【Editor's Note】Recently, a sperm whale died stranded in the waters of Dalian, and after research and decision of the Dalian Municipal Party Committee and the Dalian Municipal Government, the sperm whale was made into a specimen and stored in the Dalian Natural History Museum.
The topic of the sperm whale instantly became a hot topic, what is this sperm whale, and what puzzles are around it? Dalian Public Cultural Service Center and relevant experts from Dalian Natural History Museum jointly planned the "Whale" Strange World series of activities, and successively launched a series of popular science essays and interactive activities on sperm whales.
In the first popular science lecture hall of the "Whale "Strange World" series of activities, we invited Wu Lei, assistant researcher of Dalian Natural History Museum, and Sun Feng, deputy research librarian, to interpret the echo positioning system of sperm whales.
Many netizens may wonder, how can such a large sperm whale have such small eyes and can see things clearly in the water?
In fact, as the diving champion of the animal kingdom, the sperm whale does not have much use in the dark and deep sea, and the sperm whale relies on an auditory system called "echo positioning" to explore the environment and hunt, and does not rely too much on vision.
Origin of the echolocation function of sperm whales
Sperm whales are toothed whales (a type of whale named after adults with teeth) and are part of a large family of marine mammals. Due to the limitations of the study conditions, no more than 12 species of marine mammals capable of echolocation have been clearly confirmed, all of which are small toothed whales, most of which are dolphins (captive dolphins are more convenient to study).
Based on anatomical studies of the structure of sound emitted and received and an analysis of cetacean recordings, it is reasonable to infer that all toothed whales should have echolocation functions, a gift from a long evolution to sperm whales.
Of all the mammals in the world, only about 20% of species have echolocation functions, most of which are various types of bats.
It may be because the speed of light is much faster than the speed of sound, unless it is to adapt to a dark environment or to solve the problem of self-positioning and target positioning in a visually limited environment, vision may be a more effective positioning scheme.
The marine environment, especially the deep sea environment, is such a visually limited environment.
Compared to the speed of sound propagation in the air of 340 m/s, the speed of sound in water is as high as 1450 m/s to 1550 m/s, depending on the temperature, salinity and pressure of seawater, so many marine mammals use hearing as a primary sense to compensate for the problem of poor vision in water.
The ancestors of cetaceans were land animals whose auditory system was no different from that of ordinary land mammals, with an external auditory canal, eardrum, and ossicles.
The earliest cetacean bucky whales used the same auditory system as land mammals in air, while bone conduction auditory mechanisms were used in water. However, Bucky whales' hearing in the water is very poor in both sensitivity and direction, far from being comparable to today's sperm whales. Later Remington whales and proto whales gradually developed sound transmission systems similar to those of modern whales. Until the modern toothed whale, the function of making and receiving sound was further improved, and a powerful echolocation function evolved.
How sperm whales make and receive sounds
Modern research has shown that the source of sound in the "echolocation" system of toothed whales is a structural complex (MLDB complex) linked to the upper nasal passage, usually defined by the more descriptive term "vocal lip", also using the acronym MLDB.
Toothed whales, with the exception of sperm whales, have two pairs of vocal lips, located on either side of the top of the head, and in front of the vocal lips is the frontal bulge, which is composed of low-density lipids and connective tissue, and has the function of a sound lens. Structures such as the vocal lip and posterior capsule form a vocalization system, which combines with the frontal bulge of the sound conduction structure so that the emitted sound converges and is transmitted to the water.
Sperm whales have larger vocal systems and more complex structures. Its vocal lips are large and, located at the front of the head. Its vocal path is: after sounding by the vocal lip structure, it is transmitted through the cetyl organ to the prefrontal balloon, in which the vocal pulses are reflected forward, and then converge into a bundle through the fat lens array of waste brain oil.
Although there are significant differences in the heads of sperm whales and other toothed whales, the vocal mechanism is homologous. Sperm whale's waste brain oil is homologous to the frontal ridge of other toothed whales, and the whale wax organ of sperm whale is homologous to the right posterior capsule of other toothed whales.
Based on existing research on cetacean behavior, we now know that toothed whales are sensitive to hearing and sensitive to a wide frequency range.
The external auditory canal of ordinary mammals is a sound conduction channel that connects the outer and middle ears, but the external auditory canal of toothed whales is extremely narrow, and the baleen whale is completely blocked, so much so that people debate whether the external auditory canal of whales is functional. So if it doesn't go through the external auditory canal, how do toothed whales like sperm whales hear sounds?
Different scientists have their own conjectures. Norris first proposed the hypothesis in 1964 that the sound-receiving pathway of toothed whales is extraordinary and can be called "jaw hearing".
The posterior part of the mandible of the toothed whale is tilted outwards and is very thin, even translucent. In the mandibular foramen on both sides of the mandible, there is fat directly connected to the middle ear. These fats are low-density sound channels that conduct sound directly from the jaw to the middle ear. Norris speculated that the auditory channel of the toothed whale was that the sound was first received by the extraverted part of the lower jaw and then transmitted directly to the middle ear.
Cranford et al. proposed another auditory pathway in 2008, arguing that sound first enters the head and passes through the throat area before being conducted to fat in the jaw.
The structure of the middle ear of the toothed whale is also peculiar, with the tympanic membrane shrinking to a calcified ligament (the tympanic ligament) at the apex connected to the hammer bone. Numeirah and other scientists first proposed a mechanical model of the middle ear of the toothed whale in 1999: the incoming sound caused the drum bone to vibrate, and the vibration of the drum bone was then transmitted to the oval window and the inner ear liquid through the ossuary bone chain. This model fits well with behavioral audiogram data from some toothed whales. Cranford et al. later refined the model by vibration analysis, by adding soft tissue background analysis. Vibration analysis showed that the interaction of ossicles and other structural parts of the middle ear was more complex than previously reported.
How sperm whales use sound to communicate and hunt
Different species of whales make different signature sounds. For example, dolphins will whistle, humpback whales will make charming "songs", and sperm whales will make clicking sounds.
The clicking sound emitted by the sperm whale can be used for echo localization, which is also a "language" of communication between sperm whales, and each sperm whale also emits its own unique clicking sound, which may be an identification signal for individual sperm whales.
The clicking pulse emitted by the sperm whale is not only high in sound source energy level, but also highly directional, so Berkovich and Yabrokov first proposed the conjecture in 1963: sperm whales can stun prey with sound. This conjecture was subsequently supported by the research work of scientists such as Bozin. Norris and Mohr calculated that sperm whales could emit 265 decibels of clicking pulses, which exceeded the shock wave pressure (presumably 240-250 decibels) needed to stun small fish, but no examples have been observed.
As sonar and anthropogenic noise increase, so does the impact of humans on the marine sound environment. Some scientists (Richardson et al., 1995) have speculated that some whale behavioral anomalies are related to the noise generated by these human activities, but how exactly this affects them is still controversial. There is a need to conduct in-depth research on the vocalization and reception systems of marine mammals such as sperm whales, both to enrich our knowledge and provide benefits such as biomimicry, but also to better manage the relationship between human behavior and marine mammals such as sperm whales.
*Text part reference Marine Mammalology [Beauty] Annalisa Bertha Ocean Press, 2019
*Image:
1. Sperm whale ecological figure 2
http://lifecatalog.ru/images/p/phy/Physeter-macrocephalus--4.jpg
2. Sperm whale ecological figure 1
http://news.cgtn.com/news/3d3d774e786b6a4d30457a6333566d54/img/458f5dfbaaf54c318e3c32870bca6419/458f5dfbaaf54c318e3c32870bca6419.jpg
3. Schematic diagram of the dolphin's head Quoted from Marine Mammalology [Beauty] Annalisa Bertha Ocean Press, 2019
4. Cross-sectional diagram of sperm whale vocal system Quoted from Marine Mammalology [Beauty] Annalisa Bertha Ocean Press, 2019