Marine mammals feeding
How do marine mammals use or make sound when feeding?
Marine mammals use sound in a variety of ways while feeding. One well-known example is echolocation, in which animals produce short pulses of sounds that are reflected back when they strike an object. Animals can use information in the returning echo to learn about their environment, such as the distance to potential prey items. The farther away the object is, the longer it takes for the echo to return. However, both low and high frequency sounds have been associated with specific feeding behaviors.
Echolocating for prey
As an echolocating animal gets closer to its targeted prey item, the rate at which it produces clicks gets faster and faster. A series of echolocation clicks is called a click train. As the interval between the clicks gets shorter leading up to a capture attempt of a prey item, the click train starts to sound like a buzz.
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 soccer field). The beam of the echolocation clicks is also very directional and can be controlled with a slight turn of the animal’s head.
Blainville’s beaked whales are silent near the surface, but are known to produce sounds at depth. These deep-diving whales produce mid-frequency broadband sounds during the decent phase of their foraging dives. These sounds range in underwater dB, are different than echolocation signals, are produced before the onset of foraging clicks, and prior to the deepest point in a dive. Scientists have determined that Blainville’s beaked whales actually produce two distinct types of foraging clicks: search clicks and buzz clicks. Each of the click types occurs during a different phase of the foraging dive. Search clicks are emitted during the approach phase, at intervals of 0.2-0.4 s and a frequency range of 26 to 51 kHz. Since strong echoes are received from these clicks, scientists believe they function to enhance prey detection and classification. When the target is about one body length away (2-5 m), the whale switches to buzz clicks, short bursts of sound at a frequency of 25-80 kHz or higher. Buzz clicks are highly repetitive, and scientists estimate that the whales may click 300 or more times in the last 3 m of their approach to the target.
Coordinated feeding activities
Toothed whales and baleen whales produce a variety of other sounds associated with feeding. Humpback whales display a wide variety feeding strategies. They also possess a broad acoustic repertoire, and scientists have discovered a variety of acoustic signals associated with different feeding behaviors. In the western North Atlantic Ocean, humpback whales produce “paired burst” sounds when engaged in bottom-feeding behaviors and other humpback whales were nearby. The sounds were short (<0.25 s), broadband pulses with peak energy below 1 kHz. Tagging data show whales in this area feed on sand lances living in the seafloor, making flat-bottom dives with multiple rolls in coordination with one another.
Scientists in Southeast Alaska have observed large groups of humpback whales producing sounds at depth, preceding group lunge-feeding. In many of the events observed, whales were silent at the surface before the feeding event. When members of the group dive to feed, they start to produce low-level vocalizations. Over the course of the dive, the level of vocalizations increases, culminating in loud “feeding cries” , and then, almost immediately, a large group of whales lunge to the surface, with mouths agape, in a coordinated manner. Feeding cries range in duration from 0.4 to 8.2 s, with a short, frequency-modulated, introductory and ending component. The main body of the call ranges in fundamental frequency from 360-988 Hz.
Humpback whales have developed another lunge-feeding technique called “bubble netting”. With this method, animals swim in an upward spiral or loop, while simultaneously blowing a ring of bubbles underwater. The bubbles startle the fish and cause them to densely aggregate, which allows them to be more efficiently captured. Bubbles may be produced in continuous streams or short bursts, and create sounds associated with the bubble generation, and explosive bursts as they rise to the water surface and break. It is not clear whether it is the sound or the sight of the bubbles that startle the fish.
Acoustic signals may also play a role in synchronizing cooperative feeding events observed in Hawaiian spinner dolphins. These dolphins forage offshore at night in highly coordinated groups. Using a multibeam echosounder to track animal movements, scientists have observed the dolphins moving together in strict geometrical patterns, with tight timing, to herd prey (small fish, shrimp, and squid) into a very dense patch. Pairs of dolphins then “take turns” feeding within the prey aggregation. Clicks consistent with echolocation have been recorded and correlated with the timing and shape of foraging stages in spinner dolphins. The clicks were produced most often when the animals were changing their geometric formation (e.g. from a tight line into a circle). Scientists hypothesize the dolphins may use the clicks during foraging transitions to gain information about the prey field or the changing positions of other members in the group.
Other feeding strategies
Bottlenose dolphins 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 their sea grass habitats. A dolphin will lift its tail and lower body out of the water and slap it 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 visually and acoustically.
The spectrogram below is an example of a kerplunk from a foraging dolphin in Australia. The example starts with 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.
Scientists have found dolphins in Florida to produce “pop” sounds as an additional means to flush fishes from sea grass beds. Both pops and kerplunks produce low-frequency sound energy that can startle fish. Pops differ from echolocation clicks in that they have more energy at lower frequencies, are longer in duration, and are not produced in trains.
Bottlenose dolphins in Scotland also appear to use sound to startle and/or disorientate their prey. When feeding on salmon, these dolphins produce “brays”, a type of burst-pulsed, low frequency (< 2 kHz) call. Scientists believe the brays stun the fish. The calls may also advertise a feeding event to other dolphins and coordinate their activities.
Passively listening for prey
Another foraging strategy is listening for sound produced by other animals. Transient killer whales cue into the vocalizations of their marine mammal prey. Fishes also make a variety of sounds that both cetaceans and pinnipeds can hear. Seals and sea lions may initially detect fishes by listening for them, and then use their sensitive whiskers to track the path of the fishes as they try to escape. Foraging seals can even hone into signals produced by acoustic tags placed on/inside fish, the “dinner bell” effect.
Additional Links on DOSITS
- Baleen Whales
- Beaked Whales
- Beluga Whale
- Bottlenose Dolphin
- Humpback Whale
- Killer Whale
- Marine mammal communication
- Multibeam Echosounder
- Sperm Whale
- National Geographic Crittercam. 2013, Video Reveals Surprising Humpback Feeding Behavior. (video shows bottom-feeding by humpback whales)
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