Trivers-Willard hypothesis
Encyclopedia
In evolutionary biology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers
Robert Trivers
Robert L. Trivers is an American evolutionary biologist and sociobiologist and Professor of Anthropology and Biological Sciences at Rutgers University. Trivers is most noted for proposing the theories of reciprocal altruism , parental investment , facultative sex ratio determination , and...

 and Dan Willard, predicts greater investment in males by parents in good conditions and greater investment in females by parents in poor conditions (relative to parents in good condition). The reasoning for this prediction is as follows: assume that parents have information on the sex of their offspring and can influence their survival differentially. While pressures exist to maintain sex ratios at 50%, evolution will favor local deviations from this if one sex has a likely greater reproductive pay-off than is usual.

Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value: namely, varying maternal condition. In polygynous
Polygyny
Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...

 species males may mate with multiple females and low-condition males will achieve fewer or no matings. Parents in relatively good condition would then be under selection for mutations causing production and investment in sons (rather than daughters), because of the increased chance of mating experienced by these good-condition sons. Mating with multiple females conveys a large reproductive benefit, whereas daughters could translate their condition into only smaller benefits. An opposite prediction holds for poor-condition parents – selection will favor production and investment in daughters, so long as daughters are likely to be mated, while sons in poor condition are likely to be out-competed by other males and end up with zero mates (i.e. those sons will be a reproductive dead-end).

The hypothesis was used to explain why, for example, Red Deer
Red Deer
The red deer is one of the largest deer species. Depending on taxonomy, the red deer inhabits most of Europe, the Caucasus Mountains region, Asia Minor, parts of western Asia, and central Asia. It also inhabits the Atlas Mountains region between Morocco and Tunisia in northwestern Africa, being...

  mothers would produce more sons when they are in good condition, and more daughters when in poor condition. In polyandrous
Polyandry
Polyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered...

 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 reversed: parents in good condition will invest in daughters in order to have a daughter that can out-compete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get out-competed and will instead invest in sons in order to gain at least some grandchildren.

"Condition" can be assessed in multiple ways, including body size, parasite loads, or dominance, which has also been shown in macaques (Macaca sylvanus) to affect the sex of offspring, with dominant females giving birth to more sons and non-dominant females giving birth to more daughters. Consequently, high-ranking females give birth to a higher proportion of males than those who are low-ranking.

Assumptions

The Trivers–Willard hypothesis rests on certain assumptions:
  1. Parental condition is associated with offspring condition;
  2. Differences in offspring condition will persist into adulthood;
  3. Being in condition differentially affects the mating success of one sex more than it does the other.


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.

In their original paper, Trivers and Willard were not yet aware of the biochemical mechanism for the occurrence of biased sex ratios. Eventually, however, Melissa Larson et al. (2001) proposed that a high level of circulating glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 in the mother's bloodstream may favor the survival of male blastocysts. This conclusion is based on the observed male-skewed survival rates (to expanded blastocyst stages) when bovine blastocysts were exposed to heightened levels of glucose. As blood glucose levels are highly correlated with access to high-quality food, blood glucose level may serve as a proxy for "maternal condition" . Thus, heightened glucose functions as one possible biochemical mechanism for observed Trivers–Willard effects.
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