The usual rules of sex roles, then, are ones of male display and female choosiness. This stems from that long-ago difference in investment between the sexes—the large resource-rich egg and the small streamlined sperm. Furthermore, in those species in which parental care is necessary for the offspring to survive—which means all of mammals and birds, and a good proportion of reptiles, amphibians, fish, and insects as well—males tend to put more effort into what happens before sex, and females put more effort into what happens afterward.26
