are some species such as the Mormon cricket, pipefish seahorse and Panamanian poison arrow frog males invest more. Among the species where the male invests more, the male is also the pickier sex, placing higher demands on their selected female. For example, the female that they often choose usually contain 60% more eggs than rejected females. This links Parental Investment Theory (PIT) with sexual selection: where parental investment is bigger for a male than a female, it's usually the female who competes for a mate, as shown by Phalaropidae and polyandrous bird species. In these species females are usually more aggressive, brightly colored, and larger than males, suggesting the more investing sex has more choice while selecting a mate compared to the sex engaged in intra-sexual selection. Females as a valuable resource for males The second prediction that follows from Trivers' theory is that the fact that females invest more heavily in offspring makes them a valuable resource for males as it ensures the survival of their offspring which is the driving force of natural selection. Therefore, the sex that invests less in offspring will compete among themselves to breed with the more heavily investing sex. In other words, males will compete for females. It has been argued that jealousy has developed to avert the risk of potential loss of parental investment in offspring. If a male redirects his resources to another female it is a costly loss of time, energy and resources for her offspring. However, the risks for males are higher because although women invest more in their offspring, they have bigger maternity certainty because they themselves have carried out the child. However, males can never have 100% paternal certainty and therefore risk investing resources and time in offspring that is genetically unrelated. Evolutionary psychology views jealousy as an adaptive response to this problem. Application of Trivers' theory in real life Trivers' theory has been very influential as the predictions it makes correspond to differences in sexual behaviors of men and women, as demonstrated by a variety of research. Cross-cultural study from Buss (1989) shows that males are tuned into physical attractiveness as it signals youth and fertility and ensures male reproductive success, which is increased by copulating with as many fertile females as possible. Women on the other hand are tuned into resources provided by potential mates, as their reproductive success is increased by ensuring their offspring