Exploring Our Fluid Earth

Mammals have adapted to survive in every habitat where other animal taxa occur. The exception to this rule is deep-sea benthic habitats, although some cetaceans do dive deeply. Marine mammals have many adaptations that allow them to survive in various aquatic habitats. Figure 6.18 shows some examples of mammal adaptations.

Fig. 6.18. (A) Bats—the second largest mammalian group in terms of species diversity—can fly with the use of webbed wings.

Image courtesy of U.S. Bureau of Land Management (BLM)

Fig. 6.18. (B) Flipper forelimb appendages on a humpback whale

Image courtesy of Kevin Galens, Flickr

Fig. 6.18. (C) Humpback whale fluke

Image courtesy of Amy Kennedy, National Oceanic and Atmospheric Administration (NOAA)

Fig. 6.18. (D) Brown fur seal flipper extended out of the water

Image courtesy of Paul Mannix, Flickr

Swimming

Marine mammals have several adaptations for swimming. An obligate swimmer is any species that spends its entire life in water. All whales and dolphins are obligate swimmers. Unlike pinnipeds, otters, and polar bears, cetaceans cannot survive on land for extended periods of time. Similarly, all sirenians are obligate swimmers. Both cetaceans and sirenians are well adapted to swimming through millions of years of evolution by natural selection. They possess smooth streamlined bodies with very little hair and flipper-shaped fore limbs (Fig. 6.18 B). Cetaceans and sirenians also have wide, flat muscular tails (Fig. 6.18 C). These adaptations help cetaceans and sirenians swim efficiently through the water.

Other marine mammal groups have adapted to living both at sea and on land. Pinnipeds, otters, and the polar bear are not obligate swimmers because they can live on dry land. Pinnipeds and otters have powerful webbed limbs that allow them to swim quickly (Fig. 6.18 D). They also have powerful hind limbs that allow them to walk on land. Polar bears are truly marine mammals because they spend much of their time swimming in the open ocean, although they, too, can live for extended periods on land. They have adapted broad paws on their powerful fore limbs that allow for swimming hundreds of miles over several days. Unlike the obligate swimmer groups, pinnipeds, otters, and the polar bear have dense, thick coats of fur to keep them warm in and out of the water.

Generating and Retaining Heat

Endothermy is the process of generating heat from the chemical digestion of food. The word endothermy comes from Greek root words meaning heat within. Endothermic animals are sometimes described as being warm-blooded. All mammals are endothermic and use a variety of mechanisms to maintain steady, homeostatic internal body temperatures. Homeostasis is the condition of a body system that is actively regulated to remain consistent. If a mammal’s body temperature begins to fall, it can shiver or increase its metabolic rate of converting food energy to heat. If a mammal begins to overheat, it can secrete sweat or increase blood flow to the skin to cool off. These mechanisms allow mammals to thrive in a wide range of environments.

Marine mammals, by definition, spend most, if not all, of their time in the ocean. Ocean water is much colder than the internal body temperature of most mammals. Additionally, many marine mammal species live in polar climates or dive down into cold deep waters.

Fig. 6.19. Blubber cut from a beluga whale (Delphinapterus leucas), a species of odontocete cetacean

Image courtesy of Jai Mansson, Flickr

The primary way that marine mammals have adapted to maintain their internal body temperatures in these cold environments is with insulating layers that retain body heat. Blubber is the dense layer of fat tissue under the skin of almost all marine mammals (Fig. 6.19). Exceptions include the polar bear, sea otter, and marine otter. Some marine mammals have more blubber than others. Harbor porpoises typically only have 2.5 to 3 centimeters (cm) of blubber. Bowhead whales can have up to 50 cm of blubber. Although polar bears lack true blubber, they do have a similar layer of thick fatty tissue—up to 11 cm thick—under their dense fur.

Land mammals keep warm in cold climates with thick layers of fur hair covering their bodies. In the water, fur also serves to keep mammals warm by trapping a layer of warm air near the skin. The sea otter has the thickest fur of any mammal with up to 165,000 hairs per square centimeter of skin. They maintain this fluffy insulation by constantly grooming themselves and each other. Pinnipeds employ a combination of blubber and dense fur to retain body heat.

This combination of endothermic heat production and thick insulating blubber and fur allows marine mammals to survive in some of the coldest environments on Earth.

Diving Adaptations

A typical human can hold their breath underwater for less than two minutes. With extensive training, some competitive free-divers can hold their breath for up to 12 minutes (Fig. 6.20). In the case of marine mammals, a few minutes is a relatively short period of time for the activities they must carry out under water—hunting for food, breeding, socializing, and navigating. While most marine mammals make several short dives, some regularly make long deep dives (Fig. 6.21). California sea lions dive up to 200 meters (m) and spend approximately two minutes at a time underwater. Elephant seals spend 90 percent of their time submerged, averaging 20 minutes per dive and routinely feeding at depths of 300–600 m (Fig. 6.22). Sperm whales (Fig. 6.21) are some of the deepest diving organisms on the planet. Individual sperm whales have been recorded diving to 2,250 m and staying under water for almost 90 minutes. Sperm whales routinely forage for prey for nearly an hour at depths of up to 1,000 m. The deepest diving mammal known is Cuvier’s beaked whale (Ziphius cavirostris). A tagged individual of this species was recorded to dive to 2,992 m.

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Fig. 6.20. Trained human free-divers can hold their breath for almost 12 minutes, far longer than the average human.

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Image courtesy of Sylvain7171, Wikimedia Commons

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Fig. 6.21. Examples of deep diving marine mammals

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Image by Byron Inouye, adapted from J. Cousteau, The Ocean World, 1979.

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Fig. 6.22. (A) Map illustrating the movements of a male northern elephant seal (Mirounga angustirostris) wearing a time-depth recorder

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Image courtesy of Marine Mammal Laboratory, Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration (NOAA)

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Fig. 6.22. (B) Graph illustrating the time and depth of diving behavior by the same male northern elephant seal

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Image courtesy of Marine Mammal Laboratory, Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration (NOAA)

Diving is complicated by a drastic increase in pressure underwater. For every increase of 10 m in depth below sea level, the pressure increases by one atmosphere (atm). Air compresses under pressure. For example, a balloon filled with one 1 liter (L) of air at the surface decreases in size 50 percent when moved 10 m below the surface. Mammals have air-filled spaces in their ears and lungs, all of which have the potential to collapse under high pressures. Some marine mammals have adaptations that fill the air spaces in their ears with blood, which, being a liquid, does not compress significantly under pressure. There are also several species that have evolved structural adaptations like flexible rib cages that allow their airways and lungs to collapse as they dive and then re-expand as they surface.

Increases in pressure associated with diving also present physical problems for mammals in terms of the gas solubility. As pressure increases, the partial pressure of each gas in air can increase to toxic levels for the organism. The partial pressure of a gas, like N₂ or O₂, in a gas mixture like air is the pressure of that gas at the volume of the entire mixture. At the surface where the total pressure is 1 atm, the partial pressure of N₂ is 0.79 atm and of O₂ is 0.21 atm. At 100 meters depth, the partial pressure of N₂ is 8.69 atm and of O₂ is 2.31 atm. Oxygen becomes toxic to humans at 1.6 atm or greater partial pressure. Nitrogen gas at 3 atm or greater partial pressure can impair scuba divers and lead to variety of symptoms resembling alcohol intoxication including euphoria and hallucinations. Scuba divers refer to this impairment as nitrogen narcosis. Another issue that marine mammals face is that at increased depths, gas is absorbed in the tissues and blood of organisms. The higher the partial pressure of gas and the longer the animal stays underwater, the more the tissues become saturated. While this is not a problem at depth, when the animal returns to the surface, they must off-gas, or breathe out the gas they have absorbed in their tissue. If the organism surfaces too rapidly, the gases can bubble into the tissue causing damage and even fatality. This is known as decompression sickness. For this reason, scuba divers must slowly return to the surface to off-gas. One of the most important behaviors of marine mammals adapting to the problems of high-pressure diving is to expel most of the air out of their lungs prior to diving. This behavior can reduce the impact of gas toxicity and decompression sickness.

All mammals, including marine mammals, need to breathe to provide oxygen to cells, tissues, and organs so they can function. Unlike fish, marine mammals do not have gills to extract oxygen directly from the water. They must instead deal with the lack of oxygen associated with holding their breath when they are under water. A lack of oxygen results in a decreased metabolism and a larger reliance on anaerobic respiration to power cellular machinery. Anaerobic respiration provides a much lower energy output than aerobic respiration. Another consequence of anaerobic respiration is the accumulation of the toxic byproduct lactic acid. Most mammals can tolerate a small amount of lactic acid for a short period of time. During deep dives, where anaerobic respiration is common, many marine mammals have adaptations that aid in lactic acid tolerance. For example, many deep-diving mammals have increased levels of enzymes that help break down lactic acid. Another adaptation to holding their breath is that many marine mammals have increased the amount of oxygen that can be stored in their internal tissues compared to their terrestrial counterparts, primarily the lung, muscle, and blood. To do this, the blood and muscle have higher concentrations of oxygen carrying molecules called hemoglobin and myoglobin. Marine mammals also have, in general, increased blood volume compared with a human or other terrestrial mammals. In humans, the blood volume is seven percent of the body mass while in marine mammals it ranges from 10 to 20 percent.