Exploring Our Fluid Earth

Endothermy: Heat from Food

All mammals are capable of maintaining internal body temperatures regardless of the air temperatures around them through a process called endothermy. Endothermy allows animals like birds and mammals to survive in cold environments. However, this process of converting food into heat energy often demands large quantities of food. With the exception of sirenians (the dugong and manatees), all marine mammals are predatory in nature. They feed on other animals in order to survive. However, it may be surprising to you that some of the largest animals on Earth feed on very small animals. The blue whale grows to over 30 meters in length and has a large mouth with powerful jaws. Despite its size, the blue whale feeds on small shrimp-like animals called krill (Fig. 6.23 A). Figure 6.23 shows some examples of common prey for mysticete whales feeding using baleen.

Fig. 6.23. (A) Northern krill (Meganyctiphanes norvegica)

Image courtesy of Øystein Paulsen, Wikimedia Commons

Fig. 6.23. (B) Large school of sardines, Mactan Cebu, Philippines

Image courtesy of Juuyoh Tanaka, Flickr

Cetacean Feeding

Fig. 6.7. (A) Close-up image of baleen inside the mouth of a mysticete whale

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

The two main groups of cetaceans use two different feeding strategies based on their anatomy. Mysticete whales have rigid baleen structures in their mouths (Fig. 6.7 A). When feeding, they gulp large mouthfuls of seawater into their large expandable throat pouches (Figs. 6.23.1 and 6.24 A). Then they push the seawater out of their mouths and through the filter-like baleen structures. Any prey in the seawater—such as krill (Fig. 6.23 A) or small fish (Fig. 6.23 B)—get caught in the baleen and are subsequently swallowed. It is interesting to note that even the largest blue whales can only fit items smaller than a beach ball down their throats. Different mysticete whale species feed on different prey. Bowhead and right whales are known as “skimmers” because they move at the ocean surface with their mouths open, skimming for plankton. Gray whales are considered “suckers” because of the way they slurp and scrape mud off the seafloor when feeding. They filter out the mud and seawater through their baleen and swallow mouthfuls of small crustaceans and worms.

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Image caption

Fig. 6.23.1. Diagram illustrating how mysticete whales filter food particles using baleen

Image copyright and source

Image courtesy of Martin-rnr, Wikimedia Commons

Some groups of humpback whales employ a unique strategy for catching small fish such as sardines that swim in large schools (Fig. 6.23 B). Humpback whales work cooperatively in two groups to trap and capture these fast-swimming fish. One group corrals the school of fish by exhaling a ring of air bubbles around them—a behavior known as bubble-netting (Fig. 6.24 B). The fish inside the bubble-net crowd together into a tight ball as the ring of bubbles floats upward and drives them to the ocean surface. While the first group of whales continues to produce the bubble-net, a second group swims up the center of the bubble-net and engulfs the fish (Figs. 6.24 C and 6.24 D).

Fig. 6.24. (A) Humpback whale (Megaptera novaeangliae) feeding on juvenile pollock fish, Brothers Island, Alaska

Image courtesy of David Csepp, National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA)

Fig. 6.24. (B) Aerial view of two humpback whales bubble net feeding

Image courtesy of Allison Henry, National Oceanic and Atmospheric Administration (NOAA) Fisheries

Fig. 6.24. (C) Humpback whales engulfing prey after employing the bubble-net technique, Lynn Canal, Alaska

Image courtesy of Georgia Evelyn Stants, Wikimedia Commons

Fig. 6.24. (D) Humpback whale lunging in the center of a bubble net spiral

Image courtesy of Christin Khan, National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA)

In contrast to the filter-feeding methods of mysticetes, odontocete whales use hard, sharp teeth to directly bite their prey. Figure 6.25 shows some examples of odontocete whale predatory feeding behavior. Odontocete whales are often among the top predators of their ecosystems. The killer whale (Orcinus orca; Figs. 6.25 B and 6.25 D) is known to feed on gray whales, great white sharks, sea turtles, whale sharks, dolphins, sea otters, and pinnipeds. Dolphins have been observed attacking and eating sharks and smaller odontocetes such as porpoises (Fig. 6.25 C).

Fig. 6.25. (A) Amazon River dolphin (Inia geoffrensis) catching a fish with its sharp teeth

Image courtesy of Dennis Otten, Wikimedia Commons

Fig. 6.25. (B) Killer whale teeth

Image courtesy of Karyn Christner, Flickr

Fig. 6.25. (C) Bottlenose dolphins (Tursiops sp.) attacking a harbor porpoise (Phocoena phocoena), another odontocete, Scotland

Image courtesy of Peter Asprey, Wikimedia Commons

Fig. 6.25. (D) Killer whale (Orcinus orca) attacking a Weddell seal (Leptonychotes weddellii) on sea ice, Antarctica

Image courtesy of Robert Pitman, National Science Foundation (NSF) Office of Polar Programs

Marine Carnivoran Feeding

As cute as seals and sea otters appear, it is important to remember that all carnivorans have sharp teeth, strong jawbones and powerful jaw muscles (Fig.6.26 A). Recall that this group shares common ancestors with tigers, lions, and grizzly bears. Some pinnipeds have adapted for highly specialized diets. The leopard seal is one of the top predators of Antarctica (Fig. 6.26 A). Leopard seals (Hydrurga leptonyx) use their large sharp teeth to capture penguins and even other seals as prey. Both male and female walruses (Odobenus rosmarus) have long tusks protruding from their mouths (Fig 6.26 B). These tusks are simply elongated front “canine” teeth adapted for digging. Walruses dive as deep as 80 to 100 meters to the seafloor. They use their long tusks to dig into soft sediments and uncover buried invertebrate prey such as clams and crustaceans. It has been estimated that an adult walrus can eat 3,000 to 6,000 clams in one day.

Fig. 6.26. (A) Leopard seal (Hydrurga leptonyx) yawning to reveal sharp teeth, Aitcho Island, Antarctica

Image courtesy of ravas51, Flickr

Fig. 6.26. (B) Walrus (Odobenus rosmarus) skull

Image courtesy of Mike Peel, Wikimedia Commons

Polar bears primarily feed on ringed seals (Pusa hispida) and bearded seals (Erignathus barbatus). Polar bears ambush seals and small odontocete whales when they surface to breathe at ice holes.

In contrast to other marine mammals, sea otters primarily use their fore limbs to capture prey. They spend most of their time floating on the ocean surface, but dive down to the rocky seafloor to capture their prey. Common food items for the sea otter include sea urchins, crustaceans, molluscs, and fish. The sea otter is one of the few animal species known to use tools. They use rocks to dislodge prey attached to the seafloor as well as to crack open mollusc shells.

Sirenian Feeding

Manatees and dugongs are the only herbivorous, or plant-eating, marine mammals. They use their muscular upper lips to dig seagrass roots from the sediment (Fig. 6.27). Sirenians grind seagrass and algae using large flat molar teeth. These molars grow continually to replace teeth worn down by abrasion. This “marching molar” feature is shared by elephants, a close evolutionary relative of the sirenians.

Fig. 6.27. (A) Two West Indian manatees (Trichechus manatus) dig in the sand.

Image courtesy of Jim P. Reid, U.S. Fish and Wildlife Service (USFWS)

Fig. 6.27. (B) A dugong (Dugong dugon) grazing on seagrass, Marsa Abu Dabab, Egypt

Image courtesy of Matthijs Rouw, Flickr