To illustrate, let us contrast mainland opossums living in the south of the USA with opossums living on a nearby island, where they had been isolated from predators for thousands of years. Austad [ 2 ] predicted and found support for the hypothesis that the island opossums would senesce at slower rates than their mainland cousins. Under the idea that islanders faced lower extrinsic mortality due to lower rates of predation, he found that island opossums died at slower rates with advancing age meaning the average islander lived a longer life and also scored lower on a physiological measure of stress wear and tear to their tail tendon fibers.
As other examples, bats tend to outlive their similarly sized mammalian cousins such as squirrels, a pattern consistent with theory since bats can more readily escape would-be predators by flying away, enabling them to invest in maintenance functions [ 2 ]. Tortoises, like those Charles Darwin encountered in the Galapagos Islands, tend to lead long lives, in part because they hold protection hard shells literally on their backs.
Such examples illustrate how fertility and mortality schedules shape the rate of senescence loss of function with age among organisms. The disposable soma theory articulates an evolutionary perspective on aging, by considering the life history trade-offs involved with maintenance. These trade-offs ultimately impact reproductive function, not as a result of population control as some have argued , but as a result of natural energetic concessions [ 3 ].
Once an organism reaches an age when survivorship in the wild is highly unlikely, the state of its somatic integrity, or otherwise, is inconsequential. What light does this evolutionary perspective shed on human aging? Humans senesce at slower rates than our great ape cousins, and apes, in turn, senesce at slower rates compared with other primates such as lemurs and both old-world monkeys e.
As an example, wild chimpanzee mortality data from five field sites in Africa showed that almost every chimpanzee was dead by age 50 [ 4 ]. By contrast, recent studies of human hunter-gatherers and horticulturalists such as the! Kung of the Kalahari, the Hadza of Tanzania, the Ache of Paraguay and the Tsimane of Bolivia have found that these human populations have longer lives than our ape cousins living either in captivity or the wild [ 5 ]. Of course, recent human hunter-gatherers also have technological and cognitive shields controlled use of fire, projectile weapons, knowledgeable elders against environmental insults that our more distant foraging ancestors lacked.
How far back, then, we can project mortality patterns from recent foragers to more distant foragers is an open question. The available bioarchaeological samples e. When combining this line of evidence with other paleoanthropological data on rates of development, brain size estimates and archaeological evidence of tool use and subsistence, along with the comparative ape and contemporary hunter-gatherer data, the best guess would be that the slower rate of human senescence evolved in the genus Homo at some point within the past 2 million years [ 6 ].
A more specific educated guess would be that only with modern humans ourselves within the past , or so years have people been living long enough to even contemplate a postreproductive existence [ 7 ]. Considerable debate has arisen concerning why human females, in particular, live well beyond their reproductive capacities [ 8 , 9 , 10 ].
An extended life span gives rise to postreproductive female existence. While some argue that the cause of extended human life spans is grandmaternal care i. Several arguments favor the latter byproduct view, focusing on the fact that selection tends to be relatively weak or absent for any traits specific to postreproductive years of life. The comparative nonhuman evidence convincingly demonstrates that postreproductive female lives are hardly unique to humans.
However, even among forager groups, humans appear to spend a significantly longer proportion of their lives in postreproductive states than is the case in postreproductive captive animals. Interestingly, peri- and postmenopausal gorillas no longer maintained the same social affiliations with a silverback adult male gorilla , though whether that is because females were not interested in doing so or males were not as interested in the females, or both, we do not know [ 13 ].
Put another way, the overall theoretical and empirical evidence is most consistent with the view that human senescence has a number of parallels with captive nonhuman animals and has slowed recently in hominin evolution perhaps only with modern humans ourselves. This has given rise to common and extended postreproductive lives that can be spent having sex, among other things.
While most of the focus on human reproductive aging has centered on females, male senescence can also be understood in an evolutionary perspective [ 15 , 16 ]. Estimates of age-specific fertility in forager societies such as the! The fact that human sexual and reproductive behavior typically occurs within long-term slightly polygynous bonds is noteworthy, as this social context differs from our great ape cousins and was likely derived within the past 2 million years of Homo evolution [ 16 , 18 ].
Yet, male reproductive potential does decline with age, with age-specific fertility quite low by the 50s and onwards. In a prospective fecundability study of couples women aged 18—40 years , both men and women experienced increased infertility with age [ 19 ]. Humans, like most other species of mammals, also exhibit sex differences in senescence, which ultimately can be traced to sexual selection. Across various studies of mammals and birds in the wild, polygynous species typically display sex differences in mortality males having shorter lives, on average , whereas among monogamous species, male and female mortality profiles are similar [ 20 ].
Human males tend to live shorter lives on average than females, although at lessened intensity compared with, say, orangutans or gorillas. An evolutionary perspective on senescence helps make sense of age-related changes in the proximate mechanisms regulating sexual response [ 21 , 22 ]. For both females and males, general frailty declines at slower rates than our closest primate cousins, making it easier for older humans to remain sexually active they are alive, and better able to function, though subject to the influence of declining health.
Female peak fecundity occurs at ages 25—35, declines toward menopause and ceases altogether at menopause. With advancing age, the ovary reduces its capacity to both produce a fertilizable egg and release sex steroids that facilitate sexual response. Age-related declines in estrogen, especially during the menopausal transition, negatively impact vaginal lubrication, elasticity and vasocongestion and can also reduce sexual desire. Male reproductive function reveals declines in testosterone by the early 30s onwards, although there is also population variation in the pattern of decline.
Male declines in sexual desire and erectile function are more pronounced from the 60s onwards and may, in part, be traced to age-related declines in testosterone. All said, the mechanisms of female and male sexual function decline with age, patterns that can be understood with respect to the evolutionary background highlighted above and that would anticipate age-related changes in actual sexual behavior.
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These age-related changes in reproductive physiology also point to age-related shifts in aggression, courtship and childcare e. In parts of India, a couple is supposed to stop engaging in sex once their eldest son marries, a sexual recognition of shifting family priorities. In the 19th century Oneida commune of upstate New York, young men were sexually kept by postmenopausal women.
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Sexual Conflict in Humans
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Cognitive development: Biological theories. Anderman Ed. An evolutionary perspective on sex differences in mathematics and the sciences. Williams Eds. Levin Eds. Greenwich, CT: Information Age. Educating the Evolved Mind: Reflections and Refinements. Evolution of fatherhood. Biological and evolutionary contributions to developmental sex differences. Reproductive BioMedicine Online , 15 Supplement 2 , One mate or two? Life history traits and reproductive variation in low-income women. Acta Psychological Sinica, 39 , An evolutionary perspective on learning disability in mathematics.
Developmental Neuropsychology, 32 , The motivation to control and the evolution of general intelligence. Simpson Eds. New York: Guilford Publications.
Aging and Human Sexual Behavior: Biocultural Perspectives – A Mini-Review
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Developmental Review, 26 , Sex differences in social behavior and cognition: The utility of sexual selection for hypothesis generation. Conservation Land Management. Go to Conservation Land Management. Series: Science Masters. By: Jared Diamond Author. Click to have a closer look. Select version. About this book Customer reviews Biography Related titles. Images Additional images.
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