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.

Figure 11.5 Opossum with her babies

Their data are summarized below.

Figure 11.6 Sex ratios of pouch young from control and provisioned females. Sex-ratio difference is significant (P(one-tail) = 0.007, binomial test). Control females, n=256; provisioned females, n=270.

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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.

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Introduction to the Evolution & Biology of Sex Copyright © by Katherine Furniss and Sarah Hammarlund is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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