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Marine mammals have evolved throughout time to overcome the challenges caused by living in am aquatic habitat. 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 of this paper, contains two sub-orders: Sub-order Odontoceti is the toothed whales (dolphins and porpoises), and sub-order Mysticeti includes all of the baleen whales (Bert 1). These orders of marine mammals are all polyphyly, which means that they evolved separately from one another, yet have adapted to their shared environment in several similar ways (Feldhamer 47). Marine mammals characteristics that have evolved include a large body size, streamline shape, insulation of blubber and/or fur, reduction of the size of external appendages, and many adaptations for diving and orientation. The sirenians and the cetaceans are the only mammals that are entirely aquatic (Feldhamer 272). Thus their adaptations have more fully evolved than that of the carnivores, who still 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 were extinguished from the earth, 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 mammals from terrestrial, to archaeocete, to the baleen and toothed whales is quite extraordinary.
Cetaceans have evolved in response to 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, which slows down the mammal. Humans are very slow swimmers because of 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 the most efficient 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 a reduction in surface area and a smooth body surface. Being such large mammals creates a low surface to volume area. 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 ears) 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. As a matter of fact, some dolphins have a turn over rate of epidermal cells that is as high 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. Having a more streamlined, fusiform body shape can reduce this wake, and in turn, reduce drag (Reynolds 185).
At the surface, wave drag is caused by the production of waves that slow the mammals down. Wave drag reaches a maximum at a depth of half body diameter. To reduce this, marine mammals have adapted their behavior. They will swim away from the surface while submerged, and while near the surface they “porpoise”, which is common to witness while on a boat when you see dolphins swimming with your boat continually jumping out of the water (Reynolds 185). This helps prevent the mammals from coming into too much direct contact with the surface waves.
Another adaptation that has made locomotion easier for the cetaceans is their method of propelling 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. Movement to water changed the mode to quadrupedal paddling, then to alternate pelvic paddling, simultaneous pelvic paddling, undulation, and finally caudal oscillation. Undulation no longer uses just the limbs for movement, but movement now includes the use of the spinal column. The most advanced propulsion method, caudal oscillation, is a lift-based propulsion that increases thrust, efficiency, power, and speed (Berta 173-215).
Now that the cetaceans can swim and more easily move under water, it is important to understand how it is that they are able to dive so long and so deep. The two main problems in adaptation are oxygen conservation and tolerance of 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 they are, the more oxygen they can store per kilogram of body weight. 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 water for so long. However, the exact opposite is true. A mammal in the ocean actually wants a low lung volume to avoid nitrogen exchange (which will be discussed later). 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 planned activity (Walker 1083). Bradycardia is a term used for slowing down heart rate. Marine mammals have exquisite control over their cardiovascular systems, and some cetaceans can actually slow their heart rates to 5% of the pre dive rate. The longer the anticipated dive, the slower the heart will beat. Even though they can hold a larger amount of oxygen in storage, marine mammals have adapted peripheral vasoconstriction to further conserve oxygen to stay submerged 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 withstand hypoxia. While diving, these tissues receive a constant and steady supply of oxygen. If a dive is longer than 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 are more likely to make multiple short dives instead of one long deep dive (Berta 225,239).
For every ten meters of depth under water, pressure increases by one atmosphere (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 have marine mammals adapted to withstand this pressure increase? They have the ability 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 blood. Actually, some carnivores (seals) will even 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 a mammal living in water can conducted heat towards or away from the body 25 times faster than a terrestrial mammal can. So, how exactly does a marine mammal warm itself or cool off when needed? The most obvious answer is the insulation value of the blubber and fur on the animal. 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. In addition, 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 near 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, the testes are intraabdominal creating a more stream-lined shape. However, the core temperature of the body is too warm for spermatozoa to survive. The testes are actually located in a cavity that is surrounded by locomotor muscles which creates a very warm temperature in that region of the body. To keep cool, returning blood from the superficial veins of the flukes and flippers is shunted to the testes. 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 environment. 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 object into the mandible of the odontocete (Feldhamer 279). Not only can objects be detected, but they also can be discriminated from one another. In other words, not only can odontocetes detect whether a fish or dolphin is in the distance, but they also can “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 have made 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. However, the numerous adaptations made to solve the problems encountered by land mammals that have returned to the sea are quite remarkable. When you sit back and think just how difficult a process it would be for us to adjust to aquatic life, it is amazing how many adaptations these mammals have made.
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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