The Duck Penis, Part I: An Existential Quandary



Existence precedes tumescence, one might say of penises. Thus, dear reader, before getting into the nitty-gritty of the duck penis and duck erections, I would like to first address the unusual existential quandary they pose.

The quandary is this: Only about 3% of bird species have a penis (Briskie and Montgomerie 1997). In the other 97% the external genitals of both males and females is a nubby little orifice called the cloaca, and mating is a simple matter of bumping nubs together for few seconds in a so-called “cloacal kiss.”

However, nearly all duck and waterfowl species do have a penis. You may ask: why should this be the case? Why the Being of the duck penis and not the Nothingness of the cloaca? What is the meaning of the existence of the duck penis?

Penis scientists have struggled for decades to explain the duck penis. Some early hypotheses (King 1981, Lake 1981) were that the duck penis might exist to prevent water damage to sperm or it may help maintain genital contact in aquatic environments – a feat to accomplish, especially while swimming (trust me).

But any hypothesis setting out to explain why ducks might have “evolved” a penis is to assert the Absurd. As Briskie and Montgomerie (1997) point out, the real mystery is not “why do ducks have a penis?” but rather “why don’t most birds have a penis?”, because birds are the only vertebrate class that is predominantly penis-less. Also, as recently revealed by Prof. Patricia Brennan – the inspiration for Duck Penis Month – birds with penises belong to a diversity of lineages and yet all have a penis very similar in morphology, suggesting that a common ancestor for these lineages had a penis, too (Brennan and Plum 2012, summarized here; see also Brennan et al. 2008). Which is to say having a penis should be the default setting for birds.

Think about it: a penis is absolute evolutionary boon for males. All other things being equal, a male with a penis is more likely to get a female pregnant than a male without one, simply because it is better able to position its sperm closer to the egg.

For females, however, the existence of a penis can be the source of great Angst and Despair – evolutionarily speaking. A female is burdened with raising the offspring of any male that is able to force a mating with his penis, effectively eliminating any freedom she has to choose a mate. But if the cost of wasting an egg is low, females can regain the evolutionary upper hand by simply choosing to aborting or abandoning those eggs impregnated by unwanted matings. Then, because forced matings are no longer a winning reproductive strategy for males, the evolutionary costs of having a penis – the costs due to injury or disease, say – will outweigh the benefits, and selection will favor males without a penis. This is what Briskie and Montgomerie refer to as their female choice hypothesis.

But if the cost is too high for females to waste eggs, the above scenario cannot play out, and there is still a large enough benefit for males to retain a penis. Moreover, a penis can be an added benefit if many other males mate with the same female, since it can help to flush out sperm from competitors, a role which the penis is observed to perform in many other animals (see, for instance, Waage 1979). This hypothesis, that the penis aids in ensuring paternity for males, Briskie and Montgomerie call the sperm competition hypothesis.

To support these hypotheses, Briskie and Montgomerie show that in bird species which have penises the females lay larger and hence more costly eggs than in species without a penis, as would be predicted by the female choice hypothesis. Further support comes from the variation seen in duck penises themselves. A study led by Prof. Brennan – which we will examine in more detail in a later post – found that larger and more elaborated duck penises belong to species where competition among males over females is more fierce and where males are more likely to engage in forced matings, as predicted by the sperm competition hypothesis (Brennan et al. 2007).

Thus we are lead to a conclusion from which I believe there is No Exit: the duck penis is most likely to exist because of intrasexual selection between males, and a lack of intersexual selection by female choice to select against it.

References

Briskie, J. V., & Montgomerie, R. (1997). Sexual selection and the intromittent organ of birds. Journal of Avian Biology, 73-86. (pdf)

Brennan, P. L., Prum, R. O., McCracken, K. G., Sorenson, M. D., Wilson, R. E., & Birkhead, T. R. (2007). Coevolution of male and female genital morphology in waterfowl. PLoS one, 2(5), e418.

Brennan, P. L., Birkhead, T. R., Zyskowski, K., Van Der Waag, J., & Prum, R. O. (2008). Independent evolutionary reductions of the phallus in basal birds. Journal of Avian Biology, 39(5), 487-492. (pdf)

Brennan, P. L., & Prum, R. O. (2012). The erection mechanism of the ratite penis. Journal of Zoology, 286(2), 140-144. (summary at Nature.com).

King, A. S. (1981). Phallus. – In: King, A. S. and McLelland, J. (eds). Form and function in birds, Vol. 2. Academic Press, London, 107-147.

Lake, P. E. (1981). Male genital organs. – In: King, A. S. and McLelland, J. (eds). Form and function in birds, Vol. 2. Academic Press, London, 1-61.

Waage, J. K. (1979). Dual function of the damselfly penis: sperm removal and transfer. Science, 203(4383), 916-918.