Back to the Beach: Adaptations that Enabled Terrestrial Mammals to Return to the Water

Abstract

Modern aquatic mammals are extremely diverse. This includes classic marine mammals such as cetaceans (whales, including dolphins and porpoises) and sirenians (manatees, dugongs), but also many amphibious mammals that spend some time on land (e.g., seals, otters, hippos, polar bears). Aquatic mammals retain terrestrial‐based systems that have been vastly modified for existence in water, but this alone does not make them extreme. However, within this group are some notable extremists: the cetaceans. They have the strangest anatomical aquatic adaptations – the ones with the “‐est” adjectives – enabling the deepest dive, slowest pulse, longest apnea, shortest exhalation, largest nose, narrowest mandible, strongest suction, biggest hyoid, widest mouth, stretchiest gular region, hairiest moustache, loudest sound, lowest song, longest tooth, highest vocalization, fattiest ear, tiniest external auditory meatus, densest tympanic bulla, curliest tongue, smoothest skin, stiffest cervical vertebrae, bumpiest forelimb, heaviest testes, and fastest copulation. For example, the sperm whale can dive to the bottom of the ocean and hold its breath for nearly an hour. Its head that is nearly one‐third of its body length, and is comprised of an enormous nose modified for sound production. Its mandible may plow the sea floor, and the hyoid and tongue create suction to engulf prey. The blue whale is the biggest mammal that ever existed, and has the widest mouth for engulfing prey. It has the largest larynx capable of making the loudest and lowest vocalizations. The narwhal has an elongated tooth that is likely the basis for the legend of the unicorn's horn. The dolphin uses modified nostrils to generate ultra high frequency sounds for echolocation, and uses acoustic fats as pinnae to receive sounds. A whale calf can curl the tongue into a straw for channeling milk when nursing underwater. Whale skin is very smooth enabling hydrodynamic water flow, and compressed/fused cervical vertebrae eliminate friction from lateral head deviations. The elongated flippers of humpback whales have bumps on the leading edge that increase efficiency. The porpoise is the smallest whale but has the largest testes and longest penis that can be rapidly extruded for the quickest mating. These amazing anatomical aquatic adaptations make cetaceans one of the most extreme groups of animals on Earth.