The quest for the biological roots of violence—and there may be others besides serotonin and noradrenaline—draws on a wide variety of research, including studies of insects, monkeys, reproduction, and heart disease, in addition to brain chemistry.
None provides a more vivid example of the environmental-genetic link to violent aggression than the Grand Canyon Tiger Salamander, nature’s version of Dr. Jekyll and Mr. Hyde.
The salamanders live in ponds along an isolated rim of the Grand Canyon. When water and food are plentiful, the salamander is in its Dr. Jekyll form—a gregarious, peace-loving insect eater. But when the water begins to dry up, food becomes scarce and living conditions become unbearably cramped. Then some of the salamanders go through an amazing Mr. Hyde transformation.
Environmental pressures rapidly alter the function of some of their genes, creating changes in their physical shape and making them aggressive. Muscles enlarge to make their heads and mouths bigger and they grow a new set of huge teeth, adaptations that allow them to attack and eat other salamanders.
They become cannibals, but only for a short time. Once they’ve gobbled up enough salamanders to reduce crowding, they turn back into Dr. Jekylls. Their heads shrink to normal size, their cannibal teeth disappear, and they dine on insects again.