Life history theory

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Life history theory is an analytical framework widely used in evolutionary biology, ecology, psychology, and evolutionary anthropology which postulates that many of the physiological traits and behaviors of organisms may be best understood in terms of effects of natural selection on the key maturational and reproductive characteristics that define the life course.

Examples of these characteristics include:

• Age at weaning
• Age of sexual maturity or puberty
• Adult body size
• Age specific mortality schedules
• Age specific fecundity
• Time to first sexual activity or mating
• Time to first reproduction
• Duration of gestation
• Litter size
• Interbirth interval

Variations in these characteristics reflect differing allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions, especially growth, body maintenance, and reproduction. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another. For example, resources spent growing to a larger body size cannot be spent increasing the number of offspring. In general terms the costs of reproduction may be paid in terms of energy being diverted away from body repair and maintenance and by reducing investment in immunological competence. To understand these life histories researchers often begin by imagining a Darwinian Demon, a hypothetical organism for whom such trade-offs do not exist.

Thus the allocation of resources involves trade-offs. These trade-offs and strategies can be compared between species. Two of the most well-known trade-offs involve number of offspring (few or many) and timing of reproduction (accelerated maturation and reproduction versus delayed, allowing for larger size and more complex social supports). The extremes at the species level of these fundamental dimensions of reproduction were recognized long before life history theory, and are traditionally termed r/K selection theory. An r-selection strategy is the production of a large number of offspring (of whom only a minority may survive) as early in life as possible. The K-selection strategy is to produce a smaller number of "fitter" offspring with higher survival chances.

According to life history theory the individuals of a species are able to make limited shifts in reproductive strategies in response to the prevailing environments. Depending on abundance of resources and probable individual longevity, individuals consciously or unconsciously shift their reproductive strategy in one direction or the other to take advantage of available resources or to compensate for resource shortage or uncertainty.

Perspectives

Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility. A number of statistical predictions have been confirmed by social data and there is a large body of scientific literature from studies in experimental animal models, and naturalistic studies among many organisms.

See also

• Behavioral ecology
• Darwinian Demon
• Evolutionary developmental psychology
• Evolutionary physiology
• Human behavioral ecology
• Parental investment
• Reproductive effort
  • Mating effort
  • Parental effort
  • Nepotistic effort
• Somatic effort

References

• Charnov, E. L. (1993). Life history invariants. Oxford, England: Oxford University Press.
• Kozlowski, J and Wiegert, RG 1986. Optimal allocation to growth and reproduction. Theoretical Population Biology 29: 16-37.
• Roff, D. (1992). The evolution of life histories: Theory and analysis. New York:Chapman & Hall.
• Stearns, S. (1992). The evolution of life histories. Oxford, England: Oxford University Press.

Further reading

• Ellis, B.J. (2004). Timing of pubertal maturation in girls: an integrated life history approach. Psychological Bulletin. 130:920-58.
• Kaplan, H., K. Hill, J. Lancaster, and A.M. Hurtado. (2000). The Evolution of intelligence and the Human life history. Evolutionary Anthropology, 9(4): 156-184..
• Quinlan, R.J. (2007). Human parental effort and environmental risk. Proceedings of the Royal Society B: Biological Sciences, 274(1606):121-125.
• Vigil, J. M., Geary, D. C., & Byrd-Craven, J. (2005). A life history assessment of early childhood sexual abuse in women. Developmental Psychology, 41, 553-561.
• Walker, R., Gurven, M., Hill, K., Migliano, A., Chagnon, N., Djurovic, G., Hames, R., Hurtado, AM, Kaplan, H., Oliver, W., de Souza, R., Valeggia, C., Yamauchi, T. (2006). Growth rates, developmental markers and life histories in 21 small-scale societies. American Journal of Human Biology 18:295-311.
• Derek A. Roff (2007). Contributions of genomics to life-history theory. Nature Reviews Genetics 8, 116-125.
• Freeman, Scott and Herron, Jon C. 2007. Evolutionary Analysis 4th Ed: Aging and Other Life History Characteristics. 485-86, 514, 516.
• Kaplan, H.S., and AJ Robson, 2002. The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers. PNAS 99: 10221-10226.
• Kaplan, H.S., Lancaster, J.B., & Robson, 2003. Embodied Capital and the Evolutionary Economics Of the Human Lifespan. In: Lifespan: Evolutionary, Ecology and Demographic Perspectives, J.R. Carey & S. Tuljapakur (eds.) Population and Development Review 29, Supplement 2003, Pp. 152–182.