Relative Brain Size
Absolute brain size is important, but it can’t be the sole measure of intelligence. Small-bodied creatures such as birds have minute brains, especially compared to the much bigger ones of large-bodied creatures such as elephants. Yet in many respects birds act “smarter” than elephants, probably because the former have a lot less body to monitor and control. In fact, much of an elephant’s large brain consists of motor cortex—enormous numbers of dedicated neurons enabling those huge hulks to put one leg in front of the other without tripping. Hence the reason why most neurobiologists take as a better measure of intelligence a comparison of brain and body sizes.
Ratios of brain-to-body mass for many animals having similar overall stature show a clear separation of reptiles from mammals. Figure 7.13 shows this comparison, noting that a constant brain-to-body-mass ratio would display a diagonal line from lower left to upper right. For any given body mass, mammals consistently have higher brain mass, usually 10-100 times larger than those of modern reptiles of comparable size. Likewise, the brain masses of our prehuman ancestors (the early primates) also were greater, relative to body mass, than those of all other mammals.
The creature having the largest brain-to-body-mass ratio is Homo sapiens, namely, ~0.022. Dolphins come next (~0.016, which is also the value for H. habilis), followed by the apes, especially the chimpanzees (~0.006). The human brain is about as big as the genes can currently make it and still be safely delivered during childbirth—3 or 4 times bigger, relative to body weight, than the brains of our closest relatives, the great apes. These are data, not sociological sentiments.
Brain-to-body mass ratios then provide a useful index of the intellectual capacities among a range of animals. The systematically different ratios of Figure 7.13 virtually prove that the evolution of mammals from reptiles ~200 million years ago was accompanied by a major increase in relative brain size and intelligence. These ratios furthermore show that additional neural evolution paralleled the later emergence of human-like creatures from the rest of the mammals a few million years ago.
How Smart Are the Dolphins?
More than any property, the brain most clearly distinguishes humans from other life on Earth. The development of speech, the invention of technology, and the rise of civilization are all products of the human brain’s rapid advancement. But what about other forms of life? Are there creatures on our planet today with comparable intelligence—animals having neural capacities enabling them to communicate, act socially, or make tools?
Brain-to-body mass ratios imply that, apart from humans, dolphins (Figure 7.14) are the smartest animals now on Earth. As a numerical measure of intelligence, their just-noted brain/body ratio (~0.016) matches that of archaic humans of ~2 million years ago and exceeds that of the australopithecines of 3-4 million years ago. Laboratory tests do imply that dolphin intelligence, to the extent that it can be realistically gauged, does lie somewhere between that of humans and chimpanzees. Biologically, dolphin evolution seems not too different from ours, yet culturally they are far behind us, perhaps because they live in the water.
Dolphins were not always aquatic creatures. Along with whales and porpoises, dolphins are members of a family of mammals whose ancestors were once land-dwelling. Owing to keen competition among many 4-legged amphibians ~50 million years ago, the dolphins’ ancestors returned to the sea, possibly either in search of food or because land niches were becoming too crowded. Some disadvantages would have undoubtedly accompanied such a seemingly backward move, but that ancestral decision—really an adaptation to change—probably saved them from extinction.
Dolphins, as we know them today, are well adapted to the sea. Their exceptionally strong bodies are streamlined for deep diving and speedy locomotion. They have extraordinary hearing beyond the range of humans, as well as an uncanny sonar system resembling a kind of underwater vision. This advanced system of echo location, now being studied by human naval officials for military purposes, may employ a kind of acoustical radar to map the position and movement of objects in their watery environment.
Interestingly enough, almost every year hundreds of dolphins (and whales too) beach themselves, especially along the outward-jutting Cape Cod off the New England seacoast. Most likely, their navigational beacons go awry, causing them to temporarily lose their way. Or, just perhaps, these dolphins are trying to make their way back onto the land. Are we sure ours is a humanitarian gesture when we so quickly “rescue” them and dump them back into the sea, or are we unwittingly keeping them out of our land-based niche?
Dolphins also have a well-organized social structure. They travel in schools or families and assist each other when in trouble; females often act as midwife for another dolphin. They’re not at all hostile, being extremely friendly to other dolphins as well as to humans. Dolphins seem to be the exception to the unwritten rule that all friendly species are inherently aggressive as well—though they certainly are known to ram sharks in a coordinated way if threatened, ganging up on the predator to protect their own.
In addition to their unparalleled ability to navigate underwater, dolphins communicate with one another by means of a series of whistles, quacks, squeaks, clicks, and other noises often resembling Bronx cheers. Although we can hope to communicate with them someday, the human range of generating and hearing noise is relatively limited (20 - 20,000 Hz) when compared to the dolphins’ much wider auditory range (2000 - 80,000 Hz). They are known to be able to produce and hear sounds within our audible range, but to do so requires them to grunt and groan at frequencies lower (bass) than normal. Most of the sounds normally made by dolphins are inaudible to humans, making it improbable that their way of expressing meaning overlaps ours at all. Not inconceivable, dolphins in captivity may have been trying to communicate with us for years. If so, they must be quite discouraged by our lack of response.
Interspecies communication will not be easy, whether among humans, dolphins, or chimps. Empirical findings to date nonetheless suggest that some common ground exists for future cultivation of, especially, dolphin-human links. At the least, it seems that both parties are interested in such a collaboration
The following video shows how the animals take extreme measures to hunt for food.
Saturday, May 24, 2008
Relative Brain Size
Posted by James Sheehan at 6:58 PM