Understanding Brains by Comparing Taxa

UNDERSTANDING BRAINS BY COMPARING TAXA THEODORE H. BULLOCK* Long before the human species appeared, the pinnacle and greatest achievement of evolution was already the brain—as it had been before mammals appeared, before land vertebrates, before vertebrates. From this point of view, everything else in the multicellular animal world was evolved to maintain and reproduce nervous systems—that is, to mediate behavior, to cause animals to do things. Animals with simple and primitive or no nervous system have been champions at surviving, reproducing , and distributing themselves, but they have limited behavioral repertoires . The essence of evolution is the production and replication of diversity—and more than anything else, diversity in behavior. The great fact that nature presents for us in the animal world is the variety of brains, the millions of species with distinct behaviors. It is the thesis ofthis piece that we cannot expect to understand ourselves or how nervous systems mediate behavior unless we gain some insight into the tremendous range of brains, from simple to complex—by far the greatest spectrum of any organ system. To put the brain into perspective we will have to tap this variety, to learn how it evolved, to distinguish what is old from what is new in the most advanced nervous systems, and what the relevant differences between taxa are in their brains and behaviors. Plasticity is one of the most notable features, especially of advanced brains. This is a broad term which includes learning in the widest sense plus biasing during embryogenesis and recovery, compensation, or adjustment after injury. Some forms, such as regeneration in the central nervous system, are less notable in advanced brains. Although the capacity to change in an adaptive way is clearly important to paramecia, worms, and snails, we humans have taken this heritage and developed it to such a height that human learning and its accumulation cannot be contained in the largest libraries. ?Neurobiology Unit, Scripps Institution of Oceanography, Department of Neurosciences , School of Medicine, University of California, San Diego, LaJolla, California 92093.© 1984 by The University of Chicago. AU rights reserved. 003 1-5982/84/2704-0389101 .00 510 I Theodore H. Bullock ¦ UnderstandingBrains My purpose in this essay is to parade a double handful of examples from the rich variety offered by the zoo, the aquarium, and the field, to highlight this thesis and bring out with each species a point or two of general interest about the comparative neuroethological approach to the study of the most complex systems known (except for communities of brains), and especially to the study of their plasticity. Humpback Whales My first example takes us beyond either zoo or aquarium to the open sea. Picture yourself, like the ethologist Roger Payne and his associates, in a small boat off the island of Maui listening to hydrophones hanging over deep water, now and then donning fins and mask to slip in and see who is making those moans and whistles. Patient observation and listening have given evidence suggestive of a form of plasticity in the song of the humpback whale [1-3]. This 10-15-minute sequence of rumbles, glissandos, whistles, and other sounds is repeated over and over as a stereotyped performance characteristic of many lone male singers. The song can be divided into five to eight themes, each a repeated pattern of units. AU over the North Pacific the whales sing the same song, theme for theme, with the possibility that some details are consistently distinctive for the individual. The plasticity is noticed by comparing the song over time—here a few months. In successive months some sounds may drop out of the song while others are modified and incorporated into new themes. The same changes are incorporated in the same way into the songs ofall the singers in the population, like fads among teenagers. This suggests simultaneous social pressures for novelty, for conformity, and perhaps even for individuality; the whale's brain is capable of great memory, variety, and plasticity. We do not expect to see physiological study of vocal or auditory processing in the brain of humpback whales in the near future, but the ethological approach and findings will surely influence measurements, which are now...