Primatologists at UC Davis weren't surprised to see that the number of nuclear families dropped below 25 percent in the 2000 United States census. If they were to follow the biological path of their closest primate relatives -- in fact of most male mammals -- human fathers wouldn't be hanging around anyway, says anthropology professor Alexander Harcourt, who studies cooperation among primates. "Most male mammals mate and shove off," Harcourt says. "There's no evolved biological function for a human father required for the healthy development of a child," agrees Peter Rodman, an anthropology professor who studies the evolution of social systems. Instead, he argues that human fatherhood is defined by culture and, like culture, the role varies throughout the world. "There is so much variation in humans. In many cultures fathers are absent from their children," Rodman says. On the other hand, fathers still play central roles in caring for the family in this culture, despite the decline of the nuclear family, Rodman says. "We live in a system with no extended family, no grandmothers, no elderly men to help raise the children." Because human infants take a great deal of resources to survive childhood, Harcourt believes a major role for human fathers centers on providing resources. "For primates, if a female has a sister or two, she can provide for her young and be protected. Then the male is not that necessary -- except, as happens in humans, when he has sequestered all the resources," Harcourt concludes.