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Female Water Striders Expose Their Genitalia Only After Males 'Sing'

Date:
June 11, 2009
Source:
Public Library of Science
Summary:
Biologists report that by evolving a morphological shield to protect their genitalia from males' forceful copulatory attempts, females of an Asian species of water strider seem to "win" the evolutionary arms race between the sexes. Instead, females only expose their genitalia for copulation after males produce a courtship "song" by tapping the water surface.
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A male Gerris gracilicornis is producing courtship signals by tapping the water surface with their middle legs.
Credit: Chang Seok Han

Chang Seok Han and Piotr Jablonski at Seoul National University, Korea have found that by evolving a morphological shield to protect their genitalia from males' forceful copulatory attempts, females of an Asian species of water strider seem to "win" the evolutionary arms race between the sexes. Instead, females only expose their genitalia for copulation after males produce a courtship "song" by tapping the water surface.

150 years after the publication of Charles Darwin's On the Origin of Species, Han and Jablonski used common insects, water striders, to study the intricacies of evolutionary conflict between males and females. The mechanisms for the way Darwinian natural selection, acting separately on males and females, result in different traits in males than in females (for example, different body sizes to guarantee the highest number of offspring during an individual's lifetime) are already quite well understood.

Sometimes, however, a behavioral trait, such as mating frequency, depends on both the male and the female characteristics. Natural selection favors higher mating frequency in males than in females in many animals, including humans. This leads to an evolutionary "arms race" where males evolve adaptations that force females to mate, while females evolve defenses against males' attempts.

As in the arms races between countries and political powers, it is rare for one sex to "win" in this evolutionary race.

However, in the study by Han and Jablonski, carried out at the Laboratory of Behavioral Ecology and Evolution at Seoul National University, females of an Asian species of water striders, Gerris gracilicornis, do seem to win this race as they have evolved a morphological shield behind which their genitalia are hidden from males, protecting them against the males' forceful attempts to mate.

In an apparent response to the female adaptation, after the violent mounting onto the female's back (typical in water striders), males of this species produce courtship signals by tapping the water surface with their middle legs. It is only after receiving the male's "song" that females expose their genitalia for copulation


Story Source:

The above story is based on materials provided by Public Library of Science. Note: Materials may be edited for content and length.


Journal Reference:

  1. Han et al. Female Genitalia Concealment Promotes Intimate Male Courtship in a Water Strider. PLoS ONE, 2009; 4 (6): e5793 DOI: 10.1371/journal.pone.0005793

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Public Library of Science. "Female Water Striders Expose Their Genitalia Only After Males 'Sing'." ScienceDaily. ScienceDaily, 11 June 2009. <www.sciencedaily.com/releases/2009/06/090611110831.htm>.
Public Library of Science. (2009, June 11). Female Water Striders Expose Their Genitalia Only After Males 'Sing'. ScienceDaily. Retrieved May 24, 2015 from www.sciencedaily.com/releases/2009/06/090611110831.htm
Public Library of Science. "Female Water Striders Expose Their Genitalia Only After Males 'Sing'." ScienceDaily. www.sciencedaily.com/releases/2009/06/090611110831.htm (accessed May 24, 2015).

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