Living in the Ocean: Adaptations of Cetaceans and other Marine Mammals Final Draft

This discussion topic submitted by Stephanie Fridman ( Steph10034@aol.com) at 10:54 am on 6/9/00. Additions were last made on Tuesday, June 4, 2002.

Marine mammals have evolved throughout time to be able to withstand problems caused by being aquatic mammals. There are three orders of marine mammals that are extant. Order Carnivora includes sea otters, seals, sea lions, and walrus. Manatees and dugongs are included in the order Sirenia. Order Cetacea, which will be the center of this paper, contains two sub-orders. Sub-order Odontoceti is the toothed whales (dolphins and porpoises) and Mysticeti is all of the baleen whales (Berta, 1). The separate orders of marine mammals are all polyphyly. This means that they all evolved separately of one another and still adapted by in several similar ways (Feldhamer, 47). Characteristics that marine mammals have which have evolved are a large body size, streamlining shape, insulation of blubber and/or fur, reduction of the size of external appendages, and many adaptations for diving and orientation. Along with the sirenians, the cetaceans are the only other group of mammals that are entirely aquatic (Feldhamer, 272). This is why their adaptations have evolved to a higher level than that of the carnivores, who come to land to mate and give birth.
It is believed that the cetaceans are related to quadruped terrestrial ungulates such as the cows and pigs (Coffey, 12). After the dinosaurs died out, ancient carnivores were believed to re-enter the water and evolve into cetaceans (brown, 5). The early aquatic form of the cetaceans is known as sub-order Archaeoceti. Archaeoceti are ancient whales with features that were intermediate during the transition from primitive terrestrial mammal to fully aquatic whales (Feldhamer, 292). This transition of the marine mammal from archaeocete to the baleen and toothed whales is quite extraordinary and will be explored in this paper.
Cetaceans have evolved to deal with problems caused by swimming and locomotion, diving, thermoregulation, and orientation in the ocean. Swimming in the water is a much easier process when there is a reduction of drag to slow the mammal down. Humans are very slow swimmers because of all the drag created by our arms, legs, hair, and other external body parts. Viscous or frictional drag, pressure drag, and wave drag all need to be reduced to allow for a faster and easier swim. Viscous drag is caused by water flowing across the body surface. The effects of this type of drag are felt in what is known as the boundary layer, the layer where the body interferes with the water and therefore fluid is moving more slowly. The bottom line is, the more surface area to the mammal, the more boundary layer and the higher the viscous drag. Physical adaptations for reducing boundary layer are to reduce surface area and have a smooth body surface. Surface area is lowered first because the marine mammals are such large mammals. On average, marine mammals have a surface area that is 23% less than other mammals. A smooth body surface is obtained by having no hind limbs; instead the cetaceans have caudal flukes. There are no pinnae (external ear) and no external genitalia found on the whale or dolphin. They have adapted to have a smooth integument and a loss or modification of hair. Cetaceans also have a certain pattern of blubber deposition to make for a more smooth body surface. As a matter of fact, some dolphins have a turn over rate of epidermal cells that is as low as every two hours (Reynolds, 185)!
Pressure drag is usually greater than viscous drag and is caused by a disruption of flow around the body. When swimming, the cetacean creates a low-pressure wake at the rear of the body. Creating a more streamlined, fusiform body shape can reduce this wake, and in turn, reduce drag (Reynolds, 185).
At the surface, the production of waves caused wave drag. Wave drag reaches a maximum at a depth of half body diameter. To reduce this, marine mammals have adoptions in behavior. They will swim away from the surface while submerged, and while near the surface they “porpoise” (Reynolds, 185). It is common to witness porpoising out on a boat when you see dolphins swimming with your boat continually jumping out of the water.
Another adaptation that has made locomotion easier for the cetaceans is how they propel themselves through the water. During the evolution of cetaceans, propulsion has changed from drag-based to lift-based. As land mammals, the locomotor mode used was called terrestrial quadruped. The movement to water changed the mode to quadrupedal paddling, then to alternate pelvic paddling, simultaneous pelvic paddling, undulation (no longer are just the limbs used for movement, but movement now includes the use of the spinal column), and finally caudal oscillation. Caudal oscillation is a lift-based propulsion that increases thrust, efficiency, power, and speed (Berta, 173-215).
Now that the cetaceans can swim and move more easily under water, it is important to understand how it is that they are able to dive so long and so deep in the water. The two main problems to adapt to are oxygen conservation and dealing with the extremely high pressures at depth. Oxygen must be conserved in the body for long periods of time during diving. This is accomplished by bradycardia, peripheral vasoconstriction, and increased oxygen storage. An advantage to being large mammals is that the bigger you are, the more oxygen you can store per kilogram of body weight, and also the amount of oxygen used is not proportional to weight. It is often thought that marine mammals must have an extremely large lung volume to be able to stay under the water for so long. However, the exact opposite is true. As a mammal in the ocean, you actually want a low lung volume. So, to compensate, the mammals store huge amounts of oxygen in their blood and also some in the muscles. Up to 80-90% of oxygen supply utilized during prolonged diving is stored attached to hemoglobin and myoglobin. However, the amount of oxygen that is stored in the lungs, muscle, or blood is variable and dependent on the order of marine mammal and their specific activity (Walker, 1083). Bradycardia is a term used for slowing down your heart rate. Marine mammals have huge control over their cardiovascular systems and some cetaceans can actually slow their heart rates to 5% of the pre dive rate. The longer the dive is planned, the slower the heart will beat. Even though there is a larger amount of oxygen storage, marine mammals have adapted peripheral vasoconstriction to conserve oxygen for even longer periods of time. Peripheral vasoconstriction reduces oxygen going to non-critical tissues. These tissues are able to endure what is known as hypoxia and are adapted for low oxygen supply. The brain and the heart are critical tissues not able to handle hypoxia and still while diving receive a constant and steady supply of oxygen. If a dive is longer then the amount of oxygen conserved for the dive, then lactic acid will build up. Aerobic dive limit (ADL) is the maximum breathold possible without an increase of lactic acid concentration. To achieve ADL, cetaceans and other marine mammals have a large body size and make multiple short dives instead of long deep dives (Berta, 225,239).
For every ten meters of depth under water, pressure increases by one ATM (Feldhamer, 276). A huge problem at depth is atmospheric nitrogen, which under pressure dissolves into the blood and causes a condition known as the “bends” (Coffey, 41). So, how is it that marine mammals handle this pressure increase? They have adapted to collapse their thoracic cavity, lungs, and alveolar sacs. Cetaceans actually have very weak and flexible rib cages. When diving, the thoracic cavity is collapsed so no air can get in. When this collapse occurs, there is still air (including high nitrogen levels) in the alveolar sac, which is the site of gas exchange. Marine mammals have adapted to this by creating a cartilage build up in the bronchioles. This allows for alveolar collapse and storage of the air in the bronchioles. This is important because nitrogen is no longer at the site of gas exchange and cannot be absorbed into the body. Actually, some carnivores (seals) go even further than that and will exhale respiratory gases before they dive to ensure no nitrogen absorption in the body (Reynolds, 38).
Water is 25 times more conductive than air. This means that living in water, heat can be conducted towards or away from the body 25 times faster then a terrestrial mammal. So, how exactly does a marine mammal keep warm or cool off when needed? The most obvious answer is all of the blubber and fur on the animals. Mammals in the ocean actually decrease the thermal conductance of their integument. Also, being such a large size, the favorable surface-to-volume ratio slows the loss or gain of heat (Feldhamer, 274-275). However, this alone does not quite do the job. Marine mammals have adapted a system known as a counter-current heat exchange. They have a central artery and circumarterial veins known as the PAVR system that is used to keep the blood warm, by keeping it neat the center of the body. When the mammals get slightly warm, to cool off the blood they have an extra set of veins known as the superficial veins. This venous system runs near the surface and is located mainly in the flukes, flippers, and other appendages and brings the cooled blood back to the body core (Berta, 147).
In cetaceans, to obtain a more streamlined shape, the testis are intraabdominal. The core temperature of the body is too warm for spermatozoa to survive. The testies are actually located in a cavity that is surrounded by locomotor muscles and generate a lot of heat while swimming. To keep cool, returning blood from the superficial veins of the flukes and flippers is shunted to the testis. This is also true for the uterus in female cetaceans due to the sensitivity of the fetus to heat change (Berta, 149).
Odontocetes, toothed whales, have an extra adaptation for orientation in the water called echolocation. Echolocation is transmitting sound and receiving echoes from objects in the environments. The three main processes in echolocation are sound production, sound reception, and signal processing. The process of how odontocetes interpret signals is not known. In a very general explanation, toothed whales can focus emitted sound in a single direction through the “melon” in their forehead, “bounce” it off of an object in the distance, and receive signals from the objects into the mandible of the odontocete (Feldhamer, 279). Not only can objects be detected, but they can also be discriminated from one another. In other words, not only can odontocetes tell weather a fish or dolphin is in the distance, but they can also “look inside” the object (Reynolds, 291). This was first discovered when it was found porpoises could avoid and sometimes even escape from nets (Scheffer, 71-72).
The adaptations cetaceans and other marine mammals make to the ocean have evolved over hundreds of thousands of years. As humans, we take for granted that whales, dolphins, manatees, and seals just live in the ocean. But when you sit back and think just how difficult a process it would be to adjust to aquatic life, it is amazing how many adaptations these mammals have made.
References
Berta, A., and J. L. Sumich. 1999. Marine Mammals Evolutionary Biology. Academic Press, London.

Brown, L. N. 1991. Sea Mammals: Atlantic, Gulf, and Caribbean. Windward Publishing, Inc., Miami, Fl.

Coffey, D. J. 1977. Dolphins, Whales, and Porpoises: An Excyclopedia of Sea Mammals. Collier Books, New York.

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy: Adaptation, Diversity, and Ecology. McGraw-Hill Company, USA.

Reynolds, J. E., and S. A. Rommel. 1999. Biology of Marine Mammals. Smithsonian Institution Press, Washington and London.

Scheffer, V.B. 1976. A Natural History of Marine Mammals. Charles Scribner’s Sons, New York.

Walker, E. P., F. Warnick, S. E. Hamlet, K. I. Lange, M. A. Davis, H. E. Uible, and P. F. Wright. 1968. Mammals of the World: Second Edition. The Johns Hopkins Press, Baltimore.


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