contrast, tropical species have thinner blubber layers because their thermoregulatory demands are lower. Blubber is not simply a passive layer of fat. It is a metabolically active tissue that plays a crucial role in energy balance. During long migrations, many whale species rely heavily on their blubber reserves for energy. Gray whales, for example, fast for much of their migration between Arctic feeding grounds and Mexican breeding lagoons, losing up to 30% of their body weight during the journey. Nursing mothers burn through blubber reserves particularly rapidly, as whale milk is extremely rich in fat, containing 30 to 50% fat compared to about 4% in cow's milk. The rete mirabile system works in conjunction with blubber to manage body temperature. When a whale is active and generating excess metabolic heat, blood flow to the skin surface increases, allowing heat to dissipate into the water. When heat conservation is needed, blood flow to the skin decreases, and the insulating blubber layer traps warmth close to the body core. This dynamic system allows whales to maintain a stable core temperature of approximately 97 to 99 degrees Fahrenheit regardless of external water temperature. Whale skin itself is also highly specialized. It lacks hair (except for a few sensory hairs on the heads of some species) and sweat glands. The outer layer, or epidermis, is remarkably thick and turns over rapidly, helping to reduce drag and prevent the buildup of parasites and barnacles. In some species, the skin can regenerate from wounds with remarkable speed, with injuries from predators or entanglement healing over in weeks to months.

The whale skeleton tells the story of a dramatic evolutionary transformation from land to sea. Despite being fully aquatic, whale skeletons retain many features inherited from their terrestrial ancestors, modified extensively for life in water. The most striking skeletal feature is the presence of vestigial pelvic bones, small remnants of the hip structure that once supported hind legs. These bones serve no locomotor function but provide attachment points for reproductive muscles. Some whale specimens have been found with tiny vestigial leg bones still attached to the pelvis, a vivid reminder of their evolutionary heritage. Whale flippers are modified forelimbs that contain the same basic bone structure as a human arm and hand: humerus, radius, ulna, carpals, metacarpals, and phalanges (finger bones). However, these bones are shortened and flattened, and the entire structure is encased in a rigid, paddle-shaped flipper. In humpback whales, the flippers can reach up to 16 feet in length, making them the longest appendages of any animal. The flippers are used for steering, stability, and thermoregulation, but not for propulsion. Propulsion comes entirely from the flukes, the broad, horizontal tail fins. Unlike fish tails, which move side to side, whale flukes move up and down, powered by massive muscles along the spine and lower back. The flukes contain no bones; they are made of dense, fibrous connective tissue. The shape and size of flukes vary among species, reflecting different swimming styles and speeds. Fin whales, the fastest of the great whales, have relatively narrow, crescent-shaped flukes optimized for speed. The whale skull has undergone dramatic restructuring compared to its terrestrial ancestors. In baleen whales, the upper jaw is arched to accommodate the long baleen plates, and the lower jaw is bowed outward to create a vast mouth cavity. In sperm whales, the skull is massively asymmetrical, with a huge basin on top that houses the spermaceti organ. In toothed whales generally, the skull bones have shifted to create a concave dish shape that focuses and directs echolocation clicks. The vertebral column of whales is more flexible than that of most land mammals, allowing for the powerful undulating movements that drive swimming. However, the neck vertebrae in many whale species are fused together, providing rigidity for the head during swimming. A notable exception is the beluga whale, whose unfused neck vertebrae allow it to turn its head in all directions, a unique ability among cetaceans.