Dolphins keep a complex social network which includes a few close associates (such as mothers and calves or pair-bonded individuals) and some more casual relationships within a larger group. Dolphins hunt together for food, pods coordinate their movements to herd prey. They have a very acute sense of hearing which they use to communicate with one another, listen out for predators or threats in the area and find food.
Why are we concentrating only on dolphins in this post? Because generally, sounds of baleen whales are very different from those of odontocetes, with a wide range of types and quantity of phonation.
Hearing and vocal sounds
Sound travels 4.5 – 5 times faster through water than through air, also it carries much better underwater than light. For this reason dolphins rely on sound for communication with their environment and other individuals more than any other mode. We distinguish two categories of sounds dolphins produce:
- pure or single tones:
- whistles (low frequencies, 0.25-50 kHz): used for mutual communication. Each individual has its own recognizable whistle, which contains information about identity, location and emotional state;
- pulsed sounds:
- echolocation clicks (high frequencies about 40–130 kHz): used for sonar purposes, such as orientation, detecting and following prey. A dolphin can determine very accurately of its prey’s size and location. It has been observed that most toothed whales echolocate by producing a click and then listening for echoes from surrounding targets to return before producing the next one. Echolocation is not a form of communication, but rather a method of seeing the world through sound. By listening to the information coming back, dolphins get a mental image of objects in their environment;
- impulsive sounds or burst pulses, which are clicks in high frequencies (human ears detect them as a connected sound), tend to be produced during social interactions.
Shorter wavelengths have better spatial resolution, hence high frequencies are better suitable for detecting small objects than are low frequencies, and are thus used in echolocation. Whereas high frequencies attenuate rapidly and do not carry very far.
Both mysticetes and delphinid odontocetes have a vocal fold structure in their larynx. These folds may be used in the production of high-pitched whistles. Echolocation sounds are produced in nasal passages, in air spaces between the lungs and the blowhole and are directed forward through the melon (fat tissue organ). Returning sounds are received through fat tissue in the lower jaw. Sound is then translated to inner ear and from there to the brain.
Different whistles are produced in different situations: during social situations, when separated from the group, when excited, happy or panicked. Each individual dolphin produces its own signature whistle – a stable unique whistle structure that a dolphin seems to develop during the first year of its life and it remains stable for many years. Recent studies have demonstrated that common bottlenose dolphins spread over distances of hundreds of meters to kilometers may remain in acoustic contact with one another through whistles. Dolphins appear to be able to produce not just their own signature whistle, but also the signature whistle of other individuals they are in close relationships with.
Dolphins also produce a number of non-vocal sounds for communication. Non-vocal in this sense means any sound not produced using the organs within a dolphin’s vocal area.
- Tail slaps: hitting water surface with the flukes produces extensive, low-frequency underwater (and aerial) sound that can transmit great distances in the water. It may mean many things – for example, a signal to leave an area. But most observers agree that tail slaps often convey a threat or distress. –> I witnessed this myself when we were monitoring a group of bottlenose dolphins (Tursiops truncatus) for means of photoidentification. The group was obviously feeding and after carefully following them for an hour, maybe less, one of them started hitting the water surface with its fluke extensively. In that moment no translation was needed, we knew immediately we weren’t welcome anymore. We carefully took the rest of the photos we needed and left their area.
- Flipper slaps: slapping the pectoral fins to sound. Dolphins may slap their flippers on the surface of the water, or onto their own body (e.g., their belly), this way they produce sounds under water that likely carry a communicative message.
- Percussive sounds of jaw claps and teeth gnashing: most likely a sign of aggression.
- Breaches: many cetacean species leap vigorously into the air which includes a part of or the entire body leaving the water before crashing back into the surface. A breach produces underwater sounds upon reentry that carry for several kilometers. Breaching may occur for a variety of reasons, most likely it is a communicative signal to help cetaceans remain in acoustic contact. Spinner dolphins produce dramatic spinning leaps which also produce loud sounds upon re-entry, this aerial behavior seems designed to produce noise and given that many of these leaps are performed at night, its primary purpose is very likely the generation of noise.
- Bubbles: dolphins often blow bubble streams under water in a variety of social situations, and while these are also visual signals, the production of a large bubble cloud mainly produces a distinctive noise that can likely be heard over short distances.
Dolphins do not generally rely on visual communication, although they have excellent eyesight both above and below the water’s surface. There are many natural and synthetic particles floating in the water column that make it difficult for dolphins to communicate visually over long distances. However, when close to each other, body language does play a role in communication and dolphins also communicate by touch. Calves swim close to their mothers, brushing their bodies, which is also a behavior observed in individuals that maintain close relationships. Dolphins also use touch in rough, aggressive ways during courtship and when establishing dominance. They use their teeth to scratch and/or hurt each other’s skin.
One very important and disturbing factor in sea life communication is man-made sounds – noise. Unfortunately, industrialized world has created many sources of noise underwater. It is caused by:
- activities that produce underwater sounds incidentally, not purposefully: traffic, motorized shipping, construction, infrastructure in the sea, offshore drilling, underwater blasting, constructing or operation of power generating devices;
- several other types of underwater sounds created purposefully: sonars of many types operating at frequencies ranging from very high to low, sonars for navigation, air gun pulses for discovering oil and natural gas sources, pingers used to locate objects underwater and to alert marine mammals to the presence of fishing nets, electronic acoustic harassment devices to keep marine mammals away from mariculture facilities and widely varying sounds used for ocean science measurements over short and long distances, seismic research, military experiments.
Fish and marine mammals have evolved with the rich physical and biological cacophony of nature and are presumably well adapted to those sounds. Most anthropogenic sounds first appeared in the past about 100 years ago and in some parts of the world are increasing in intensity and geographical extent year by year. Most marine mammals rely on underwater sound for communicating, finding prey, avoiding predators and navigating. Other senses are available to them, but sound is the most important one at distances or in environments where the senses of touch, taste, and sight are not available.
Anthropogenic sounds can result in a variety of effects whose consequences to marine mammals can range from nil to severe, depending on the type and received level of the sound:
- tolerance: no obvious responses, continuing activities without moving away. However, responses can be subtle so a detailed behavioral and physiological study is needed before it is legitimate to conclude that there is no overt response;
- changes in behavior or activity: common when marine mammals are exposed to human-made sounds. Sometimes the effects are subtle, detectable only by detailed observation and statistical analysis, e.g., changes in surfacing/respiration/dive cycles. More conspicuous effects include changes in activity, e.g., from resting or feeding to alert, facing toward the noise source, and so on;
- avoidance reactions: exposure to strong man-made sounds can disable effective communication and food search or other “normal” activities and it makes the animals swim away;
- hearing impairment: animals (including humans) exposed to strong sounds can incur a reduction in their hearing sensitivity. This impairment is often temporary if the sound levels are not too high or too prolonged. However, repeated exposure to strong sounds, or a single brief exposure to an extremely strong sound (e.g., nearby explosion), can cause permanent hearing impairment with loss of orientation and possibly stranding, resulting in death;
- nonauditory physiological effects and stress: exposure to very strong underwater sounds can cause resonances in lung cavities and other types of nonauditory physiological effects. Chronic exposure to strong noise also causes stress reactions.
As everywhere, human presence is (too) strong and aggressive in the oceans and seas. Since sounds are transmitted much faster and further through water, man-made sounds pollute this world in large scales. There are ways to lower this type of pollution – starting with each individual, ourselves. Be realistic, be humble – how much energy and how many physical things do you really need to live a healthy quality life? You already know the answer – by far not as much as we use (I’m looking at myself here as well). Together we can lower our environmental impact. Nature is our home, let’s keep it healthy, safe and clean.
Encyclopedia of marine mammals
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