Key Terms in Evolutionary Biology of Sex, Gender, and Sexuality: Parental investment theory

= Definition =

Parental investment theory was developed by Robert L. Trivers, an American evolutionary biologist and sociobiologist. He defined “parental investment” in 1972 as any time, energy, resources, and risk that a parent uses for the benefit of their offspring. David Barash added to this, “Every expenditure of this sort carries a cost along with it, and an element in this cost is a reduction in the ability of the investing parent to rear additional successful offspring” (Barash 1981, pg. 56). According to this theory, the sex investing the most and having the most to lose (the female) in reproduction will be the choosier sex, causing the opposite sex (the male) to be more competitive and aggressive in pursuing it.

= Example(s) of use in context =

In The Whisperings Within (1981) David Barash introduces the biological differences between the male and female that lead to different reproductive investments. Human females produce a small number of large sex cells, eggs, while men are able to produce a large number of small sex cells, sperm (Barash pg. 46). Females are the ones who gestate offspring and must undergo all the mental and physical changes that come with pregnancy as well as nursing the offspring once it’s born (Barash pg. 47). Pregnancy also limits the amount of offspring a female can produce, and when she is pregnant, she cannot reproduce again (Barash pg. 56). As Barash writes, “For males, reproduction is easy, a small amount of time, a small amount of semen and the potential evolutionary return is very great is offspring are produced.” Once a male has copulated, it does not limit his ability to copulate again and for the best evolutionary success, it makes the most sense for him to find another female to mate with. Therefore, it is much more costly for a female to be involved in reproduction than for a male, she has a lot more to give and a lot more to lose.

= History =

Robert Trivers begins his paper “Parental Investment and Sexual Selection” (1972) by discussing how Charles Darwin’s approach to sexual selection was easily confused because he didn’t present a structure in which he could relate key variables of the theory. Trivers says that his paper is “…a general framework within which to consider sexual selection. In it I attempt to define and interrelate the key variables” (Trivers pg. 137). Trivers references Angus John Bateman’s study on fruit flies, Drosophila melanogaster, in 1948 in which he stated that since male Drosophila invest a small amount of metabolic energy when producing one sex cell, a male’s reproductive success is limited by his ability to fertilize eggs with his sperm (Trivers pg.138). A female’s reproductive success is limited by her ability to produce eggs, which requires a large investment of metabolic energy (Trivers pg. 138).

= Debates =

Many evolutionary biologists have greatly criticized Bateman’s principle because they believe it doesn’t apply to a vast number of species in the animal kingdom and also includes statistical errors, poor genetic techniques, and oversights (Snyder &amp; Gowaty 2007). Snyder and Gowaty say that Bateman’s study “…had flaws, including the elimination of genetic variance, sampling biases, miscalculations of fitness variances, statistical pseudo-replication, and selective presentation of data. We conclude that Bateman's results are unreliable, his conclusions are questionable, and his observed variances are similar to those expected under random mating” (Snyder and Gowaty pg. 2457). Zuleyma Tang-Martinez and T. Brandt Ryder also critiqued Bateman and consequently, Trivers, in 2005. They believe that Bateman’s notion that female reproductive success peaks after one mating is not justifiable because in many species, females that mate with many males have a higher reproductive success than monogamous females (Tang-Martinez &amp; Ryder 2005). They also say, “Among Bateman’s most important popularizers is Trivers, whose influential paper on parental investment perpetuated the stereotypes of indiscriminate males and sexually restrained females. Trivers assumes that male and female parental and sexual behaviors can be explained by anisogamy...” (Tang-Martinez &amp; Ryder pg. 824).

= References  =

Barash, David. The Whisperings Within. Penguin, 1981. Print. Hewett, Caspar. "The Great Debate: Sexual Selection".The Great Debate: Home Page. Web. &lt;http://www.thegreatdebate.org.uk/sexualselection.html&gt;.

Snyder, Brian, Gowaty, Patricia Adair, s  "A Reappraisal of Bateman's Classic Study of Intrasexual Selection", Evolution 61: 2457-2468 (2007).

Tang-Martinez, Zuleyma, Ryder, T. Brandt "The Problems with Paradigms: Bateman's Worldview as a Case Study", Integr Comp Biol 45: 821-830 (2005).

Trivers, R.L. (1972). "Parental investment and Sexual Selection". In B. Campbell (Ed.), Sexual Selection and the Descent of Man: The Darwinian Pivot. (pg. 136-179). Chicago, IL: Aldine.

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= Author =

Erica Lupinacci