A number of studies have sought to explain the evolution of menopause in humans. Williams (1957) suggests that menopause eliminates the risk that a woman dies in childbirth, and by ceasing menses the infertile woman can provide more child care for the existing children. Diamond (1992, 1996) shares the same view, and emphasizes the relation between menopause and the long juvenile dependency period of humans. Rogers (1993) classifies the theories into two categories, the first relating menopause to the mortality of women in childbirth, and the second relating menopause to women’s “opportunity cost of fertility”, referring to activities (such as baby-sitting and guarding) in which women are more capable when not pregnant themselves. Rogers uses empirical human data to show that the first hypothesis is likely to be rejected, but the second hypothesis is compatible with human demography.
It is certainly possible that menopause is non-adaptive, meaning that it simply represents an uncontrolled degenerative loss of homeostasis associated with senescense. If that is the case, then understanding menopause is equivalent to understanding post-reproductive survival. The existence of old-to-young transfers, as shown by Lee (2003), can sustain the survival of sterile old-ages in an evolutionary context, hence such transfers may also provide a non-adaptive explaination for menopause. However, the hypothesis of non-adaptive menopause has been seriously challenged by Diamond (1996) and Sherman (1998) on theoretical grounds, and it is not supported by empirical data. In this paper, we explore a theory of adaptive menopause. A brief look at menopause in non-human species provides a useful background.
Some scientists (Carey and Gruenfelder 1997, Sherman 1998, Peccei 2001) discuss menopause in non-human species, such as some whales and dolphins. Carey and Gruenfelder (1997) discuss the case of post-reproductive dolphins and whales, in which the older males or females guard or baby-sit their grandchildren, allowing the younger parents to forage, and that some post-reproductive Hawaiian spinner dolphins can even produce milk to feed the baby. In these examples, caring for babies is mainly the grandparents’ job, since foraging is something they are “not able to do as efficiently” (p.145). For humans, however, lactation and child care are often the mother’s job, while the old post-reproductive grandmother specializes in hunting and for-aging (Gibbons 1997, Peccei 2001, Hawkes 2003). Although the activities of older humans, dolphins and whales differ somewhat as described above, a common characteristic, as pointed out by Carey and Gruenfelder (p.144), is the specialized division of labor.
In the whale and dolphin examples, the old grandmothers devoted care of juveniles, which implies that they do not have time to participate in the pack hunting with the other adults in their group. Thus, in order for them to survive, some kind of ex ante division of labor and ex post food sharing between the old grandmother and the adult mothers must be in the background. It is therefore intriguing to explore the possible co-evolution of menopause and the division of labor. Empirical evidence also shows that the few species which do have menopause also have a rather long juvenile dependent period (Carey and Gruenfelder 1997, Hawkes 2004), during which the adults provide protection, guard and care. We shall consider how this extensive time demand of the youth changes the life history trajectory, in particular sterility in old age. Finally, why do only a few “intelligent” species have menopause? And why do species with menopause tend to have lower adult mortality? The purpose of this paper is to explore the relationship between menopause and the division of labor, and to provide preliminary answers to these questions.
Our strategy is to set up a model of optimal life history in which age specific fertility, growth and survival are choice variables. For the species we consider, children need both food and safety to survive, and the provision of the latter needs the adults’ input of time, for which the opportunity cost is the lost food from hunting and foraging. Thus, the adults of all ages can use their time either in baby-sitting or foraging, and they can allocate their disposable energy either to maintaining/survival or to fertiltiy. Given this opportunity cost structure, we introduce an intergenerational division of labor, and study the possible optimal choice of menopause in old age and its correlation with the division of labor. We show that species in which juveniles require extensive inputs of adult time are likely to have menopause as an optimal life history strategy. Since longer time inputs from adults may often imply more intensive juvenile learning,we suggests that menopause may be more likely to appear in more intelligent species.
The remainder of this paper is as follows. Section 2 sets up a model of optimal life history with overlapping generations. The third section presents the intuitive background needed to support the choice of menopause. Section 4 introduces the division of labor and derives the condition for its selection. Sections 5 and 6 consider when menopause may arise as the chosen strategy and how such a division of labor co-evolves with the optimal age-specific survival probability and fertility at mature ages. The final section contains some discussion and conclusions.
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Menopause and the Specialization of Labor: An Economic Analysis
