Food and Predators
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Habitat Food and Predators Social Spacing Mating System Photos Links References Home |
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What do dolphins eat? Bottlenose dolphins eat a wide variety of fishes, squids, and crustaceans depending on their location. They eat approximately 4% to 5% of their body weight in food per day, meaning they spend a great deal of time searching for food and feeding. Dolphins have very large home ranges in which they roam to find food. Often times they are specific plentiful areas that they will return to to feed. If food sources become scarce dolphins will migrate to a new location with a more stablef ood source. Dolphins will feed together or alone; however, feeding generally seems to be behavior that is associated with the pod. Often times dolphins will surround a school of fish and then take turns darting through the middle of the school to feed (Barros and Odell, 1990). Dolphin pod behavior is explained further in the social spacing section of the website. At times dolphin herds participate in whole-herd searching for food. The herds surround fish using synchronous movements and then individually feed. In addition to whole-herd searches, searching can be conducted by the smaller individual groups of dolphins. This type of searching allows the dolphins to spread out and cover much wider areas. Also, dolphin scouts often break from the group and search for fish. When one dolphin locates a fish and moves towards it, usually the rest of the herd approaches that individual immediately and general feeding begins. The actual technique of catching the fish involved a "carousel" pattern that involves the dolphin swimming around and encircling a group of fish. This feeding behavior demonstrates at least one reason why group formation is important to bottlenose dolphins (Bel'kovich, 1991).
Figure 1: A school of fish being hearded by dolphins (Image Courtesy of http://www.charliephillipsimages.co.uk)
What are the natural predators of dolphins? Large sharks and killer whales are the only natural predators of dolphins. Tiger sharks, bull sharks, and great white sharks are the main species that attack dolphins. Humans have also posed a major threat to dolphins in recent years(in Japan the dolphin population was almost completely dessimated during the 1980's and 1990's).Predation risk may have been an important factor that led to the evolution of group living in dolphins. Furthermore, risk of shark predation appears to influence both habitat use and group sizes of dolphins in a variety of locations. In Sarasota Florida 31% of bottlenose dolphins bear bite scars while 36.6% of bottlenose dolphins in Moreton Bay Australia bear bite scars. These are very significant numbers, and could have easily spawned a group defense system among dolphin populations. Interestingly enough, Heithaus and Lawrence found that coastal dolphins exhibited more bite scars than pelagic dolphins. This seems to contradict theories that coastal dolphins remain safer from predators because of the shallow water. Also, few age and sex differences were noted in scarring rates. This implies that all dolphins are equally vulnerable to attack. Some scientists believe that shark attacks on dolphins are simply acts of aggressive behavior because dolphins and sharks utilize the same food sources. This has not been proven however, and more conclusive research needs to be carried out (Heithaus and Dill, 2002).
Figure 2: Great white shark (image courtesy of www.sullacresta.it)
How do dolphins defend themselves against predators? Dolphin calves are the most vulnerable to predators. Dolphin pods serve as a means of predatory defense because there are more eyes to look out for predators. Keen eyesight, speed, and agility make predator detection and escape easier for dolphins. Although the dolphins most common response to predators is flight, the bottlenose dolphin will sometimes exhibit aggressive defensive behavior against sharks. They do this by battering the soft underbelly of the shark with their snouts. This can result in killing the shark. Bottlenose dolphins also use whistles and clicks to warn other dolphins of predator presence.
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