Monday, March 22, 2010

Evolution of Sex

There is obvious "asymmetry between males and females" (Coyne 157) in the sexually reproducing world. Compare the male and female mating strategies of various species. Include both behaviors and anatomical features in your explanation. How does the idea of parental investment explain these stark differences between males and females?


  1. One major difference between males and females in terms of their parental investment is that: "Males have nothing to lose by mating with a "substandard females." (157) According to Lindemann, a human male releases on average 280 million sperm into the female during intercourse. In addition, according to Campbell, a male continues to produce sperm throughout adulthood, infact hundreds of millions of new sperm are made each day throughout the seminiferous tubules. The process of spermatogenesis includes mitotic divisions that bring the primordial germ cell to a primary spermatocyte. Meiosis then divides the cell into secondary spermatocyte, then spermatids and eventually a complete sperm cell. Females on the other hand, begin with an oogonium which divides by mitosis and becomes a primary oocyte which is arrested in prophase I until puberty where the oocyte divides by meiosis and eventually matures to a complete ovum at fertilization. The process of oogenesis stops when the woman undergoes menopause around age 50 and ovulation and menstruation end. Women are borm with all the primary oocytes they will ever have, and thus they want to find the best mate to fertilize their eggs because they have a limited amount of eggs and a limited amount of time before menopause. As Coyne puts it, "the evolutionary difference between males and females is a matter of different investment" (157) Females have a larger investment becuase their eggs are more "expensive" than sperm, they must invest a lot of time and energy in pregnancy, and they spend more time, in most species, and effort caring for their children than do males. One example of a male anatomical feature is in the stalk eyed flies. Males try to impress the females with their long stalks during courtship, and research shows the females tend to choose the males with the longer stalks. Also, in zebra finches, the males tend to be more patterned and colorful in order to attract the females. One reason the females may not be so colorful is so that they do not stand out to predators (because they have invested so much enregy in reproduction as compared to males). Overall, the differences between the amount of energy the male or female puts into reproduction and how much they have to "lose" can be shown by their anatomical features.

  2. Here are my sources:

  3. The above comment provides a good explanation of the investment that a female must make in child care and the reasons for a male's bright colors and impressive anatomical features. However, there are also behavioral adaptations in males that influence mating. Coyne states that it is advantageous for males to be promiscuous (159). This is because in general, males don't do the majority of child care and produce sperm throughout their lifetime. To pass on their genes to the most possible offspring, males should try to mate as many times as possible during their lives. As a result, many species are polygamous and exhibit a system called polygyny, in which a male takes several females as mates (Campbell 1134). In these species, the male is often highly ornamented to attract females. For example, male elk grow large antlers as a show of strength to attract females, who must choose their mates carefully so as to make the best use of their limited supply of eggs (158). Male elk use these antlers in sparring battles against other elk, with the stronger one winning and claiming the watching females as mates. Another sexual behavior adaptation is the unusual mating ritual of the blue-footed booby, in which the male flaunts his bright blue feet in a dance for the female (Wikipedia). If the female likes the male's feet and dance, she chooses to mate with him. Over time, natural selection has selected this specific ritual as the most successful for males to attain a mate. In general, males tend to be more aggressive than females because they must compete for mates, while females can simply wait for the males to come to them (159).
    However, there are some species where the male contributes greatly to child care. One example is the seahorse, in which the males are impregnated by the females. The reason for this is that it allows for shorter birthing intervals, thus allowing more pregnancies and more offspring to be produced (Wikipedia). Most species where the male carries at least some of the load of child care tend to be monogamous, i.e. pairing one male with one female. In this case, the male only has one mate, so he will work harder to protect her and the offspring that they produce (Campbell 1134).