Mind of the Raven: Investigations and Adventures With Wolf-Birds (42 page)

A
LL VERTEBRATE ANIMAL BRAINS
consist of fore-, mid-, and hindbrains. These divisions have different functions, with the hind- and midbrains responsible mainly for integrating and processing sensory information and organizing movement and attention. The forebrain is the locus of conscious activity, playing important roles in sensation, learning, memory, and mood. There is relatively little variation among species in hind- and midbrains, but fore-brains vary greatly, and in such animals with large brains as humans, it is the forebrain that accounts for the large brain size.

In general, the greater the average brain volume of a species, the more information the animals can handle. Large animals require bigger overall brain size than small animals simply to control their bodies; and in general, brain size increases proportionally to body mass or
volume. If brain size is greater than what would be predicted by body size alone, that is called the “residual” factor, and is a measure of the brains’ “encephalization.” Humans are some of the most encephalized animals in the world, second only to some species of dolphins.

Some birds also have high cephalization. In the 1940s, the Swiss zoologist Adolphe Portman compiled data on brain volume in birds, reporting that the corvids as a whole, which include ravens, crows, jays, magpies, and nutcrackers, had one of the highest encephalization indexes, scoring a 15. The raven scored a 19, the highest member of the corvid group and therefore the highest of any bird. All other passerine, or perching, birds ranged from 4 to 8.

1A Bluejay. 1B Raven. 1C American Crow
.

The potential information-processing power of the brain is presumably related to the number of units, or neurons, it contains, and the complexity of their interactions. Brain volume is closely correlated with the number of neurons, and the complexity of interconnections of neurons is independent of brain size and of species. Encephalization is
thus probably a fairly objective measure of behavioral flexibility. We intuitively infer that intelligence is correlated to brain size, and this inference is generally supported by a variety of criteria. It is also true, however, that we can’t credibly claim that one species is more intelligent than another unless we specify
intelligent with respect to what
, since each animal lives in a different world of its own sensory inputs and decoding mechanisms of those inputs.

Primates live largely in a visual world, and in general have large areas of the brain devoted to processing visual information. Humans, in contrast to other primates, additionally have large brain areas committed to auditory processing, speech, and language. The large fore-brains of some dolphins and killer and sperm whales are thought to be devoted to echolocation. Echolocation alone, however, does not explain their huge brains, because bats and some other whales and dolphins echolocate superbly with very small brains. Birds’ encephalization could be necessary to coordinate flight, yet such insects as dragonflies manage exquisite flight (and walking) coordination of their four wings (and six legs) operating independently all with a brain smaller than a pinhead. Why would a raven need a large brain to coordinate two wings?

Brain tissue is metabolically as active and hence as expensive as muscle, and it is active day and night. Our brain accounts for only about 1.5 percent of our body weight, but it demands about 20 percent of our energy supply. At any one time, this energy is used mainly by those neurons that are active, and we can determine the regions of the brain where neural activity is concentrated using a modern technique called PET (positron emission tomography) imaging. PET scans provide pictures of the regions of the brain where neural activity is most intense, moment by moment. For example, different areas light up when we hear, see, speak, or generate words. When we see an object, a specific area of our brain indicates neural activity. When we later
think
of that object, the same area lights up. That suggests to me that thinking involves in some way the same or some of the same neurons that are involved in processing and storing incoming information, indicating a suspiciously close link to memory.

Animals have evolved to minimize energy use whenever possible. Large, metabolically expensive brains would only have developed and be maintained for very compelling reasons. As already mentioned, we can infer that sensory processing and motor coordination alone do not explain why dolphins, humans, and some birds have such large brains. One suggestion that neurobiologists have made is that the often limited stimulus load that the animal
accepts
from the environment is considerably less demanding, in terms of number of neurons required, than
what is done
with the stimuli—how large stores of memories are projected and manipulated. Different animals in effect not only see different worlds (because they have different sensors and different sensitivities), they also handle the incoming information in different ways, to
create
different worlds in their heads. For example, a bat and a dolphin both live in a world where pressure waves and vibrations of air and water, respectively, are highly important to their survival. A bat uses pressure waves as information to plot an interception with flying insects. A dolphin, however, could to use them not only to intercept prey but also to plot the ocean floor, to navigate over thousands of miles, to distinguish individuals in a herd, and maybe even to discern the moods of other dolphins (a popular theory), and to track individual dolphins for mutual interactions.

An animal may extract enormous amounts of information from the environment, organize it, and give it meaning. Before the animal encounters the patterns of vibrations or pressure waves, for example, there is no sound. The animal’s sensors detect these vibrations or pressure waves, making them stimuli; the brain then interprets these stimuli to perceive them as sounds, and to manipulate or sort them to create “stories” and scenarios out of them. Since the brain creates or specifies the animal’s unique world, it is difficult to apply the same intelligence tests across species. Perhaps the only objective criterion is brain volume itself.

In June 1988, on a canoe trip down the Noatak River in Alaska, my companions and I found the remains of a dead raven behind a trapper’s cabin, the only human structure we saw along four hundred miles of river. At the time, I saved the bird’s skull as a curiosity, or perhaps as a memento of the trip. Later that year, one of my five tame ravens unexpectedly died. I had nicknamed this bird the “cretin” because I had the
subjective impression it was incredibly dumb relative to the other birds. The cretin’s skull showed an injury, possibly a peck, which could have resulted in abnormal development, and possibly death. I saved the skull, and on a whim compared it with the Alaskan raven’s skull. Both skulls were almost identical in length, but the Alaskan’s had a strikingly larger and more rounded brain case. I next weighed a known volume of sugar and filled the brain cavities with sugar through the foramen magnum, then weighed the sugar to determine brain volume. The Noatak raven’s brain volume was 18 cubic centimeters, while the cretin’s was only 11.8 cc. I had never before seen two
similarly sized
skulls of the same species with such an enormous difference in brain volume. With numbers like that, you immediately wonder whether the cretin’s brain was abnormally small, or the other’s abnormally large.

Pursuing the possibilities, I called my friend and ornithological colleague Fran James at Florida State University in Tallahassee, who steered me to Phil Angle of the Smithsonian Institution in Washington, D.C., Phil loaned me a boxful of raven skulls from their collections. I measured brain volumes as before, determining that my Noatak raven skull was close in volume to the others from Alaska. That is, it was not abnormally large. The Alaska ravens, with an average volume of 17 cc, had a higher brain volume than the ravens from the western United States, with mean volume 13.1 cc (see Table 27.1). Brain volume of the Maine ravens overlapped both, with a mean of 15.5 cc. Alaskan ravens are larger than western ravens, and perhaps their larger brain volumes can be attributed to their larger body mass.

The northern raven’s absolute brain volume of about 17 cc is twice that of an American crow and nine times that of the common or rock pigeon. Both crow and pigeon weigh about 400 grams versus the raven’s 1,200 to 1,400 grams. In contrast, a domestic chicken (Rhode Island Red) weighed for comparison had a pea-sized brain volume of 3.1 cc, even though its body weighed twice that of a raven (see Table 27.2). These numbers reinforce general prejudices that pigeons and chickens don’t come close to ravens in intelligence, but crows probably do.

Yet brain mass in any one individual is not constant. Recent research with some birds shows that the mass of the hippocampus, the
portion of the brain devoted to the specific functions of singing and food caching, increases and decreases seasonally with use. Birds can grow and shrink brain tissue as needed, thereby avoiding expensive maintenance of tissues not in use.

There has been much discussion of why some animals have large brains. Most of this discussion has centered on human brain evolution, but the same ideas probably apply generally. One thing stands out: The large brain of hominids appeared rather suddenly. Furthermore, it did not evolve uniformly in all of the hominids. For millions of years, our ancestors, such as
Australopithecus afarensis
, walked bipedally and had an essentially modern human form, but had a brain volume just marginally larger than a chimpanzee’s. For millions of years, that smaller brain sufficed. Suddenly (in evolutionary terms), a small brain wasn’t good enough anymore for one small group of ancestral hominids. Something changed for them alone. What was it? The one correlation we have is that as hominids became meat-eaters, they became larger in body size, and brainier. The others who remained largely vegetarian remained small-brained. Is this change in diet a clue?

The diet connection is strong, but interpretations of it differ. The prominent anthropological argument acknowledges the diet connection and attempts to explain it by saying that meat from large animals—large amounts of high-powered concentrated food—was needed to power that metabolically costly brain, so we turned to scavenging and hunting. I think that particular explanation is backwards, because it assumes a large brain is a good thing. It isn’t, necessarily. Both ravens and hawks are meat-eaters, but hawks are small-brained, and they are very effective and successful predators. Diet alone therefore does not explain raven brain evolution. Another explanation is that a protein diet
enabled
an expensive brain, and that some strong selective pressure, such as sociality, then drove the evolution of increased encephalization.

Much recent research in mammals has converged to indicate that perhaps the major driving force behind the evolution of increased brain size is social complexity. In turn, social complexity increases inordinately when individual recognition becomes possible and the animal tracks not just others, but myriad
specific
others. Ravens, like other corvids, and like
dolphins and most primates, are highly social. As I have indicated (Chapter 14), they apparently recognize one another. Furthermore, not only do nonbreeding subadult ravens form coalitions against breeding adults, but adults may cooperate in pairs and perhaps in coalitions of pairs (Chapters 9 and 10). Ravens also are exposed to interactions with dangerous carnivores as they attempt to get meat from them (Chapters 19, 20, and 21). All of these interactions require instant reactions or choices that can be made much more quickly and safely in the head, rather than overtly. In short, they may require consciousness, the ability to examine, evaluate, and make mental choices before committing to action.

Table 27.1 Average Skull Measurements and
Brain Case Volumes of Ravens (
Corvus corax
)