The Grandmother Effect

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Science  20 Jan 2006:
Vol. 311, Issue 5759, pp. 305-307
DOI: 10.1126/science.311.5759.305d

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Women's postreproductive years, the immaturity of newborns, and strong kinship networks combine to make it reasonable that the grandmother effect could be important in the evolution of human life-span structure. For guppies, on the other hand, with live-born young that require no further care and with a lack of familial structures, the female postreproductive life phase is not likely to be a target of evolutionary forces. In fact, because guppies produce eggs throughout adulthood, a nonreproductive late-adult phase seems unlikely.

Nonetheless, guppies do have a postreproductive life phase, and, as Reznick et al. show, this phase seems to be an accident of extended reproductive life span rather than a point of evolutionary leverage. Guppies that face high rates of predation mature earlier and reproduce more often than guppies in more benign bends of the river. When reared in the lab, safe from predators, these two populations continued to show different life histories, and the predator-intense family of guppies lived longer. Many of these individuals had a postreproductive life phase, suggesting that reproductive senescence precedes somatic senescence. The length of the postreproductive life phase seemed to be an accidental outcome of the generally longer reproductive life span that the high-predation environment had brought about. — PJH

PLoS Biol. 4, e7 (2006).

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