Parental Investment Strategies On Agro-pastoralists In The Eastern Tibetan Plateau

Human behaviour is one of the results of natural selection,the study of behaviour also requires the guidance of evolutionary theory.Human behaivoural ecologists have a long stand interest of ecological effects on trade-offs of reproductive benefit and cost,among which parental investment affecting directly on parents' reproductive success,is thought to be one the most important trade-offs.Trivers and Willard hypothesized that parental investment might be biased according to different conditions of the parents,and we tested that parents might adjust their investment strategies to differences of ecology and social factors.In this paper,We examined the factors affecting parental investment in the agro-pastoral Amdo populations in the eastern Tibetan Plateau.We collected demographic and social-economic data as well as self-narrative offspring sex preferences of parents in a demographic survey in 2016.We used Cox mixed-effects model to analyze interbirth interval to study the factors affecting interbirth intervals as a measure of parental investment behavior.Our results showed that where male income was significantly higher than female income,parents' preference and investment behaviour all favored sons.It is possible that when men can create more economic resources,the parental investment is more male-biased.We also found that wealthier families have larger interbirth intervals,it is possible that the fertility rate in wealthier families is lower than in poorer families.Older mother have larger interbirth intervals,the reason for the slowing of reproduction may be part of the mother's aging process,or it may be an evolutionary effect from the competition of siblings.The male-biased parental investment we found in our study may be due to the son creating more economic resources.Therefore,this gender preference may reflect cultural norms that have not changed for a long time,reflecting the importance of the male role.