资料题：Scientists still don’t know（  ）。

It is no secret that fathers tend to spend less time than mothers caring for their children.It is equally well known that men tend to have shorter lives than women.That the two facts might be connected, however, is surprising.But research just published by John Allman of the California Institute of Technology and his colleagues suggests that they are.Male chimpanzees take no interest in any infants who might be around.Female chimps areextremely promiscuous(混杂的) and males, therefore, have little idea who their children are.Dr.Allman's group found that female chimpanzees live, on average, about 40% longer than males.Male gorillas form stronger bonds with particular females, and thus have more confidence that those females' children are also their own.Male gorillas frequently play with their infants and protect them from threats, but they still have a lesser role than females in rearing their off-spring.Female gorillas live 12.5% longer than males.The acid test, however, Was to gather data from species where the main burden of parental care falls on the male.The researchers managed to find two monkeys where the father is very new age- -the titi(绢毛猴)monkey and the owl monkey.Not surprisingly, the bonds between males and females are unusually strong in these species, 80 the male's confidence in his fatherhood is high.In both titi and owl monkey's, the father assumes the critical duty of carrying his young children around.Infant titi and owl monkeys spend much of their time clinging tenaciously(顽强地)to their fathers' fur.They are transferred to their mothers only during brief musing periods.Gratifyingly for Dr.Allman's theory, in both owl monkeys and titi monkeys, males live about 20% longer than females.The rest of the argument that the non-care-giving sex is trading a shorter life for some evolutionary advantage earlier on also makes some sense, at least for males.The more“available" that females are, due to lack of a permanent pair bond, the more they are worth competing for.Suchcompetition, whether it involves direct conflict between males, or showing of to females, is not cost-free.But if it results in extra offspring, then it is worth it, evolutionarily speaking.That, however, suggests that female it and owl monkeys must be involved in a life shortening trade off of their own.Just what this is remains unknown.But it is at least heartening to discover that natural selection, despite its pitiless motivations, has found a way to reward nurturing parents。