Emlen and Oring developed a system for birds, but this can be used for other taxa. Different hypotheses suggest why. Increased parental investment in rearing young raises the reproductive success of the male. Females may refuse to mate with mated potential partners or harm rival females. Pregnancy block in some species of mice is a good example of this.
Birds tend to be monogamous whereas mammals tend to be polygamous.
Predicts that females who mate with multiple males will be more likely to have viable young. To forage there requires mating with him. Males control access to multiple females. Resource-defense Polygyny . Males control access to females indirectly by monopolizing critical resources.