Evolution of Paternal Investment

REPRODUCTION INVOLVES TRADE-OFFS between mating and parenting (Trivers, 1972; Williams, 1966), and attendant conflicts between males and females and parents and offspring (Hager & Johnstone, 2003; Trivers, 1974). Conflicts arise because the ways in which each sex and each parent distribute limited reproductive resources is not always in the best interest of the other sex or offspring. Still, males and females have overlapping interests, as do parents and offspring, and thus the evolution and proximate expression of reproductive effort reflects a coevolving compromise between the best interest of the two sexes and of parents and offspring. For the majority of species, the evolutionary is males invest more in mating (typically competition for access to reproductive females) than in parenting, and females invest more in parenting than in mating (Andersson, 1994; Darwin, 1871), although there are readily understandable exceptions (Reynolds & Székely, 1997). Females benefit from male-male competition and the male focus on mating, because their offspring are sired by the most fit males, and successful males benefit because they produce more offspring by competing for access to multiple mates than by investing in parenting. The basic pattern is especially pronounced in mammals, where male parenting is found in less than 5% of species and where females invest heavily in offspring (Clutton-Brock, 1991). The reasons for the large mammalian sex difference are related to the biology of internal gestation and obligatory post-partum suckling, and the associated sex differences in the opportunity and poten-tial benefits of seeking multiple mating partners (Clutton-Brock & Vincent, 1991; Trivers, 1972). Given this, the phenomenon of human paternal investment is extraordinary and the focus of this chapter (see also Draper & Harpending, 1988; Flinn & Low, 1986; Geary, 2000; Geary & Flinn, 2001; Marlowe, 2000). Human paternal investment is considered in terms of the benefits of providing care to children and the costs of investment from the males’ perspective, as well as cost-benefit tradeoffs from the females’ perspective. In the first section, I provide an introduction to these trade-offs in nonhuman species, and discuss them in relation to human paternal investment in the second section. In the third section, I discuss the