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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
The largest of all toothed whales (Odontocetes) are the relatively elusive deep-diving sperm whales (Physeter macrocephalus), noted for their large conspicuous heads that measure one-third of their body length. These enormous whales can reach up to 20 m in length and 50,000 kg in weight. Their dives can exceed 1,000 m into oceanic waters in search of their preferred food source, squid. Sperm whales forage continuously day and night with diving times averaging 40 to 50 minutes although durations up to 138 minutes have been recorded[1] Surface intervals of around 10 minutes between dives are used to release carbon dioxide and take in more oxygen. But how are these large marine mammals able to locate their prey in the dark ocean twilight zone, provide sufficient oxygen for their tissues, and survive the pressures and cold temperatures of the deep ocean?
A feature of all toothed whales is their capacity to use echolocation to find their prey.[2] Echolocation is the whale’s ability to “see” by emitting specialized sounds to assess their environment and then listening to the sound waves as they rebound from distinctive objects or prey. “Monkey lips,” or special valves, along with small fat bodies located in the upper nasal openings of the sperm whale’s head are unique sound producers. Air is forced though the monkey lips, causing them to vibrate in a manner similar to human vocal chords. The sound, which is higher than frequencies heard by humans, is then directionally transmitted toward their prey by the spermaceti organ. This organ, which takes up much of the sperm whale’s headspace, is filled with low-density lipids and acts as an acoustic lens. The short duration clicks produced as a result of this sound-production mechanism become more like a buzzing sound as the whale approaches its prey. Based on prey characteristics, sperm whales can change vibration frequency, interval, and duration to provide a three-dimensional image.
In sperm whales, sound is not directed into the middle ear through the ear canal as it is in humans but through highly sensitive fat tissue associated with the jaw. Sound is then conducted to the tympanic bulla in the inner ear. The tympanic bulla is suspended by connective tissue in a mixture of mucus, fat, and air that actually separates the middle ear from the skull and thereby focuses the sound waves to enhance reception. The fact that the membrane of the tympanic bulla is also reinforced with bony ligaments seems to likewise improve hearing of high frequencies.
Breath-holding sperm whales also have a number of physiological mechanisms to counteract compression and subsequent damage to tissues resulting from increasing hydrostatic pressure when diving at depth for prey.[3] The pressure within all the body’s airspaces must match that of the ambient pressure so as to avoid any distortion or injury. With depth, the air-filled cranial sinuses become engorged, thus eliminating the air so as to prevent sinus squeeze. Moreover, the reinforcement of peripheral airways in sperm whales allows for a gradual collapse of the lungs with the air being pushed into the upper airways and stopping the exchange of gases in the blood. Limiting gas exchange is advantageous since it reduces nitrogen absorption and subsequently prevents nitrogen narcosis.
The lungs of sperm whales only store 5% of their total oxygen and are therefore not considered an important oxygen store. Instead the large volume of blood in sperm whales (200–260 mL/kg) acts as a substantial oxygen store.[4] Furthermore, much of the sperm whale’s oxygen supply is due to elevated levels of the oxygen-binding proteins hemoglobin and myoglobin in the blood and muscles, respectively. Sperm whales can likewise reduce their oxygen use and energy consumption by slowing down their heart rate and metabolic rate when diving. Their streamlined and torpedo-like body along with extensive use of gliding provides minimal drag and reduced workload for the muscles, thus decreasing oxygen usage.
The ability to maintain body-core temperature (thermoregulation) is another essential physiological mechanism of the warmblooded sperm whales, which forage in cold and extremely conductive waters. Their blubber (which may be as thick as 250 mm) aids in insulating the whale’s organs.[5] Furthermore, their low body surface-tovolume ratio also reduces potential heat loss. An additional challenge sperm whales face is potential heat loss from their poorly insulated body appendages such as flukes and flippers, which contain limited blubber. To overcome this, they use a countercurrent system,[6] which involves arteries and veins that run in parallel but opposite directions. The cool incoming venous blood from the flukes and flippers that are exposed to the cold water is warmed by the outgoing arterial blood from the heart, forming an efficient heat-transfer mechanism. Conversely, when under heat stress, whales have a superficial venous system in their skin that is not warmed by outgoing arterial blood and thus enables the whale to cool down.
Sperm whales are amazingly designed for foraging and living in deep oceanic waters. The absence of any of the above interconnected mechanisms would affect their ability to survive in the hostile environment of the dark, deep, and cold mesopelagic zone. The ingenuity and integration of these efficient physiological systems speak to me of a wise Creator who took great care in implementing His designs.
NOTES
[1] SL Watwood, PJO Miller, M Johnson, PT Madsen, PL Tyack. Deep-diving foraging behaviour of sperm whales (Physeter microcephalus). Journal of Animal Ecology 2006; 75(3):814–825. doi:10.1111/ j.1365-2656.2006.01101.
[2] Ibid.; S Hooker. Toothed whales (Odontoceti) diving. In: Wursig B, Thewissen JGM, Kovacs KM, editors. Encyclopedia of marine mammals. 3rd ed. Cambridge (MA): Academic Press; 2018, pp. 1004–1010. doi:10.1016/B978-0-12-804327-1.00261-2.
[3] GL Kooyman, PJ Ponganis. Diving physiology. In: Wursig B, Thewissen JGM, Kovacs KM, editors. Encyclopedia of marine mammals. 3rd ed. Cam-bridge (MA): Academic Press; 2018, pp. 262–267. doi:10.1016/B978-0-12-804327-1.00108-4.
[4] PJ Ponganis. Circulatory system. In: Wursig B, Thewissen JGM, Kovacs KM, editors. Encyclopedia of marine mammals. 3rd ed. Cambridge (MA):Academic Press; 2018, pp. 191–194. doi:10.1016/ B978-0-12-804327-1.00091-1.
[5] K Evans, MA Hindell, D Thiele. Body fat and condition in sperm whales, Physeter microcephalus, from southern Australian waters. Comparative Bio-chemistry and Physiology–Part A: Molecular and Integrative Physiology 2003; 134(4):847–862. doi: 10.1016/ S1095-6433(03)00045-X.
[6] Ponganis, op. cit.
Christine Jackson is an associate professor in earth and biological sciences at Loma Linda University. She holds a PhD in Marine Ecology from the University of Tasmania. She has published a number of scientific papers and is currently working on the trophic ecology of toothed whales using biochemical techniques.