The Dolphin Brain
by Kenneth W. LeVasseur
Lire aussi :
Dolphin Mental Abilities Paper
Cetaceans have brains that range in size from approximately 200 grams for the smaller species (Ganges River dolphin etc.) to over 9000 grams for the largest brain on the planet (the sperm whale).
The bottlenosed dolphin has a brain ranging between 1200 and 2000 grams.
Human brains have ranged in size from 1000 to 1800 grams.
A chimpanzee’s brain is about 350 grams and a gorilla’s brain is about 600 grams.
Jerison (1973) explains that cetacean ancestors had large brains as long as 15-20 million years ago.
Homo sapiens’ large brain goes back 100,000 years.
Ridgway (1986) makes the point that many odontocete cetaceans of similar size have grossly divergent brain sizes.
For example, Orcinus, 5.55 meters long has a brain 5617.7grams and Ziphius, 5.49 meters long has a brain 2004 grams, slightly more than one-third the mass of the orca brain.
The same type of discrepancy is seen among primates.
Soviet dolphin researcher A. G. Tomlin in 1968 pointed out that dolphin can react very quickly to acoustic signals — more quickly than humans do — most probably because, in the underwater environment, sound travels about 4.5 times faster than it does in air.
On this side of the Pacific, Ridgway (1986, p. 53) has confirmed the dolphins fast neural processing of acoustic signals :
» Dolphin auditory transmission is fast, not only in the cochlea but in the CNS as well. . . . Brain stem transmission time (BTT) is considerably faster in Tursiops Truncatus (a dolphin species with mean brain weight around 1500 g) than in humans or domestic cats and is similar to that of the rat. Despite a much longer nerve pathway, BTT in the dolphin is equal to or faster than that in much smaller brained species. »
Aside from the speed of processing mentioned by Tomlin and Ridgway, Thompson and Herman (1977) and Herman (1980) document the dolphin’s excellent short-term memory for acoustic signals.
But hearing is not the only sense dolphins have that appears to process information quickly.
Their vision system is also unusually fast.
Dawson (1986) points out that among marine mammals in general :
« . . . A major deviation is seen as the development of an elaborate giant-cell system in layers 8 and 9 (the ganglion cell and fiber layer) of the retina. The giant-cell system and smaller cells in this traditionally ‘simple’ layer may reach its greatest development in the marine dolphins . . . . But a most striking aspect of this system is the large size of each element.
Each of the great cells generates an axon of large dimensions, which continues in the optic nerve ‘cable.’ . . . it is well established that conduction velocity along neurons is directly proportional to the cross-section diameter and to the extent of axonal myelination.
Consequently the giant-fiber system seemingly provides the marine mammal with an uncommonly rapid communication pathway with the central nervous system. »
Unlike the dolphin’s hearing, which is fast and demonstrate excellent short-term memory, this fast vision system appear to be tied closely to movement.
Herman 1980,1986, points out the difficulty dolphins have with static (motionless) visual discrimination tasks that do not allow echolocation or other forms of acoustic bridging.
Apparently, the dolphin throughout its evolution has had such little experience with static objects in its environment that there are few if any direct connections between its static imaging system and its associative areas and thus access to memory storage.
The evolution of cetaceans from ungulate/mesonychid stock is undoubtedly showing its effect here.
Modern ungulates are well known for their insensitivity to static objects, something hunters and bullfighters capitalize upon.
Herman 1980, 1986 speculates that acoustic bridging can tap into existing associative areas involved with processing sounds, thus developing a pathway for processing static visual information.
This makes sense when one considers the environment exploited by the dolphin.
Echolocation allows the perception of distant static objects at better than 150 feet, much sooner than a dolphin can see them (rarely does underwater visibility reach 150 feet), therefore once a dolphin usually sees a static object it has already been encoded through its excellent hearing system.
Once objects start moving, though, the dynamic visual experience has very good cognitive characteristics.
Here, there appear to be direct connections between the dynamic sensations of the vision system and the associative areas of the brain.
Interestingly enough because the dolphin is almost always moving, the images seen by the dolphin are also moving images and tap into the dynamic vision system with its connections to associative areas of the brain.
There certainly has been very little evolutionary pressure to develop connections for static vision with associative areas of the brain, but that may have changed.
In 1990, Herman takes another tack by using his gestural language experiments to show that the dolphins dynamic visual system is indeed rich in associative connections.
Apparently, Herman’s earlier writings, 1980, 1986, were more ambiguous than he led us to believe.
In 1990 he says :
« The remaining question is whether the dolphin’s visual system can support the cognitive function as well as the biological function.
An early view of dolphins, which I held at one time, was that the visual system primarily was constrained to serving the biological function (Herman, 1980). »
« In the remainder of this chapter, I will try toconvince the reader that this view is incorrect.
I will review several areas of study from our laboratory, studies which grow progressively more complex in character, and which have convinced me of the power of the visual system as a source of information for complex cognitive behaviors and mental processes. . .. »
The paper goes on to show that Herman is retracting his contention that the visual system somehow connects with the acoustic system, which has demonstrated connections with the associative areas of its brain, in a cross modality paradigm.
He is now contending that the dolphin has a vision system with its own direct connections to visual associative areas.
He does not make a distinction between a static visual system and a dynamic visual system, yet this distinction could be at the heart of the problem he is trying to resolve.
It may be that the dolphin has an inability to comprehend static visual images, like the ungulates from which they evolved, yet it can overcome this problem by assimilating static images through an associatively rich dynamic visual system rather than the acoustic system.
This problem with static objects may play a part in the dolphin’s reaction to nets, in particular purse seine tuna nets.
Nets are static devices, which stop the normally dynamic dolphins and confine them.
Because nets are hard to echolocate at distance and some monofilament nets are hard to detect up close, being caught in a net enclosure deprives dolphins of the movement so important for their only operational sense, their vision, to function competently.
All the dolphin perceives, visually and acoustically, is the kind of dolphin movement and sound indicative of social panic. Their echolocation system may not even perceive the net and their visual experience is not connecting to areas of the brain that might initiate an adequate response. The dolphins appear to be caught in a sensory dystopia
where nothing makes « sense ».
Of course in tuna nets, the dolphins have been totally disoriented by the speedboats and seal bombs prior to encountering the net, but the circumstances after they are set upon do not offer relief.
Ridgway (1986) has also observed how the growth of the Tursiops brain compares to the growth of the human brain.
With a 3 month longer gestation period of 12 months, Tursiops are born with a brain 42.5% of the adult brain mass (possibly at an age of 9 or 10 years – Ridgway’s estimate).
Humans are born with a brain 25% of the adult mass (at an age of 17 or 18 years – Ridgway’s data).
He says : « . . . By weaning time at 18 months, Atlantic coastal Tursiops have brains over 80% of adult mean weight, a stage not reached by human brains until year three or four. . . . »
Although Ridgway acknowledges that the longer gestation and possibly the heavier brain weight at birth might be needed to swim and engage in activities, he does not mention that the human brain at birth must be small enough to allow the head to pass through the birth canal.
Nor does he attempt to define adult.
Many cetacean species, especially the larger species, are like elephants in that they never stop growing, so a definition of adult should not be size dependent.
It might be better for the definition of adult (at least for research purposes) to be the latest marker comparable between species — the age when an individual has the ability to reproduce.
Under these different interpretations, the development of dolphins and humans to maturity may be closer than Ridgway would have us believe and thus more comparable, though possibly not directly comparable.
The principle that Ridgway is trying to document is that dolphins are between non-human primates and man in how long it takes for the brain to grow to full size.
But the comparison only does a good job in showing the profound effects that getting a large brain through such a small orifice (in order to be born) can have on the physical development of the recipient to maturity, in this case the development of humans.
This is particularly true, when the development is compared to large brained species without such a limited passage.
On the other hand, the younger achievement of maturity is most likely a reproductive advantage that propagates large brains more often and enables the development of even larger brains.
It may be that this evolutionary advantage takes many million years to develop — something that would be outside the human experience.
Morgane and Jacobs (1972), Jacobs (1974), and Bunnel (1974) point out that cetaceans have evolved a new part of the mammalian brain, called the paralimbic lobe, which is not seen in terrestrial mammals.
This extra lobe is thought to be projection area for the senses; and it is thought that this projection phenomenon performs a sensory integration function (Jacobs, 1974).
Pilleri and Busnel (1969) report that the brain layers in primates and dolphins are the most highly differentiate of all mammals.
New research (Morgane, Jacobs and Galaburda (1986) with Golgi stains of cetacean cortical tissues have show less differentiation of these tissues in cetaceans than previously thought.
This unique pattern of differentiation has been considered by Morgane, Jacobs and Galaburda (1986) to be representative of prototype mammals and is referred to as « archaic ».
But since all three orders of mammals mentioned as retaining these structures have been shown to have echolocating animals in them, this label may be too superficial.
Of the cetaceans, bats and hedgehogs (cited as having this type of archaic brain laminations), only the hedgehogs have not been shown to be echolocators, but in the hedgehog order, insectivore, some shrews and tenrecs have been shown to use echolocation Gould et al., 1964; Gould, 1965).
In 1968, Poduschka stated that hedgehogs could hear sounds higher in frequency than 24 kHz.
Sales and Pye (1974) found that hedgehogs could hear frequencies as high as 84 kHz with their optimum at 20kHz.
Since hedgehogs have been shown to have the excellent hearing needed for echolocation and live in an environment where shrews and tenrecs have demonstrated the possibility of echolocation,
testing hedgehogs for echolocation is in order.
Should the hedgehogs be shown to exploit sound in an echolocative manner, the behavior would be consistent with its brain type as demonstrated by microbats and dolphins.
With the exception of some of the river dolphins, hearing and vision are the major sensory modalities of dolphins.
The suggestions above that these senses process signals much faster than terrestrial mammals may explain the difference in brain laminations seen in the cetacean brain.
Each layer of differentiation contains various nerve cell bodies, which have axons and dendrites that take part in passing and processing neural signals by connecting with other axons and dendrites through synapses.
Whereas axons and dendrites pass signals electrically, synapses process signals chemically and thus at a slower rate subject to chemical balance.
The more nerve cells of any type, their layering, and thus the greater differentiation, means that processing the signals as they pass between the layers is slowing down the transmission of these neural impulses.
It soon becomes obvious that, the less differentiation, the faster the brain.
Because the hedgehog, bat and dolphin may all be echolocators and all have excellent hearing, especially in the higher frequencies, their brains may not have evolved the differentiations because the earlier and faster brain better served their ecological needs.
Thus, although their brains may be « archaic », this state may have been maintained due to sensory needs and a capability not seen in other land mammals.
This of course assumes that these « archaic » cells have not been evolving for millions of years.
The term archaic itself may be ill advised because it could be that the cells that make up the layers of the laminations may have evolved since the era of the proto-mammal.
Although the bodies of bats, hedgehogs and many cetaceans may not have changed much since that era, the observation does not mean that the cells of their brains have not changed because the brain cells themselves do not fossilize.
Instead, the cell types in these layers of the brain should be investigated for speed of processing neural information.
Regarding this phenomenon, Stephan, Baron and Frahm, in their 1991 book Insectovora (the first volume of the series Comparative Brain Research in Mammals) state on page 311 in conclusion of the section on adaptive radiation that,
» . . . there is no parallel between the evolutionary level of the somatic structure and the brain.
Species with large number of conservative traits may have a very progressive brain, while species with many derived features may be at a low level with respect to brain evolution.
« . . . Although the fact of mosaic evolution is well established, sparse quantitative data on detail brain structure have limited our understanding of the various phylogenetic changes occurring in Insectivora.
It appears now beyond any doubt that somatic and brain evolution can progress at different rates.
A species with a conservative ‘Bauplan’ may reach a high level of brain evolution, while a specialized and progressive species may be characterized by a very conservative brain structure. » This opinion leaves the question about the development of neural tissue with regard to body form in hedgehogs, bats and cetaceans open again, but
from a more informed position.
Testing the Neural Development Theory
An interesting test of this alternative hypothesis would look at the brains of the two groups of bats in the order Chiroptera, the Microchiroptera (microbats) and the Megachiroptera (megabats).
While all microbats are nocturnal, known to echolocate using high frequency sounds, have small eyes, large ears and often have specialized nasal outgrowths and eat insects; megabats are fruit eaters and lack the large ears and nasal specializations.
Only the megabat Rousettus is known to echolocate and it uses very low frequencies produced by the tongue.
The microbats were already well developed echolocators 50 million years ago when they first appear in the fossil record and
appear to have evolved from the same order as hedgehogs, the order Insectivora.
In contrast the first fossil megabats are only 20 million years old, appear to have primitive wing formation and may have evolved from a form of flying lemur, a primate.
Indeed the megabat vision system turned up about 30 features that were thought to be unique to primates and the whole issue is now steeped in controversy.
The opportunity presented here is to compare brain laminations between the microbats and the megabats and the compare these results with brain laminations in the echolocating megabat Rousettus.
Support for the proposed hypothesis would show increasing laminations from the microbats to the Rousettus and the most differentiation in the non-echolocating megabats.
Other results, though, may tell a different story.
In his last paragraph, Ridgway (1986) expresses his thoughts on what this fast auditory sound system and brain of the dolphin are for,
« . . . In this paper I have reviewed a number of findings, for example, the large diameter of the auditory nerve fibers, the short latency and the relatively flat latency-intensity function of the dolphin ABR waves, and the rapid temporal resolution of successive sounds, that lend support to my view that much of the great hypertrophy of the dolphin auditory system — and perhaps the entire cerebrum — results from the animal’s need for great precision and speed in processing sound. »
Although this conclusion is perfectly reasonable, is it the answer or part of the mystery?
After all, a great many dolphins are poor echolocators, indeed many of the dolphins in the most famous echolocation experiments, had to be trained at some or all of the echolocation tasks.
This training may have been upgrading a dormant sensory system, developing an unused sensory system or the result of capturing baby dolphins for research and display; the question has not been answered.
The implication is that, if the dolphin acoustic system and possibly the brain are for processing echolocation signals and these dolphins are not processing echolocation signals on their own (but
only in response to experimental requirements), then what are they doing with all that brain.
Is the circumstance similar to humans and the speech centers in their brains when for some reason a human is not taught to speak?
Or is the answer more in tune with those who feel that large brains develop to provide the individual with social advantages.
Both primates and cetaceans are generally highly social and many arguments have been presented in support of the theory that social needs stimulate brain growth (Hobos, 1986; c.f. Beer, 1986).
In this theory, as applied to dolphins, the social needs generate a brain growth that the acoustic system can then use for echolocation once it is dictated by circumstances.
Dolphins are highly acoustic animals (indeed it is their primary sensory modality).
Because sound travels 4.5 times faster underwater than it does in air, and because the dolphin’s vision system is geared to process visual information at or near the speed of their acoustic system; the respective brain structures would have to show the streamlining of evolution in enabling the required speed.
If that process keeps earlier « archaic’ structures and possibly refines them more to make them faster, then that demonstrates one of the advantages of evolution — when structures and systems that solve environmental problems (allowing advantage for the individual) work, they do not change unless required to meet further demands.
A theory that explains the dolphin’s large brain may be: the structure and quality of the dolphin’s brain enables complex social systems to exist in the aquatic medium due to the speed that sound travels underwater.
The large brain allows the social system to function smoothly enough to evolve because it is advantageous for the individuals in the gene pool.
The relationship between brains and behavior is nowhere near being completely understood but headway is being made, although rarely fast enough to keep up with the speculation it generates.
Fortunately, this speculation is good for the creative process required to solve the puzzles presented by new discoveries.
As each behavioral relationship to the brain is hypothesized, new research can be designed to fill the gaps through verification.
It is this process of verification, which keeps the speculation grounded so that relationships can be quantified and qualified, that provides more knowledge for further speculation.
Problems occur when speculation exceeds the
technological ability for verification or when it involves points of view that, although verifiable, involve areas of study where human politics, economics and opinion fear such verification.
In these cases verification may not be pursued in order to perpetuate the required illusion.
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