Phylum: Chordata (cord)
Subphylum: Vertebrata (animals with backbones)
Class: Mammalia (mammals)
There is little doubt that marine mammals occupy a special place in our view of the animal kingdom. We love, respect, admire and even worship them. And as divers, many of us admire and envy their aquatic skills.
From a taxonomic perspective, marine mammals are described in the subphylum Vertebrata and are members of the class Mammalia. Modern-day marine mammals include whales, dolphins, seals, sea lions, manatees, sea otters and walruses.
Marine mammals are believed to be descendants of creatures that once lived on land, and many species retain remnants of the legs they had in eons past. Like their terrestrial cousins, marine mammals are warm-blooded, breathe air, nurse their young and have hair. As you learned in your scuba certification course, water conducts body heat much faster than air, and that makes being constantly immersed in water a threat to land mammals. Keeping warm to maintain a core body temperature is one of the greatest challenges that marine mammals have had to overcome.
In order to combat the problems that result from being constantly immersed in water, marine mammals have developed comparatively large bodies with proportionately less surface area, and this structure provides an effective means of conserving heat. Whales further combat cold by insulating their bodies with a thick layer of blubber. These layers of fat also serve as a food reserve when times are difficult. Seals and sea lions, animals which are collectively known as pinnipeds (fin-footed), possess a layer of blubber and many species have insulating body hair as well. Sea otters and a group of seals known as fur seals lack a significant fat layer. These mammals rely upon extremely dense pelts, oils in their coats that repel water and energy obtained through their voracious feeding habits to maintain their core temperature. Every day, adult sea otters consume as much as 15 percent of their body weight in food.
Whales and Dolphins
Described in the order Cetacea (Sey-tay-sha), all whales and dolphins are known as cetaceans. The order is divided into two suborders: Mysteceti (Mist-eh-sey-tee), the baleen whales and Odontoceti (Oh-don-toe-sey-tee), the toothed whales.
Also known as the great whales because of their enormity, baleen whales lack teeth. They are filter feeders that strain plankton and small fishes from the water. When feeding, these behemoths commonly swim mouth agape through dense concentrations of krill and schools of small fishes taking in as much food as they possibly can. They close their mouth to expel the water and the food is trapped by tough, horn-like sheets of modified hair known as baleen which looks like the strands of a very large, synthetic toothbrush. Blue whales, the largest creatures on earth, finback whales, humpback whales, California gray whales, sei whales and minke whales are baleen whales. All baleen whales possess a pair of blowholes near the top of the head and through which the animals breathe.
As the name suggests, toothed whales are equipped with large, well-developed teeth. This group includes orcas (killer whales), pilot whales, beaked whales and dolphins, and all have a single blowhole.
While laymen often try to distinguish between whales and dolphins, most scientists do not as both groups are types of toothed whales. The biggest difference between the animals laymen refer to as toothed whales and dolphins is size.
Toothed whales prey primarily upon squids, crustaceans, fishes, including some large sharks, and other mammals. For many years it has been commonly accepted that sperm whales are toothed whales. After all, they have a mouthful of very large teeth that enable them to feed on giant squid and other denizens of the deep. However, recent DNA studies indicate that sperm whales might be more closely related to baleen whales than toothed whales. This fact is not intended to confuse but to illustrate that mankind’s system of categorizing and describing the world of plants and animals is far from perfect.
Though some are seen singly, most cetaceans are highly social animals that live in groups known as pods. Pod size can vary from as few as a handful to several thousand, and occasionally pods join into groups known as superpods. While much remains to be learned about the social structure of cetaceans, it is known that killer whales and other cetaceans are matriarchal societies ruled by the dominant female. Mother-calf bonds are very strong and youngsters in several species commonly nurse from and remain near their mothers for two years. In many species of both whales and dolphins, males escort and protect other members of their pod from predators such as sharks.
Cetaceans are streamlined creatures. Genitals, mammary glands and other organs are internalized to reduce drag. In some species, the flippers that are used for stability can be tucked in to the animal’s side to further increase efficiency. Whales and dolphins share an adaptation in the skin that allows them to rapidly adjust their skin at a cellular level to further reduce drag.
RELATED READ: CETACEAN SIGHTINGS: ALL ABOUT WHALES, DOLPHINS AND PORPOISES
There is no evidence of hind limbs in cetaceans, but the rear third of the body in most species is incredibly powerful and heavily muscled, a design that allows them to push their flattened tails through the water to propel themselves. While most people see cetaceans at the surface of the sea or in relatively shallow depths explored by sport divers, many species possess amazing diving skills. For example, sperm whales have been documented to reach depths as great as 10,000 feet (3,076 m), remaining submerged for as long as 90 minutes while hunting for giant squid and other prey.
As divers, we see more cetaceans from the decks of dive boats than we do underwater. Despite their size and obvious aquatic skills, many cetaceans are skittish around divers. There are, however, plenty of exceptions. Snorkelers commonly have an opportunity to swim with Atlantic spotted dolphins, bottlenose dolphins, Pacific white-sided dolphins and other curious species. And every once in a while it seems like a pod of pilot whales, a curious gray whale or humpback whale swims through a basic certification class.
Seals, Sea Lions and Walruses
Seals and sea lions are called pinnipeds. Their order, Pinnipedia (fin feet) is divided into three families:
- Otariidae (Oh-tah-rye-day) — 14 species of sea lions and fur seals.
- Phocidea (Foe-sid-ee-ah) — 19 species of true or hair seals.
- Odobenidae (Oh-doe-ben-eh-day) — the walrus, a species that occurs only in Arctic waters and is not regularly encountered by sport divers.
Sea lions and seals share many traits. But before you lump sea lions and seals together, remember that specialists are quick to point out significant differences. In fact, many evolutionary experts suggest that seals are actually more closely related to cats and bears than they are to sea lions. It is their belief that cats, bears and seals evolved from a common ancestor while sea lions did not appear until considerably later in the evolutionary process.
Sea lions can be distinguished from seals by noting the following three traits:
- Sea lions possess small, external ears that are lacking in seals. Sea lions are correctly called eared seals.
- The fore flippers of sea lions are relatively large and are used as their chief means of in-water propulsion. They use their rear flippers as a rudder to provide directional control. Seals propel themselves with their more powerful rear flippers. Seals are not able to rotate their rear flippers forward, and as a result are less agile on land than sea lions.
- Sea lions possess a coarse coat while seals have a dense, soft undercoat that is protected by coarse guard hairs.
Most pinnipeds occur in cooler waters where their primary prey of schooling fishes, crustaceans and squids is abundant, and where they are commonly encountered by sport divers. The most well-known sea lion is the species often called the California sea lion, the stars at many marine theme parks. A large California sea lion bull can easily weigh as much as 700 pounds (318 kg) and be 10 feet (3 m) long. Bulls are easily identified by their size and their sagittal crest, an arrow-shaped ridge that extends from their forehead to the rear of their skull. Mature females attain proportions of 250 pounds (112.5 kg) and 8 feet (2.4 m) in length. California sea lions commonly gather with a variety of other pinnipeds including Stellar sea lions, Galapagos sea lions, Galapagos fur seals, Guadeloupe fur seals, northern elephant seals and harbor seals.
RELATED READ: ALL ABOUT SEALS AND SEA LIONS
As large as these California sea lions might seem, they are mere dwarfs compared with southern elephant seals, a species in which males commonly reach a length of 16 feet (4.8 m) and weigh as much as 5,000 pounds (2,250 kg). These champion deep divers have been documented to make a single dive that lasted as long as 60 minutes and to reach depths up to 4,100 feet (1,242 m).
Many pinnipeds such as California sea lions are gregarious throughout the year and almost always prefer to gather in groups. However, there is a lot of intraspecies aggression during the breeding season. Breeding males (bulls) vigorously compete for females or territory, and confrontations between competing males can be violent. When establishing and defending their turf, bulls can be very aggressive toward intruding divers, and bubble blowing, teeth bearing and barking should be taken as warning signs to be heeded.
In all pinnipeds, females bear the primary pup-rearing responsibilities and often newborns depend upon their mothers and nurse for a year or more. Once the pups have acquired some aquatic skills and independence, diving at a sea lion or seal rookery can be as fun as diving gets. Life at the rookery reminds me of recess at an elementary school as games of “tag” and “chase” go on incessantly. These “games of youth” seem innocent, but they also serve as training for the serious business of adulthood when males attempt to establish breeding territories and females must decide who to mate with and who to reject.
Manatees
The name “manatee” is derived from the Greek for “sirens,” the mythical temptresses who lured sailors to their deaths. The name alludes to the fact that ancient mariners supposedly mistook manatees for mermaids. Manatees routinely weigh as much as 1,500 pounds (675 kg) and have patches of algae growing on their skin, giving them a mottled appearance, so I can only assume that if true, these poor sailors had been at sea far too long without shore leave.
Manatees are one of four species of marine mammals described in the order Sirenea. Three live in waters bathed by the Atlantic Ocean while one, the dugong, inhabits pockets of the Indo-Pacific region. All species are known as sea cows. A fifth species, the Stellar’s sea cow, was first discovered near the Bering Sea off Alaska by western man in 1741. Tragically, in only 27 years, this species was hunted to extinction.
The highly publicized and well-known American manatee inhabits the estuaries, mudflats, inlets and lagoons of Florida and several Central American nations. The grayish hued American manatee attains a size of 13 feet (4 m) and 800 pounds (360 kg), but as big as they are, they are considerably smaller than dugongs. Like all sea cows, American manatees spend the vast majority of their lives in shallow, somewhat protected waters and rarely venture into the deeper waters of the open sea.
Manatees breathe air through large nostrils found near the top of the head, a position that allows them access to life-sustaining air with minimum effort. Manatees use their broad, flattened tails for thrust and their flippers to assist in maneuverability. Rather docile animals, manatees often appear inactive, but they have been clocked at speeds as fast as 12 miles per hour (19.3 kph).
Herbivores, manatees feed upon a variety of seagrasses, especially hydrilla, the frilly seagrass despised by so many boaters. Adults consume as much as 100 pounds (45 kg) a day.
The gestation period of manatees is a relatively long 13 months, and mothers nurse calves for as long as two years. The mammary glands of manatees are under the flippers and nursing behavior is commonly observed, and if you happen to encounter a mother with a nursing calf, it is courteous not to get too close and interrupt the feeding. Mature females bear a single calf only once every three years and it is believed that a manatee reaches sexual maturity between the ages of 6 and 10 years.
Sea Otters
Sea otters are members of the weasel family and are much more closely related to terrestrial mammals than other marine mammals. Occurring along the Pacific coast of North America, sea otters range from southern Alaska to northern Baja, Mexico, with the bulk of their population inhabiting a 200-mile (320-km) stretch along the central California coastline.
Sea otters lack the layer of insulating blubber found in most marine mammals, and instead depend upon their very dense coat and voracious eating habits to maintain their core body temperature. In fact, sea otters possess the thickest coat of any animal in the world — with as many as eight million hairs in an area the size of a business card. Including their 12-inch- (30.5-cm-) long tail, males grow to nearly 4.5 feet (1.4 m) long and can weigh as much as 85 pounds (38.25 kg). Females are the smaller sex, attaining proportions of 4 feet (1.2 m) and 60 pounds (27 kg).
Sea otters prey upon a variety of invertebrates including lobsters, crabs, abalone and sea urchins, a diet which has put them at odds with commercial fishermen who pursue many of the same animals. Young otters consume as much as 35 percent of their body weight every day in order to combat the effects of cold water, and adults down as much as 15 percent of their body weight in a single day. When feeding, sea otters can often be seen on the surface using rocks as tools to bash their prey in order to gain access to the meat.
Sea otters are very capable swimmers and divers, and they dive down to catch their prey, but they tend to return to the surface to eat. Although they usually prefer to make relatively shallow dives, they can dive as long as 4 minutes to depths up to 300 feet (91 m).
Face-To-Face With The Biggest of Them All!
Blue whales are well-documented to attain proportions of 110 feet (33.3 m) and 100 tons (90,718 kg). Calculate how many school buses or train cars it takes to equal those dimensions and perhaps you will begin to have some appreciation for the size of a blue whale. But I find it really hard to convey via statistics and words the sizes of big blue whales and how small you will surely feel if and when you swim next to an animal that is more than 1,000 times your size.
Once, while shooting film for the BBC (British Broadcasting Co.) I had the good fortune to spend a few days working in extremely clear water around feeding blue whales. It is an incredible feeling to suddenly realize that even with 100-foot-plus visibility, unless you are positioned at midbody, you might not be able see an entire animal from head to tail.
I recall my amazement when a blue whale swam right past me from less than 20 feet away. First the head passed me, and then the body just kept on coming and coming and coming and coming some more before I finally saw the tail flukes. It seemed like it took forever for the entire animal to move past me.
Moments later I saw a blue whale feeding on krill. The pleated throat expanded in accordion-like fashion until I was certain the mouth could hold several Volkswagen-sized objects. Thank goodness blue whales, like most of the oceans’ larger creatures, feed on tiny planktonic organisms.
HOW MARINE MAMMALS AVOID THE BENDS
Air-breathing scuba divers are susceptible to the bends if they dive too deep for too long or ascend too rapidly. Yet marine mammals, especially some whales and elephant seals, breathe air and then dive to extraordinary depths, sometimes going deeper than 6,540 feet (1,981 m) before ascending rapidly without getting the bends. How is this possible? Although the entire process is not completely understood, scientists do know that:
- As marine mammals descend, their heart rate slows dramatically and they can cut off circulation to nonessential body parts.
- Unlike scuba divers, marine mammals do not breathe in more air during their dives.
- The lungs of marine mammals collapse as they dive, an act that forces air into bony passageways in which nitrogen is not absorbed into bodytissues and as a result, the unabsorbed nitrogen does not enter the blood.
- As the animals ascend, the nitrogen is simply exhaled.
THE PHENOMENON OF ECHOLOCATION
About 20 percent of all marine mammals, a group that includes some whales, dolphins and pinnipeds, assist their well-developed sense of sight with a sophisticated sensory ability known as echolocation that is in some ways similar to radar. Echolocating animals produce sharp sounds of varying frequencies and then analyze the returning echoes to give them information about the size, shape, distance and direction of movement of objects around them. The sounds are generally a series of “pulsing clicks” that are repeated as often as 800 times a second.
Although various species have evolved their own methods of producing and processing the echolocating “clicks,” the principles involved are essentially the same. Toothed whales possess a large, melon-shaped deposit of fat in the front of their head that focuses the clicks into a narrow, directional beam of sound. The reflected sound waves are detected by a different deposit of fat in the lower jaw. This organ sends the sound to the whale’s ear for processing by the brain.