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.
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Is it possible that at some point in the past an ancestor species (Homo habilis?) developed creches as a survival strategy? It would be well within the capacity of non-human primates-- lions, for instance, practice creching-- and it shows up not infrequently in social species under environmental stress. The problem for hominids is that creching females are usually caring for the offspring of their sisters or cousins, while hominids probably had social groups of related males and unrelated females (as both chimps and humans typically do today.) Older females who lost their ability to reproduce might have turned their mothering instincts toward caring for the offspring of the unrelated females in their group. This apparently inappropriate behavior would have had a selective advantage because the older females were, in fact, tending their own paternal grandchildren. I don't know how to test this, but it's an idea I've played around with since about the time I became a grandmother.
"Increasing family size was associated with lower offspring survival, primarily for later-born children, indicating a tradeoff between offspring quantity versus quality."
Razib, do you know if they controlled for resource quality/quantity?? Seems likely that instead of reduced survival being a product of reduced quality- it might be a result of fewer resources...