In the Light of Evolution Volume IV: The Human Condition (2010) / Chapter Skim
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11 How Grandmother Effects Plus Individual Variation in Frailty Shape Fertility and Mortality: Guidance from Human-Chimpanzee Comparisons--Kristen Hawkes
11 How Grandmother Effects Plus Individual Variation in Frailty Shape Fertility and Mortality: Guidance from Human-Chimpanzee Comparisons--Kristen Hawkes
Pages 211-230
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From page 211... ...
A grandmother hypothesis, subsequently elaborated with additional lines of evidence, helps explain both exceptional longevity and additional features of life history that distinguish humans from the other great apes. however, some of the variation observed in aging rates seems inconsistent with the tradeoffs between current and future reproduction identified by theory.
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From page 212... ...
he illustrated the latter effect with the slow senescence of indeterminate growers that continue to increase in size and rate of egg production throughout adulthood. Concluding that evolutionary life history theory predicts no postreproductive period in normal life spans, Williams then addressed the apparent contradiction posed by survival past menopause in our own species by observing that older women still investing in descendants are not literally postreproductive.
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From page 213... ...
based on the same evolutionary tradeoffs between current and future reproduction has directed attention to processes of cellular maintenance and repair that affect somatic aging rates (Kirkwood and holliday, 1979; Finch, 2007)
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From page 214... ...
judged vaupel's hypothesis to be in conflict with hamilton's forces and found those forces themselves sufficient to explain the mortality plateaus. i argue here that rather than being mutually exclusive alternatives, heterogeneity of frailty and tradeoffs between current and future reproduction explain different things.
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From page 215... ...
More vigorous elders, through greater reproductive success of their daughters, would have spread their slower somatic aging to more descendants. longer adult life spans then reduced the cost of waiting longer to mature, delaying age at maturity and increasing adult body size (hawkes et al., 2003)
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using data from five wild study sites. Average age at first birth is 13 in wild chimpanzees so the 10- to 14-year age class is included in the childbearing years.
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, patterns that must surely affect death risks. The grandmother hypothesis highlights sharing by grandmothers in particular because, as noted by hamilton, evidence that women remain healthy and productive past their fertility provides a clear link between human longevity and fitness payoffs to ancestral grandmothering.
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From page 218... ...
represent variation in iMrs and demographic aging rates in that species. The same strehler–Mildvan relationship found across human populations holds for chimpanzees.
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The evolutionary models all assume that more energy allocated to somatic maintenance pays off in future reproduction but leaves less for current reproductive effort. life history variation among individuals of the same population often seems to go in the opposite direction as well.
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From page 220... ...
older age classes are a biased subset of younger ones and that bias affects their average mortality risk. in higher mortality populations of both humans and chimpanzees, older age classes are more strongly culled, leaving proportionately fewer frail survivors.
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, exposed to two conditions of age-independent mortality. initial mortality rates are low (similar to the United states and Japan)
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numbers continue to fall across young and middle adulthood, reaching thresholds associated first with reduced fecundability, then secondary sterility, and finally menopause ~10 years after last birth. Average ages at these thresholds differ some across populations (Bentley and Muttukrishna, 2007)
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From page 223... ...
Fig. 11.4 displays the average age-specific fertilities for three hunter-gatherer populations and the conservative age-specific fertility schedule synthesized from six wild chimpanzee populations by M
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The finding indicates that mortality selection across the childbearing years culls the females with lower fertility. As the age classes shrink to almost nothing, they are increasingly biased to the less frail, more fertile females.
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other effects on heterogeneity are likely as well. Associations between regional infant mortality rates and late life morbidities led D
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. even with the imperfect association between exposure and birth date and effects of these unmeasured covariates, rates of cardiovascular disease after the age of 60 were >20% higher in those whose fetal development coincided with pan
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From page 227... ...
do not specify particular mechanisms of aging, but their incorporation in an analysis of human survival curves points to a deep history of reproductive benefits accruing to postmenopausal women in our lineage. Mueller, rose, and rauser have focused attention on the period of life in many species when mortality rates slow from an exponential increase and may become constant at succeeding age intervals.
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has hypothesized that selection pressures for distinctively human cognitive and emotional capacities arose from our evolution as cooperatively breeding apes. Unlike our nearest living relatives, human mothers accept help with babies right from parturition.
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From page 229... ...
As continuing innovations in medical and daily living technologies interact with mortality selection to produce complex dynamics in the health and welfare of elders (Manton, 2008) , the heterogeneity in ovarian and somatic aging that is an aspect of our evolved life history becomes an increasing medical as well as social, economic, and political concern of our time.
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