Differential fitness costs of reproduction between the sexes:
We found that increasing number of offspring (parity) and rates of reproduction were associated with reduced parental survivorship, and significantly more for mothers than fathers. Parental mortality resulted in reduced survival and reproduction of offspring, and the mothers' mortality was more detrimental to offspring than the fathers'. Increasing family size was associated with lower offspring survival, primarily for later-born children, indicating a tradeoff between offspring quantity versus quality. Also, we found that the costs of reproduction increased with age more for women than men. Our findings help to explain some puzzling aspects of human reproductive physiology and behavior, including the evolution of menopause and fertility declines associated with improvements in women's status....
The short of it is that the authors are arguing that menopause is a fitness enhancer because reproduction beyond a certain age may actually result in increased risk of mortality. I've commented on this before. I didn't really give much thought to menopause before talking to a physical anthropologist who explained to me that this transition in life was a proactive physiolgical change as a female's reproductive cycle "shuts down." In contrast, the male reproductive system degrades in a more conventional fashion. A tight cascade of physiological changes to me suggests some sort of adaptation. Related: Also in Scientific American.
