The Darwin Awards Next Evolution: Chlorinating the Gene Pool by Wendy Northcutt Page A
Authors: Wendy Northcutt
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will ever see,” Dr. Brennan recollects. Not only are some duck penises long, and not only are some spiral-shaped, but some are even equipped with a feathery tip! To understand the basis for all this anatomical variation Dr. Brennan decided to examine reproductive systems in female ducks.
The results would surprise almost anyone but a duck. Females did not simply have a vaginal canal long enough to accommodate the phallus of a compatible drake. In species featuring a coil in the male phallus, vaginas spiral in the opposite direction. This intensifies the degree of struggle required to accomplish fertilization without cooperation. Some females also developed branching canals, complete with musculature to divert a penis from the proper canal to an infertile sac. Without a relaxed and compliant mate, these features dramatically reduce a male’s chances of successful fertilization.
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The feathery tip of a male duck phallus, a feature of those species most extreme in their incidence of forced copulation, appears to be used to swab out seminal fluid deposited by a prior mating. Observational data indicates that females also discourage fertilization through behaviors such as using water to cleanse themselves internally after an unwanted mating.
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The emerging picture suggested that duck reproduction was as far removed from the avian cloacal kiss as the cheetah’s sprint is from the ponderous stride of the sloth. Yet crucial questions remained. Were the unusual features of ducks’ phalluses similar to the male peacock’s train, adapted to an extreme because presentation plays a crucial role in sexual selection? Or was this instead a sort of evolutionary battle with each side escalating to gain control over the fertilization process?
Bird mating behaviors have long drawn the attention of ornithologists and hobbyist birdwatchers alike. Consider some of the spectacular techniques males employ to gain the attention of receptive females.
Zebra finch (Taeniopygia guttata) —elaborate and loud bird-song
Greater bird of paradise (Paradisaea apoda) —competitive dancing displays
Northern cardinal (Cardinalis cardinalis) —eye-catching red coloration
Great frigate bird (Fregata minor) —large inflatable throat bladder
Satin bowerbird (Ptilonorhynchus violaceus) —ornately decorated love nests
Based on the mating behavior of other bird species the case for a long male duck phallus as a visual enticement had some merit. Yet this did not explain why the reproductive anatomy of female ducks seemed adapted as much to frustrate as to accommodate the exotic endowments of their counterparts. Dr. Brennan advanced the hypothesis that the real driving force behind these adaptations was “postcopulatory competition.” To gather data she and her collaborators scrutinized the behavior and anatomy of sixteen species of waterfowl.
A complex empirical process known as phylogenetic analysis was able to establish that the unusual shapes in male and female duck genitalia evolved in tandem. This view of coevolution is further supported by a simple fact confirming one of Dr. Brennan’s key predictions. From one duck species to another the complexity of genital morphology varies in clear correlation with the incidence of forced copulation! This analysis enabled scientists to make precise predictions about the shape of duck features yet to be examined. Testable predictions are an essential part of the scientific process.
All that twisty anatomy is evolution’s response to the conflicting goals of males and females. Male ducks seek to produce as many offspring as possible, while female ducks seek the best father for a clutch of eggs. Drakes may attempt fertilization via forced copulation, but hens resist by refusing to relax a complex vaginal canal or by offering up reproductive dead ends to unwelcome mates. The mechanisms male ducks evolved to usurp the prerogative of sexual selection are marginalized by female countermeasures.
The fact
The results would surprise almost anyone but a duck. Females did not simply have a vaginal canal long enough to accommodate the phallus of a compatible drake. In species featuring a coil in the male phallus, vaginas spiral in the opposite direction. This intensifies the degree of struggle required to accomplish fertilization without cooperation. Some females also developed branching canals, complete with musculature to divert a penis from the proper canal to an infertile sac. Without a relaxed and compliant mate, these features dramatically reduce a male’s chances of successful fertilization.
----
The feathery tip of a male duck phallus, a feature of those species most extreme in their incidence of forced copulation, appears to be used to swab out seminal fluid deposited by a prior mating. Observational data indicates that females also discourage fertilization through behaviors such as using water to cleanse themselves internally after an unwanted mating.
----
The emerging picture suggested that duck reproduction was as far removed from the avian cloacal kiss as the cheetah’s sprint is from the ponderous stride of the sloth. Yet crucial questions remained. Were the unusual features of ducks’ phalluses similar to the male peacock’s train, adapted to an extreme because presentation plays a crucial role in sexual selection? Or was this instead a sort of evolutionary battle with each side escalating to gain control over the fertilization process?
Bird mating behaviors have long drawn the attention of ornithologists and hobbyist birdwatchers alike. Consider some of the spectacular techniques males employ to gain the attention of receptive females.
Zebra finch (Taeniopygia guttata) —elaborate and loud bird-song
Greater bird of paradise (Paradisaea apoda) —competitive dancing displays
Northern cardinal (Cardinalis cardinalis) —eye-catching red coloration
Great frigate bird (Fregata minor) —large inflatable throat bladder
Satin bowerbird (Ptilonorhynchus violaceus) —ornately decorated love nests
Based on the mating behavior of other bird species the case for a long male duck phallus as a visual enticement had some merit. Yet this did not explain why the reproductive anatomy of female ducks seemed adapted as much to frustrate as to accommodate the exotic endowments of their counterparts. Dr. Brennan advanced the hypothesis that the real driving force behind these adaptations was “postcopulatory competition.” To gather data she and her collaborators scrutinized the behavior and anatomy of sixteen species of waterfowl.
A complex empirical process known as phylogenetic analysis was able to establish that the unusual shapes in male and female duck genitalia evolved in tandem. This view of coevolution is further supported by a simple fact confirming one of Dr. Brennan’s key predictions. From one duck species to another the complexity of genital morphology varies in clear correlation with the incidence of forced copulation! This analysis enabled scientists to make precise predictions about the shape of duck features yet to be examined. Testable predictions are an essential part of the scientific process.
All that twisty anatomy is evolution’s response to the conflicting goals of males and females. Male ducks seek to produce as many offspring as possible, while female ducks seek the best father for a clutch of eggs. Drakes may attempt fertilization via forced copulation, but hens resist by refusing to relax a complex vaginal canal or by offering up reproductive dead ends to unwelcome mates. The mechanisms male ducks evolved to usurp the prerogative of sexual selection are marginalized by female countermeasures.
The fact
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