University of Vermont Neuroscience Graduate Student, Alisha Linton, writes:
“So long and thanks for all the fish.” In “A Hitchhikers Guide to the Galaxy,” Douglas Adams depicts dolphins as the most intelligent species on Earth. So intelligent, that they are aware of the impending destruction of the Earth while humans are not. They even kindly try to warn humans of this impending destruction by doing things such as balancing balls on their noses and jumping through hoops. But sadly humans are not capable of translating these messages, and rather see them as endearing and entertaining.
While this is clearly satirical commentary and not factual representation, it does bring to light humans’ fascination with dolphins and their capacity for intelligence. They are one of the most interesting and complex creatures in the sea. They have a learning capacity greater than most other mammalian species, and they have an aptitude for communication and language equal to or greater than apes. They have been the subject of extensive studies on learning and memory, social structure, and behavior. While it is clear that dolphins are not as sentient as humans, the research shows that they do possess a remarkably large capacity for intelligence.
Part 1. Structure: Sensory Systems and Neuroanatomy
Dolphins are marine mammals and thus have developed special sensory systems to interpret and understand the world around them. Sight is hindered underwater due to the refractive nature of water; so instead, dolphins use echolocation to “see.” They use their sense of echolocation to locate prey, sense objects in their way, and communicate with other dolphins. They produce two basic types of sounds: narrow band-frequency modulated whistles, which are used primarily for communication, and broad band pulsed sounds in trains or clicks that they use for echolocation and communication. They produce these sounds through a structure called the melon. The melon is a fatty substance that sits anterior to the frontal bone of the skull and superior to the upper mandible. It is surrounded by thin muscles that are homologous to human facial muscles. These muscles are innervated by cranial nerve 7, and contraction of these muscles focuses and modulates sounds through the melon. The initial sound production is created by forcing air through the trachea, and the vibrations travels up to air sacs below the blow hole, and is then projected out through the melon.
Functionally, dolphin echolocation works just like bat echolocation, in that they send out pulses of sound, those pulses bounce off of objects in space, and then return to the dolphin as an echo. The dolphin can tell the location and distance of an object in space based on the time delay between the original sound and the echo. While the phenomenon of echolocation is very interesting in and of itself, the way the dolphin’s anatomy has adapted to receive the sounds is equally important.
Dolphin ears are specially designed to perceive sounds through water. Because there does not need to be an air-water interface, dolphins have no ear canal. Vibrations are collected through the jaw bone and surrounding fatty tissue and transmitted back to the inner ear, which is located at the very back of the lower mandible. The inner ear is completely encased in bone, called the auditory bullae, and is connected to the skull by fibrous connective tissue. The bulla contains the the tympanic membrane, inner ear bones and the cochlea. Cranial nerve 8, the auditory nerve, transmits information from the cochlea to the brain. The cochlear nerve in the dolphin has approximately twice as many nerves as that of the human, more than 67,000 compared to humans 30,000 (1). The reason for this increase in fiber density is probably the expansive hearing range of dolphins. While humans only hear between 20 Hz and 20 kHz, dolphins hear frequencies ranging from 1-150 kHz. This means that they can hear much higher frequency sounds than we can, much like a dog. Their ears are designed to hear such a range of high frequency sounds because echolocation utilizes these high frequency sounds. High frequency sounds are preferentially used for echolocation because high frequency sounds travel better in water and they are smaller and thus give more detail of objects.
To mirror this uniquely designed auditory system, dolphins have the specialized neuroanatomy to match (all neuroanatomy information from citation 2). Their cortex is of a comparable size to that of primates, with the auditory cortex being the largest functional area. The somatosensory (SCtx) and motor cortices (MCtx) are more anterior than that of the human, and the frontal lobe is relatively small. Generally, all auditory structures are increased in size as compared to humans, while many other structures are decreased in size due to their lack of importance. For instance, the olfactory bulb, tract, and cortex are non-existent. These structures disappear during embryonic development, as smell is not very useful in an underwater environment. On the other hand, the cochlear nerve (8c), trapezoid body (TB), ventral cochlear nucleus (VCN) and the inferior colliculus (IC), all structures associated with auditory processing, are hypertrophied. Additionally, cetaceans (which includes baleen whales and toothed whales, Figure 4) possess a structure called the elliptic nucleus (E), which is thought to be a hypertrophied nucleus of Darkschewitsch. It receives input from the striatum (Str), inferior colliculus, and posterior interposed nucleus of the cerebellum (PIN), and projects to the inferior olivary nucleus (IO). This nucleus appears to be involved in coordinating body movements with auditory input.
Above is a diagram of the known functional areas in the dolphin brain including the ones listed above. A few other interesting points of neuroanatomy are the size of the corpus callosum (cc), anterior commissure (ac), posterior commissure (pc) and the cerebellum (cb). The corpus callosum, anterior commissure and posterior commissure, structures that connect the right and left hemispheres, are relatively small and thin. This allows them to perform actions such as unihemispheric sleep. By putting only one hemisphere to sleep at a time, dolphins can maintain breathing and buoyancy in the water while sleeping. They would likely drown if this were not possible because they must control their breathing at all times. Additionally, the small size of the anterior commissure may also be due to the lack of the olfactory system. The anterior commissure in humans conveys olfactory information bilaterally, but since dolphins do not have an olfactory system, those fibers do not exist. Lastly, the cerebellum is relatively large as compared to the size of the cortex. This is probably due to the reliance on conveying auditory information to body movements, which is one of the jobs of the cerebellum. These are just some of the major specializations of the dolphin’s neuroanatomy
Part 2. Function: Communication and Behavior
Dolphins have one of the largest encephalization quotients (brain to body weight ratio) in the entire animal kingdom, second only to humans. This fact suggests to some that dolphins are among the smartest creatures in the world. Dolphins have proven themselves to possess above average intelligence through their behaviors. These behaviors include complex social interaction and social structures, development and use of communication, an extensive auditory memory, and ability for self-recognition.
Dolphins live within social structures called fission-fusion societies where individuals primarily associate with small groups, but the exact composition of these groups can be dynamic overtime. There have been only a few pioneering studies on this topic in the bottlenose dolphin, as they require extensive time and resources (Sarasota Bay, FL: 3; Shark Bay, Western Australia: 4; Moray Firth, Scotland: 5). From the analysis of these various communities, we know that females are variable in their associations (some living in close pods while other are loners), and males tend to form very strong associations and bonds that last for a very long time. Making and maintaining these social associations is crucial for tasks such as finding food, protection from predators and rearing young (6). In order to maintain connection over such a dynamic social structure dolphins have developed specific ways to communicate that allow them to maintain social bonds over time.
To identify himself or herself, each dolphin develops their own “signature whistle,” which is equivalent to a name. Caldwell and Caldwell were the first researchers to identify the signature whistle as something distinct to the individual (7,8). The development and function of these whistles is perhaps the most significant indication of intelligence.
First of all, these whistles are thought to develop through vocal learning. Vocal learning is the ability to reproduce sounds heard from others, and comprehend their specific function. It is an ability only seen in humans, great apes and some avian species. The signature whistle is developed by the end of the first or second year of life and appears to be modeled on the calls of community members (9, 10). Additionally, the stages of whistle development by calves follow similar stages as human and avian voice development. There is evidence of overproduction of whistles, vocal play and attrition (11). A study by Fripp et al. set out to determine to what degree vocal learning impacts signature whistle development. They followed 6 dolphin calves and their moms within the Sarasota dolphin community off the coast of Florida (~140 dolphins). They found that the signature whistles of the calves most closely matched whistles of other dolphins within the community that they rarely associated with (were within 50 meters of <5% of the time), and tended to not resemble the whistle of the mother. Additionally, the signature whistles of the calves were more similar to the whistles of Sarasota dolphins than of Tampa dolphins. From this study it is clear that dolphins tend to develop whistles by listening to other community members, indicating an importance for social context in vocal learning.
Vocal learning is also an important part of song birds learning their species specific song. Studies raising zebra finches in isolation found that they still develop a song, but it is not the same as their wild cousins’ species song (12). Thus, there is a clear genetic link to developing a song, but vocal learning and culture cues are required to learn the “correct song.” There have not been studies that raised dolphins in isolation to see if they would develop a song on their own, but given the importance of social vocal learning, one could hypothesize that they may not develop one unique signature whistle. When there is no societal pressure to differentiate yourself from others, why would you develop your own song?
Secondly, through studying the signature whistle it is clear that dolphins have remarkable auditory memory. These signature whistles have been shown to actually convey identity information that is remembered by other dolphins. Janik et. al. found that dolphins are more likely to turn their heads toward the whistle of a kin as opposed to a non-kin (13). This indicates that not only do signature whistles contain identity information, but that identity information is used by a receiver to determine how much they should react to said whistle. Furthermore, this memory can last for decades. A study by Jason Bruck found that dolphins react strongly to signature whistles of individuals they used to know, even after 15 years of separation (14). This evidence of long term memory supports the pattern that long term memory is crucial for large-brained, socially-oriented species. Ape species such as chimpanzees, and elephants are also thought to have long term social memory. All of these species exist in complex social hierarchies, therefore the development of a communication system as well as long term memory for this communication system is crucial. Moreover, those two traits are key in defining these species as “intelligent”, because we define an “intelligent species” as possessing close to human traits.
Lastly, the idea of mirror-self recognition is thought to be a sign of complex cognition, and thus intelligence. Humans and apes are the only other species shown to develop this ability. It is believed that the ability to recognize oneself in the mirror means you have the ability to tell the difference between your reflection and another individual. The classic way to test this is to put a mark on the creature somewhere, place them in front of the mirror, and see if they notice the mark on their body. In a study by Reiss and Marino (15) marks are placed on dolphins who had been raised in captivity and they are given a reflective surface. It was found that the dolphins spent more time in front of a reflective surface when there was a mark on them as opposed to no mark. Additionally, they were faster to get to the reflective surface after a mark was put on them. So, this study proved that these dolphins not only have a sense of self-recognition, but that they also enjoyed playing in front of the mirror. This study was the first to show a non-primate having the ability for self-recognition. This indicates that mirror self-recognition is probably a result of increased encephalization and neocortical expansion (aka cognitive complexity) as opposed to just a capacity of primates.
In conclusion, dolphins are complex creatures that have been extensively studied for decades. From the extensive research on dolphins it doesn’t seem as if we have gained much perspective of ourselves as a species, but we have learned a lot about how they are both similar and different from us and primates. It is clear that they possess a level of intelligence equal to great apes and more than almost all other marine mammals. The the common thread between us, sentient primates, true primates, and underwater mammals is a large brain to body mass index. Given that, perhaps the key to intelligence is simply more cortical gray matter.
We as humans seem to be addicted to studying creatures with traits similar to our own. Is it because by studying and understanding these animals we gain traction to understand ourselves, or is it because we fear losing our position as the superior species? Personally I would put money on the first explanation, but the truth has yet to be elucidated.
Author: Alisha Linton, UVM Neuroscience Graduate Student
Figure 1) Wikipedia. File: Toothed whale sound production. Web. https://commons.wikimedia.org/w/index.php?curid=511683. April 13, 2016.
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