Trivers-Willard hypothesis

In evolutionary biology, the Trivers-Willard hypothesis proposes that parents will invest more in the sex that gives them the greatest reproductive payoff ('grand-offspring') with increasing or "marginal" investment, and that the "optimal" avenue of investment may differ according to the parent's condition. It was formally put forth by Robert Trivers and Dan Willard.

For example, in species where some males may mate with multiple females and other males achieve few or no matings (polygynous species), parents in good condition can have many more grand-offspring if they produce and invest in sons rather than daughters, because their good-condition sons have an increased chance of mating with multiple females—a huge reproductive benefit—whereas daughters will experience smaller benefits from being in good condition.

On the other hand, the offspring of parents in poor condition are also likely to be in poor condition (all else being equal), so (among polygynous species) parents in poor condition will have more grand-offspring if they produce and invest in daughters rather than sons, because those daughters are likely to achieve at least some mating success (i.e. from the good-conditions males of other parents), whereas sons in poor condition may be outcompeted by other males and end up with zero mates (i.e. those sons will be a reproductive dead-end).

Thus, the Trivers-Willard hypothesis predicts greater investment in males by parents in good condition and greater investment in females by parents in poor condition (relative to parents in good condition). The hypothesis was used to explain why, for example, Red Deer mothers would produce more sons when they are in good condition, and more daughters when in poor condition. In polyandrous species where some females mate with multiple males (and others get no matings) and males mate with one/few females (i.e. "sex-role reversed" species), these predictions from the Trivers-Willard hypothesis are all reversed: parents in good condition will invest in daughters in order to have a daughter that can outcompete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get outcompeted and will instead invest in sons in order to gain at least some grandchildren.

One can define "good condition" in multiple ways, such as body size, low parasite loads, or dominance, which has also been shown to affect the sex of offspring, with dominant females birthing more sons and non-dominant females birthing more daughters. This effect has been found in macaques ("Macaca sylvanus") where males born to high-ranking females have greater reproductive success than those of low-ranking females. Consequently high-ranking females give birth to a higher proportion of males than those who are low-ranking (Kuesterl & Arnemann 1992).

The Trivers-Willard hypothesis rests on certain assumptions:
# Parental condition influences offspring condition;
# Differences in offspring condition will persist into adulthood;
# Being in good condition increases the mating success of one sex more than it does the other sex.

Evolutionary biologists predict a Trivers-Willard effect where these conditions hold, and no effect when these conditions do not hold. In polygynous species where some males have multiple mates and others have none (i.e. greater variance in mating success among males than females), being in good condition affects males more than females. This is reversed in polyandrous species, and possibly in species where condition is based on social status and males disperse.

No direct biochemical explanation of "how" the effect occurs has yet been given.

ee also

* Robert Trivers
* Dan Willard

References

*Trivers, R.L., & Willard, D.E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. "Science, 179," 90-92.
*Kuesterl, A.P. & Arnemann, J. (1992) Maternal rank affects reproductive success of male Barbary macaques (Macaca sylvanus): evidence from DNA fingerprinting . Behavioral Ecology and Sociobiology. 30:5;337-341