Engebi Island, on Eniwetok’s northeast reef, is the home of a wholly self-contained colony of Pacific rats living in a network of burrows in the shallow coral sands. After 1948 Engebi was exposed repeatedly to atomic detonations, and in 1952 the whole island was swept clean of growth and overwashed by waves from the thermonuclear explosion of Operation Ivy. On each of these occasions, exposure of the rat colony to radiation was intense. In 1952, by later estimates, the animals aboveground received radiation doses of 2500 to 6000 roentgens per hour, and those in burrows doses of 112 to 1112 roentgens per hour.[19] The island environment was so altered by atomic forces and by contaminated water that radiobiologists believed it impossible that any of the rats had survived. Because there was no natural route by which the island could be repopulated, scientists even considered introducing a new rat colony for study of a population growth in a mildly radioactive environment. [Illustration: _Engebi Island, Eniwetok Atoll, home of a colony of rats living in radioactive surroundings._] [Illustration: _Close-up shows one burrow in the soil._] Contrary to all expectations, however, the original colony had not been eliminated. Biologists visiting Engebi in 1953 and 1954 found the rats apparently flourishing. New generations of rats were being born and were subsisting on grasses and other plants in an environment still slightly radioactive. In 1955 analysis of the bones of rats revealed the presence of strontium-89 and strontium-90 in amounts approaching what was assumed to be the maximum amount that would not cause bodily harm. The rats’ muscle tissues contained radioactive cesium-137. But no physical malformations were found in the rats. All animals appeared in sound physical condition, despite these body burdens of radioactivity. By 1964 the rat population had so increased that it apparently had reached equilibrium with available food supplies. Questions relating to the reestablishment of the colony are intriguing. Why are new generations of these warm-blooded animals continuing to thrive after the colony was exposed to devastating nuclear effects? Is there a different dose-effect relation for these rats than for other animals? Even if it is assumed, as it must be, that some members of the colony survived the original nuclear heat and radioactivity because they were shielded by concrete bunkers or other man-made structures, how is it that there have been no observable effects among rats existing for years in an area that continually exposed them to radiation? [Illustration: _A native rat, captured alive on Engebi Island, being held by a scientist before having its toenails clipped as a means of identification. Note the animal’s healthy appearance._] Fallout and Populations