Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Terrestrial, or Tetrapod, Vertebrates — Vertebrates with Embryonic Membranes
Class Mammals, or Mammalia — Subclass Theria — Suborder Higher Primates, or Anthropoids — Order Cetaceans

Cetaceans are a highly specialized group of mammals (comprising about 80 species) that spend their entire lives in Water (Fig. 202). Whales that accidentally end up ashore are unable to return to the water on their own. Due to their aquatic lifestyle, cetaceans have evolved numerous specific adaptive features. Their bodies are spindle-shaped and streamlined, featuring a disproportionately large HEAD. There is no distinct neck constriction. The forelimbs are modified into flippers, while the hindlimbs are absent. Only two rudimentary pelvic bones remain, which are disconnected from the Axial Skeleton. The Skin is naked and hairless, except for a few sparse hairs on the head. Sweat and Sebaceous Glands are absent. There is a single pair of Mammary Glands, with the nipples opening into pocket-like recesses located in the inguinal region. This prevents the milk from mixing with water while feeding the young.

Subcutaneous fat is exceptionally well developed: in large species, the blubber layer reaches a thickness of 50 cm. This fat layer prevents body heat loss and reduces overall body density, which in large species is 1.025 g/cm³, closely matching the density of water. The resilient, elastic skin and the thick underlying layer of blubber dampen turbulent eddies generated during swimming. The Lungs are exceptionally large. In dolphins, they inhale 1 to 2 liters of air, whereas in the giant blue whale, they accommodate up to 14,000 liters. Many species can remain submerged for 15 to 45 minutes. The nostrils have shifted to the crown of the head and open only briefly during the short exhalation-inhalation breathing cycle performed after surfacing. In cool weather, exhalation propels a plume of condensed vapor and entrained water droplets, forming the characteristic "spout" by which whalers identify the species.

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Fig. 202. Cetaceans:

1 — right whale; 2 — blue whale; 3 — sperm whale; 4 — killer whale; 5 — beluga whale; 6 — Black Sea common dolphin

The respiratory tract is highly distinctive. Due to the elongation of the epiglottis and arytenoid cartilages, a tube is formed within the Oral Cavity that connects directly to the internal openings of the nasal passages, completely isolating the respiratory tract from the Digestive System. The ability to remain submerged for extended periods is ensured by a set of morphological and physiological adaptations. The number of alveoli is immense; in a 1.5-meter dolphin, there are three times as many alveoli as in a human. The small bronchioles feature specialized muscular sphincters that contract when the whale dives, trapping the air inside the alveoli and allowing a more thorough extraction of the oxygen it contains. The Hemoglobin concentration in the Blood is extremely high—20 to 50% greater than in terrestrial mammals. A reduced sensitivity of the respiratory center to carbon dioxide accumulation in the blood, combined with a high Myoglobin content, enables cetaceans to prolong their breathing pauses and utilize this time for foraging.

Among the Senses, Hearing is the most highly developed. In water, sound waves travel five times faster than in air and can be perceived over long distances. Dolphins are capable of producing and detecting not only low-frequency sounds of a few tens of hertz, but also ultrasounds up to 150–200 kHz. Toothed whales possess a well-developed echolocation ability, emitting sound waves in a directional beam. The fatty melon located on the forehead acts as a biological "acoustic lens." Vision is well developed, whereas the olfactory Organs are rudimentary.

Whales have evolved unique adaptations related to reproduction in an aquatic environment. All cetaceans give birth to very large calves capable of swimming alongside the mother immediately after birth. Newborns measure 1/4, 1/3, or sometimes even 1/2 the size of the mother. For instance, Black Sea dolphins, with a body length of 160–170 cm, give birth to calves 80–85 cm long. Sperm whales measuring 7–8 meters give birth to calves of 3–4 meters. The blue whale, which reaches 22–23 meters in length, produces calves 7–8 meters long. The ability to give birth to such large offspring is associated with the reduction of the pelvic bones.

During nursing, the calf grasps the nipple with its Tongue curled into a tube, while the mother ejects milk into its Mouth by contracting her Abdominal Muscles. As a rule, cetaceans give birth to a single offspring. Baleen whales reproduce once every 2 to 3 years, beluga whales every other year, and dolphins annually. The young grow very rapidly, which is largely due to the exceptionally high Nutritional Value of the milk. For comparison, cow's milk contains 3–4% fat, whereas whale milk contains up to 54%. Sexual maturity is reached at the age of 3 to 6 years, rarely later. Baleen whales live up to 20–50 years, and dolphins up to 25–35 years.

Cetaceans emerged no later than the early Paleogene-Neogene, likely descending from ancient terrestrial predators. Modern species are classified into two suborders.