Our broad definition of "intelligent behaviour" applies to many types of animals. It is not confined to warm-blooded vertebrates. Octopus behaviour is as "intelligent" as the behaviours of most mammals, though brain organisation is very different in mammal and octopus and the responses to stimuli are quite distinct. In some instances, intelligent behaviour appears to reach across generations. The mechanisms involved are mysterious. A honey-bee swarm returning from migration readily locates its original nest site, though no individual in the swarm has been there before, the original inhabitants having died. Do particular swarms leave distinctive chemical markers that are durable enough for their returning descendants to recognise? This hardly seems plausible, but it is difficult to see any other explanation. Salmon returning to the rivers in which their parents bred take refuge during the journey behind the same rocks that their forebears used, although alternative, equally adequate, shelter is available. These remarkable phenomena remind us that much about animal life continues to defy ready explanation.
Interesting as these reflections are, we shall focus on mammalian brains for the rest of this chapter. This will serve as a prelude to discussing "human intelligence" in chapter 17. Mammalian brains consist of three main parts: forebrain, midbrain and hindbrain. Very broadly, the hindbrain controls basic physiological processes - heart rate, breathing, body temperature, eating and drinking, sleeping and waking. The midbrain co-ordinates sensory information from body and environment and initiates appropriate responses. Parts of the midbrain are associated with emotions and with aspects of memory storage. The forebrain is the area most concerned with intelligent behaviour. In primates in particular, it is dominated by the cerebral cortex. The cerebral cortex is fairly small in mice but covers the entire brain in gorillas, chimpanzees and humans. A mouse deprived of its cerebral cortex behaves quite like a normal mouse, but a human deprived of his or her cerebral cortex is a vegetable. It is tempting to infer that the cerebral cortex is the root of intelligent behaviour. This seems to be the case for mammals, but it is unwise to generalise too much. Birds, which have no cerebral cortex at all, are often capable of "intelligent behaviour" in our sense of the phrase. But because we are now focusing on mammals, the cerebral cortex deserves particular attention.
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