Lesson Five: Dolphin Adaptations
Adaptations are the behavioral or physical characteristics of an organism that allow it to survive and thrive in its environment.
Typically categorized into behaviors, body parts, and body coverings, the adaptations of an animal are critical to their survival. Think of some adaptations that humans have to effectively survive in our environment, like opposable thumbs and lactose tolerance! Marine mammals have a wide variety of adaptations to thrive in their marine environments.
Thermoregulation
Dolphins are able to regulate their body temperatures, just like humans and other mammals. This is an important adaptation for dolphins, since they dive deep in the ocean to catch their prey, and water is an excellent conductor of heat, removing heat from the body faster than in air. Dolphins have a higher metabolic rate than other animals to generate body heat, and they have blubber – or a thick layer of fat – which insulates the dolphin. Typically, arteries carry warm blood through the body's core, whereas veins contain cooled blood from extremities (like fingers and toes in humans). In the dolphins' tail flukes, flippers, and dorsal fins, arteries are surrounded by veins. Some of the heat traveling through the arteries is transferred into the veins rather than into the environment, which allows them to conserve body heat. In cold water, dolphins are able to pull blood from their extremities (their tail, fluke, and flippers) and increase the circulation around the dolphins' core where their vital organs lie (heart, lungs, etc.). Dolphins also can easily overheat. In fact, when dolphins are out of water, their skin must constantly be kept wet to prevent overheating. This is why you may have seen a stranded mammal on the beach being covered with wet towels by marine mammal rescue teams – to keep the animal cool.
Echolocation
Dolphins use echolocation to hunt, communicate, and navigate. Among marine mammals, echolocation is unique to toothed whales (odontocetes). Bats and some bird species also have the ability to echolocate. Dolphins have a very sophisticated sonar system that allows them to perceive their surroundings underwater even in very low visibility conditions. They can even use it to locate fish that are buried under the sediment on the ocean floor. Scientists do not actually know for certain how the clicks in a dolphin are generated, since no one has been able to observe an animal echolocating on land. One theory is that the animal sends out a series of "clicks", generated from the nasal sacs in their forehead. The melon, or the 'bump' on top of a dolphin's forehead, is a fat-filled sac that not only amplifies the clicks generated by the dolphin, but focuses the clicks into a single, specific direction.
Other groups of scientists, however, believe that the sound is emitted by the larynx, and the focusing of the echolocation is achieved by sound bouncing off various areas of the skull. Either way, each of these clicks produced by the dolphin will travel until it encounters an object, in which an echo will return back to the dolphin and is received through their hollow, fat-filled lower jaw. (Imagine having your ears on your chin!). These clicks are produced in a series, which is called a click train, as more than one click-echo pair is needed for a dolphin to make an image. The dolphin hearing range is much greater than most mammals. High-frequency hearing is important for echolocation. It has been shown that dolphins are able to determine if an object is metal, glass, or rock and can even tell the difference between shapes!
Buoyancy Control and Deep Diving
Buoyancy is the force of a liquid, pushing upwards against an object submerged in it. Blubber is generally less dense than water, so the dolphin's layer of blubber helps them remain buoyant in the water.
Uncompressed Lungs
Compressed Lungs at 300 meters
Dolphins also have adaptations to dive much deeper than most mammals. For example, when humans go SCUBA diving, they are at risk of developing "the bends", or decompression sickness. This is caused when rapid changes in pressure (like coming to the surface too quickly after diving deep for long periods of time) cause gas bubbles to form in the bloodstream. (Don't worry, this is a rare occurrence and, when done correctly, SCUBA diving can be fun and safe!) Dolphins have very large lungs and dive very deeply very quickly, so why do they not experience decompression sickness like people can? Dr. Ridgway discovered that dolphins have collapsible lungs! Have you ever been swimming and try to push a beach ball or a pool float to the bottom? It's really difficult to do, right? That resistance you feel is the buoyant force. Now if you picture dolphins lung and chest cavity as a beach ball, the dolphin is able to deflate that beach ball and collapse its entire chest cavity as it dives, and then re-inflate and re-expand their chest cavity as they come back to the surface – the deeper the dolphin dives, the smaller the volume of air in their lungs gets. Some other marine mammal adaptations to diving include:
90% of air in the lungs is exchanged with every breath
High blood volume compared with terrestrial animals
High oxygen retention due to increased red blood cell count, increased hemoglobin and myoglobin, and large spleen
High tolerance for lactic acid and carbon dioxide (lactic acid is what makes our muscles feel sore!)
Collapsible lungs, limited nitrogen absorption
Hydrodynamic body
Low heat loss because they have blubber to insulate their bodies and small surface area
Countercurrent circulation in extremities
Unihemispheric Sleep
Humans exhibit "unconscious sleep"; we are not aware of our surroundings when we sleep and have a breathing reflex. Even if we become unconscious, we breathe automatically. Dolphins, however, are not able to breathe automatically; it is consciously controlled [2]. Therefore, if they were to go completely unconscious when they sleep, like people do, dolphins would stop breathing and drown. Dolphins have adapted to hemispheric sleep, meaning they only allow half of their brain to sleep at a time while the other half stays alert to continue breathing and be on the lookout for potential dangers in the environment.