They trapped and marked forty virgin female opossums in their burrows in Venezuela. Then they fed 125 grams of sardines to each of twenty opossums every two days by leaving the sardines outside the burrows, no doubt to the delight and astonishment of the opossums. Every month thereafter they trapped the animals again, opened their pouches, and sexed their babies. Among the 256 young belonging to the mothers who had not been fed sardines, the ratio of males to females was exactly one to one. Among the 270 from mothers who had been fed sardines, the sex ratio was nearly 1.4 to 1. Well-fed
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