Marine mammals feeding
How do marine mammals use or make sound when feeding?
Marine mammals use sound to aid them in finding and capturing food. They do this by producing sounds that are reflected back when they strike an object. This is called echolocation. Echolocation is important to marine mammals because it allows them to navigate and feed in the dark at night and in deep or murky water where it is not easy to see. Some porpoises may also use echolocation clicks for communication as well as for finding food and navigation. It has also been reported that marine mammals produce other sounds (such as slapping their flippers) separately or in conjunction with echolocation to help concentrate large quantities of prey or stun their prey before capture. At the present time only toothed whales, including beluga whale, sperm whale, dolphins, and porpoises, have been shown to use echolocation during feeding, but it is thought that other groups of marine mammals may have the potential to use echolocation during feeding too. Echolocation is typically used by toothed whales to capture single prey items such as fishes or squid. Bats are the only other mammals known to use echolocation for feeding.
Echolocation used in feeding is different from sounds used in communication. Sounds used for communication provide animals with information about other animals in the area. (See How do marine mammals communicate using sound?)). The signals produced by animals during echolocation provide the animal with information about what is in the environment. Toothed whales that use echolocation send high frequency click sounds into the environment. The sounds then bounce off distant objects, and the echoes are received by the animal that produced them. The animal that produced the original echolocation clicks can determine how far away an object is based on the time an echo takes to return. The farther away the object is, the longer it takes for the echo to return.
As an echolocating animal gets closer to its targeted prey item, the rate at which it produces clicks gets faster and faster. The series of echolocation clicks leading up to a capture attempt of a prey item is called a click train. As the interval between the clicks gets shorter, the click train starts to sound like a buzz.
Click either choice below to hear the spinner dolphin echolocation clicks:
Sound courtesy of Marc Lammers. Released under Creative Commons License, non-commercial attribution.
The returning echoes sound different than the original click produced by the animal. The differences between the sound of the original click and the returning echo provide the echolocating animal with information about the size, shape, orientation, direction, speed, and even composition of the object. Dolphins have an amazing ability to detect and identify a target the size of a golf ball at a distance of 100 meters ( more than the length of a football field). The beam of the echolocation clicks is also very directional and can be moved with a slight turn of the animal's head.
Toothed whales and baleen whales produce other sounds to increase their chances of success during feeding. Humpback whale have developed a feeding technique called bubble feeding. Bubble feeding involves one or a few whales blowing air from their blowhole while underwater. This produces sound as the bubbles form a cloud, curtain, or column that rises toward the surface. The bubbles trap the prey between the surface and the whales mouth. A bubble net is formed when the bubbles emitted by the whales form a ring and concentrate the prey inside. Both the sound and the bubbles work to concentrate prey so the humpback can capture more food per mouthful.
Bottlenose dolphin also make use of sound and bubbles. Dolphins foraging in seagrass beds in Australia and Florida use a technique called kerplunking to drive fishes from the protection of the sea grasses. A dolphin will lift its tail and lower body out of the water and crash it down on the water surface. This causes a loud splash and creates a trail of bubbles under the water. This startles the fishes hiding in the seagrass and flushes them from their hiding places, making it easier for the dolphin to detect them.
The spectrogram below is an example of a kerplunk from a foraging dolphin in Australia. Listen to the sound while watching the spectrogram. You first hear a series of echolocation clicks that sound like a buzz. The kerplunk happens at about 2.5 seconds and sounds like a deep splash. This is then followed by more echolocation as the dolphin scans for fishes driven from the seagrass.
Click either choice below to hear the kerplunk sound:
Courtesy of Connor, Heithaus, Berggren and Miksis.
Lastly, there is the simple mechanism of listening for sound produced by prey items. Some marine mammal species may use this technique to find prey. Fishes make a variety of sounds that cetaceans and pinnipeds may detect. Transient killer whale sit and quietly listen for sounds of other marine mammals in the area before they make their attack. In this instance, it benefits the killer whale to be as quiet as possible while listening so it doesn't scare away its prey. Listening for prey sounds is a mechanism that is also used by seals and sea lions. Once the seal or sea lion detects a fish by sounds, it waits for it to get close enough to use its sensitive whiskers to track the path of the fish as it tries to escape.