since i'm still stuck on the topic of monogamy, here's 5 alternative hypotheses that James Wittenberger and Ronald Tilson summarised in the article The Evolution of Monogamy: Hypotheses and Evidence. (JSTOR Link)

1. Monogamy should evolve when male parental care is both nonshareable and indispensable to female reproductive success. A female may be unable to rear offspring without nonshareable male parental assistance, either because she cannot provide them with enough food alone or because continuous attendance of offspring is essential to their survival. Hypothesis 1 applies only if females cannot rear anyoffspring within the species-typical social system without male assistance. Males may still copulate with other females (68), but they maintain a prolonged pair bond with only one mate. Hypothesis 1 implies that monogamy is advantageous for both sexes. It resembles Lack's (1 11) hypothesis for monogamy in altricial birds.

2. Monogamy should evolve in territorial species ifpairing with an unavailable unmated male is always better than pairing with an already mated male. Any benefits females might gain by breeding in a superior habitat, mating with a superior male, or cooperating with other females on a shared territory may be too small to compensate for the costs of polygyny. Since a major cost of polygyny is often lost male parental care, Hypothesis 2 covers cases where parental assistance is important but not indispensable to female success. Hypothesis 1 could be treated as a subset of Hypothesis 2, but the distinction is useful for pointing out cases where monogamy is clearly advantageous for both sexes as opposed to those where it may be advantageous only for females. Hypothesis 2 is derived from the polygyny-threshold model of Verner (227) and Orians (145).

3. Monogamy should evolve in nonterritorial species when the majority of males can reproduce most successfully by defending exclusive access to a single female. Sequestering individual females should be especially advantageous for males when sex ratios are male-biased, because the majority of males would then achieve higher success by claiming sole possession of one female rather than taking their chances in a promiscuous "lottery" system. Females may or may not benefit from being sequestered, but the costs of resisting the male's continual presence must exceed the costs of accepting his presence.

4. Monogamy should evolve even though the polygyny threshold is exceeded if aggression by mated females prevents males from acquiring additional mates. The criterion for distinguishing between Hypotheses 2 and 4 in territorial species is the magnitude of the polygyny threshold. If the polygyny threshold is exceeded, Hypothesis 4 applies. If not, Hypothesis 2 applies. In social animals Hypothesis 4 applies when aggression by a dominant female can prevent breeding by sexually mature subordinate females. The occurrence of female aggression is not sufficient evidence for accepting Hypothesis 4, because females may be aggressive for reasons other than maintenance of a monogamous pair bond.

5. Monogamy should evolve when males are less successful with two mates than with one. The presence of a second female may substantially reduce the success of a male's first mate by increasing competition for resources or increasing her conspicuousness to predators. If the combined success of both females is less than that possible for a single female, the male should enforce monogamy by excluding additional females. This hypothesis was first proposed by Trivers (224).



i don't think that hypotheses 1, 2 and 5 really apply to humans, and no. 4 is really funny. 河东狮吼。。。