Description & Behavior
Marine mammals evolved from their land dwelling ancestors over time by developing adaptations to life in the water. To aid swimming, the body has become streamlined and the number of body projections has been reduced. The ears have shrunk to small holes in size and shape. Mammary glands and sex organs are not part of the external physiology, and posterior (hind) limbs are no longer present.
Mechanisms to prevent heat loss have also been developed. The cylindrical body shape with small appendages reduces the surface area to volume ratio of the body, which reduces heat loss. Marine mammals also have a counter current heat exchange mechanism created by convergent evolution where the heat from the arteries is transferred to the veins as they pass each other before getting to extremities, thus reducing heat loss. Some marine mammals also have a thick layer of fur with a water repellent undercoat and/or a thick layer of blubber that can’t be compressed. The blubber provides insulation, a food reserve, and aids with buoyancy. These heat loss adaptations can also lead to overheating for animals that spend time out of the water. To prevent overheating, seals or sea lions will swim close to the surface with their front flippers waving in the air. They also flick sand onto themselves to keep the sun from directly hitting their skin. Blood vessels can also be expanded to act as a sort of radiator.
One of the major behavioral adaptations of marine mammals is their ability to swim and dive. Pinnipeds swim by paddling their flippers while sirenians and cetaceans move their tails or flukes up and down.
Some marine mammals can swim at relatively high speeds. Sea lions swim up to 35 kph and orcas can reach 50 kph. The fastest marine mammal, however, is the common dolphin, which reaches speeds up to 64 kph. While swimming, these animals take very quick breaths. For example, fin whales can empty and refill their huge lungs in less than 2 seconds. During dives marine mammals’ larynx and esophagus close automatically when they open their mouths to catch prey. Oxygen is stored in hemoglobin in the blood and in myoglobin in the muscles. The lungs are also collapsible so that air is pushed into the windpipe preventing excess nitrogen from being absorbed into the tissues. Decreasing pressure can cause excess nitrogen to expand in the tissues as animals ascend to shallower depths, which can lead to decompression sickness aka “the bends.” Bradycardia, the reduction of heart rate by 10 to 20%, also takes place to aid with slowing respiration during dives and the blood flow to non-essential body parts. These adaptations allow sea otters to stay submerged for 4 to 5 minutes and dive to depths up to 55 m. Pinnipeds can often stay down for 30 minutes and reach average depths of 150-250 m. One marine mammal with exceptional diving skills is the Weddell seal, which can stay submerged for at least 73 minutes at a time at depths up to 600 m. The length and depth of whale dives depends on the species. Baleen whales feed on plankton near the surface of the water and have no need to dive deeply so they are rarely seen diving deeper than 100 m. Toothed whales seek larger prey at deeper depths and some can stay down for hours at depths of up to 2,250 m.
Marine mammals are often very social animals. Dolphins travel in pods (schools) and catch rides on the bow waves of boats. Marine mammals are also known to help each other when one member of the group is injured. There have been accounts of members of a pod refusing to leave the wounded or dying, a trait often exploited by whalers. Cetaceans (whales and dolphins) often hunt together, often with one leading the pod to act as a scout when entering unfamiliar territory. This close knit socialization is thought to be a factor in some whale strandings when a pod follows one or more members of the group that have become disoriented due to storm, illness, or injury.
Many marine mammals also participate in yearly migrations, either in groups or individually. Toothed whales are an exception and only move about in search of food, but some baleen whales (such as gray whales) embark on extremely long migrations, moving from tropical breeding grounds in winter to feeding areas in colder waters during the summer.
Marine mammals are capable of sophisticated communication because they live in a world dominated by sound, which travels much more efficiently through water than through air. Dolphins communicate with sound to coordinate hunts; humpback whales sing to attract females. Female pinnipeds and their pups recognize each other by their “voices.” Slapping the surface during breaching can be heard for miles. Whales have no vocal cords; they warble for up to 30 minutes between breaths just by recycling air. They also emit low frequency sounds that can be heard by humans such as grunts, barks, squeaks, chirps, or even moos. These noises are thought to be associated with different moods and are believed to be used as social or sexual cues during communication. They might also serve as a signature to allow one animal to be recognized by another. Certain pods are known to even have dialects than can be distinguished from others.
Echolocation is a skill that only toothed cetaceans, bats and a few birds have perfected. They send out rapid sound pulses and listen to their echo to find prey and determine their surroundings. It is thought that sperm whales also use echolocation to stun squid with loud clicks. Clicks can be repeated at different frequencies with low frequencies traveling long distances that are highly penetrating. Toothed whales have a structure called the melon on their forehead that focuses and directs the sound waves; incoming sounds are received primarily in the lower jaw, which is filled with fat or oil that transmits the sound to the inner ear.
About 65 million years ago (mya) when dinosaurs became mostly extinct, marine mammals began to evolve from their land-dwelling ancestors. Their evolution into sea-dwelling mammals is thought to be a result of the availability of new marine food sources and a way to escape from their terrestrial predators. The fossil record for whales is not as extensive as it is for other marine mammals such as otters and pinnipeds, therefore the transition period between land and water is unclear. In 1994, the remains of Ambulocetus natans (“the walking whale that swam”) dating 49 mya were found in Pakistan in what’s left of the Tethys Sea. These whale remains showed that the animal once had strong legs with long feet, similar to modern pinnipeds, that were functional both on land and in the sea. It retained a tail, but lacked flukes, however it is still thought that this animal swam like modern whales by moving the rear portion of its body up and down. In 2001, other fossils were found that linked early cetaceans to hoofed animals (ungulates).
How Scientists Study Marine Mammals
There are a variety of ways that scientists study whales. Whale teeth have rings, a new one for every year like trees. These rings can be examined to determine whether a whale was healthy and how long it lived. DNA fingerprinting can also be used by taking a small piece of skin or muscle to identify individuals. It can even be used to identify relatives. Humpbacks have unique black and white patterns on their tails used for identification. Whales and other marine mammals can also have telltale scratches and scars that can help in identifying individuals. Various tags are used to monitor heart rate, the animal’s movements, and how deep and how fast they swim. Recent genetic analysis has led to new separations of species resulting in, for example, three species of right whales.
Otters and Polar Bears
Sea otters are the smallest of all marine mammals. They lack a layer of blubber so they protect themselves from the cold by trapping air in their extremely dense fur. Polar bears are also considered marine mammals because they are semi-aquatic and rely entirely on the sea for food.
There are three Families of pinnipeds. First are the earless, or true seals, (Phoncidae) which consist of 19 species. Next are the eared seals (Otariidae) which has 14 species including sea lions and fur seals. Last are the walruses (Odobenidae). Most of these animals are common in the Arctic and Antarctic because their primary prey is more available there than in warmer water.
This is a diverse group despite its physiological similarities. Pinnipeds vary greatly in size and the way they utilize the marine environment. They consume a variety of different food sources, including fish and cephalopods (squid, cuttlefishes and octopuses), and certain species have also been known to eat krill, crabs, and shellfish. Feeding behavior and diet may differ widely even within a specific population.
All cetaceans are marine except for a few species of freshwater dolphins. There are two different groups of cetaceans, toothed and baleen. The toothed whales (sperm whales, beaked whales and dolphins) hunt fish and have only one blow hole while the baleen whales have two blow holes and feed on plankton.
There are a number of different subgroups in this category. The 26 species of oceangoing dolphins are in every ocean except the Arctic and Antarctic. Narwhals are unique in that the males have a tusk. There are 6 species of porpoises and 18 species of beaked cetaceans, at least one of which has never been seen alive and is only known from two washed up skulls. The sperm whales are the largest of toothed whales. Orca (killer whales)and pilot whales consist of 6 species and rorquals, the largest of the whales, are comprised of 6 species.
Because of their physiological adaptations to the marine environment, cetaceans have been able to grow to enormous sizes. A bull elephant, the largest land animal, could stand on a blue whale’s tongue. The biggest dinosaur was possibly 7 m longer but weighed about 70 tons less than that of a large blue whale. Animals that feed on plankton and don’t have to chase their prey, like baleen whales, don’t expend much energy on speed and agility and therefore are able to grow to great sizes.
There are three species of manatees and only one species of dugong, which are the only vegetarian marine mammals. These animals are thought to be distantly related to elephants. They have only one pair of front flippers and no rear limbs.
All mammals are viviparous, meaning that their eggs develop inside the female and the embryo derives nutrition from the mother. Whales and pinnipeds usually mate and give birth in the spring, with pregnancies lasting between 12-18 months. Seals usually have a single pup each year. Whales, however reproduce more slowly and generally raise one calf every 1-3 years. Cetacean calves are born tail first to keep them attached to the placenta as long as possible to avoid oxygen deprivation.
Mating can be a social as well as functional activity with marine mammals. With dolphins, sex is used to establish and maintain bonds among the group. Humpback and beluga whales both take part in group matings.
Marine mammals are greatly influenced by their interactions with humans, either directly or indirectly. Fishing takes the lives of at least hundreds of whales, dolphins, and seals every year that drown when they become tangled in fishing nets. Drift nets meant for fish catch anything that goes by, including dolphins, sharks, sea turtles, seals, seabirds, and other marine life. Pollutants also collect (by a process called biomagnification) in the blubber of these marine mammals and many are also hunted by humans. Seals and sea otters are still sought after for their pelts in some places. Sirenians are exploited for their meat, which apparently tastes like veal, and for their skin and oil-rich blubber.
Whales have had one of the biggest population declines due to human hunting. Whaling can be traced back to Native Americans who hunted gray whales, but large scale whaling didn’t occur until the 1600’s when Europeans began whaling followed soon after by Americans, who dominated worldwide whaling. Large-scale whaling started from open boats where harpoons were used. The blubber was used to make soap and lamp oil and baleen was used as stays for corsets and other garments. Meat and other valuable whale parts were also sold. In the 1800’s fast steamships began to be used for whaling and even the faster whales like the blue and fin whales couldn’t escape the devastating harpoons. The low reproductive potential of these whales caused the whale global populations to be drastically reduced. The first species to be depleted was the right whale, so named because they were the “right” whale to be killed since they floated after being killed. Whaling nations even developed factory ships to process whole carcasses. Blue whales were particularly sought after because there was so much oil, about 9,000 gallons, in an average whale. As the blue and right whale populations began to dwindle, whalers moved to the fin whale and then the smaller sei whale. The disappearance of commercially profitable whales forced the whalers to move to smaller, less profitable whales.
It wasn’t until 1972 that the US issued the Marine Mammal Protection Act, which banned the hunting of all marine mammals in US waters and banned the importation of whale products into the US.
The International Whaling Commission, based in Cambridge, England, was established earlier, in 1946, under the International Convention for the Regulation of Whaling to provide for conservation of whale stocks. The IWC consists of 66 member countries that agree on policies such as: full protection for certain whale species, designation of whale sanctuaries, limits on the numbers and size of whales that may be taken, designation of seasons and areas for whaling, prohibiting the capture of suckling calves and their mothers. The IWC also facilitates whale research and publication of research findings.
In 1986, a moratorium on commercial whaling was enacted by the IWC. Nevertheless, countries such as Iceland, Japan, and Norway continue to take whales above and beyond acceptable catch limits. During the 2005 IWC meeting, Japan announced plans to add humpback whales to its list of target species in spite of the zero quota established by the IWC for humpback whales.
In 1994 the IWC declared a vast sanctuary around Antarctica, the main feeding ground for 80% of the surviving whales, however these waters continue to be exploited by whalers.
Though much progress has been made toward the protection of whales, much more is needed.
Dolphins are quickly replacing the larger whales as the most threatened of all cetaceans. Fisheries are depleting stocks of fish and squid and are therefore hunting dolphins for human food in places such as Peru because it is cheaper than chicken or beef.
Not all interactions between marine mammals and humans are bad, however. Many people believe that encounters with dolphins can be a spiritual experience. It is also believed that they can help children with behavioral disorders. In Southern Brazil a group of fishermen actually work with the dolphins to catch fish by interpreting cues given by the dolphins that reveal the location and abundance of fish; the dolphins are rewarded by the fishermen with an easy catch.
References & Further Research
Castro, Peter, and Huber, Michael E. Marine Biology. 4th ed. McGraw Hill. New York, 2003.
Miller, David. Seals and Sea Lions. Voyageur Press. 1998
Vassili, Papastavrou. Whale. Dorling Kindersley. New York, 1993.
The Whale & Dolphin Conservation Society (WDCS)