The case of Bikini already has been discussed as an example of a
predominantly aquatic environment apparently recovering from association
with nuclear experiment. Eniwetok offers an instance of the toughness of
an animal population exposed both to direct and long-range radiological
impact.
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._]