11.7 Testing Trivers-Willard in opossums
In the mid-1980s, two biologists devised a simple test of the Trivers-Willard hypothesis of sex-ratio allocation. Specifically, they trapped and marked (for future identification) 40 female Venezuelan opossums. They then supplemented the diets of 20 of these opossums, specifically by leaving a tin of sardines by their burrows every two days for several weeks. Then, they re-trapped the females, looked in their pouches (opossums are marsupials, meaning their young develop in pouches), and identified the sex of their offspring.
Their data are summarized below.
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Similar support of the Trivers-Willard hypothesis has been observed in similar studies with other organisms, but in some cases, these findings have not been observed. In some situations, explanations other than the Trivers-Willard hypothesis may better explain observed sex ratios. For example, in some organisms, males are bigger and demand more resources; it makes sense that a mother in poor condition would selectively miscarry males in favor of females.
