Dabbling ducks, such as the mallard duck, have a sex life that can only be described as quite unromantic. Mallards do not form long-term pair bonds, but instead form a new pair bond each year.
Pairing takes place in autumn, when male ducks actively court the females. The females then choose to pair with the male they find most attractive, and she nods her head back and forth to indicate to the male that she has chosen him.
The couple then pair up for the winter and come spring they start their mating rituals. Despite the bond between the pair, male mallards will frequently pursue females other than their own, and may even force matings with these additional females.
These male intruders can be so insistent that they may interrupt a female trying to mate with her preferred male, and it is not an uncommon sight to see several males simultaneously chasing a single female and then trying to mate with her.
– These chases and forced matings can have serious consequences for the female, who is at risk of injury and death. In fact, the potential consequences for females are extreme, with at least one study estimating that up to 10 percent of the female mallard ducks in a population are killed annually as a result of injury and drowning during these forced mating attempts, explains scientist Melissah Rowe at the Department of Biosciences and the Natural History Museum.
Who fathered Huey, Dewey and Louie?
The result of these behaviours is that the nest of a female mallard duck can contain eggs fertilized by multiple males, that is eggs fertilized by the chosen pair male and those fertilized by these forced extra-pair matings.
Therefore it may be a telling tale that Huey, Dewey and Louie, in the comic cartoon strip Donald Duck, live with their uncle - Uncle Donald. Among ducks it is simply too difficult to know who the father is. The normal number of eggs in a mallard duck nest (known as clutch size) varies between one and thirteen eggs.
Melissah Rowe explains that evolution is very much in play when it comes to the sex life of mallard ducks. Firstly, the male ducks compete with each other to be chosen by the females by performing a number of courtship displays, such as by shaking or flicking their head from side-to-side or raising up in the water and vigorously flapping their wings.
During the breeding season, females will mate with the male they are paired to, but may also mate with additional males, referred to as extra-pair copulations, including those matings that are forced. Consequently, the female can have sperm from multiple males in her oviduct at the same time.
Genitalia as corkscrews
The genitalia of birds are very different to those found in mammals. Most notably, birds have a joint opening – named the cloaca – through which passes both waste and semen in males, and waste and eggs in females.
A mating between the male and female is referred to as a ‘cloacal kiss’, whereby male and the female press their cloacal openings against each other, and the male subsequently ejaculates into the female reproductive tract. Typically, the ritual lasts no more than a few seconds.
Ducks, such as the mallard, differ from most other birds by the fact that they have a penis, which is shaped like a corkscrew that curls counterclockwise.
The American scientist Patricia Brennan, a collaborator of Rowe, has shown that the female mallard ducks have a reproductive tract also shaped like a corkscrew, but that it curls the other way compared to the male, it turns clockwise. Moreover, the vagina has a number of blind pouches.
This morphology appears to prevent the male penis fully everting during copulation, and thus possibly helps to prevent fertilization of eggs by unwanted males.
– One hypothesis is that the female can prioritize the sperm from her favorite male by relaxing muscles surrounding the oviduct and thus allowing the penis to bypass the ‘dead-ends’ in the vagina and ejaculate his sperm closer to the site of egg fertilisation, explains Rowe.
Cryptic female choice
Biologists call this phenomenon “cryptic female choice”, that is females are able to ‘choose’ or ‘select’ sperm from specific males. In mallards, it is thought that cryptic female choice may occur because sperm from a favoured male is placed in a more favourable location in the oviduct.
It may also occur through other processes, such as the female reproductive tract exhibiting a lower immune response to sperm from a preferred mating partner.
However, the competition doesn’t end there. As well as cryptic female choice, the sperm from rival males can compete with one another to fertilize the female’s eggs, a process referred to as sperm competition.
– In some instances, it seems that the male with the fastest sperm will have an advantage and fertilize the majority of the eggs. In some species, however, sperm size and shape may also be important such that longer sperm will be advantageous and males with longer sperm will fertilize more eggs.
– However, we still have so much to learn about which sperm and/or ejaculates traits are important to male success in the majority of species, says Rowe.
Provoked “Right wing America”
Rowe explains that Patricia Brennan has published groundbreaking research, but that she has also met some surprising obstacles along the way. In 2013, she suddenly got the attention of “Right wing America”, including the conservative TV station Fox News.
They proclaimed it was a “waste of government money” to study duck genitalia and started campaigning against Brennan and this type of wasteful spending. Moreover, they tried to use her research as an example of how the Obama administration wasted money on ridiculous research, but failed to notice that Brennan’s research was originally funded under the Bush administration.
– Just use the hashtag #duckpenisgate on Twitter and you will get an impression on exactly what Brennan was up against. Luckily, she did strike back and wrote a number of excellent hard hitting articles on the value of basic research, explains Rowe.
Also read at Titan.uio.no:
Melissah Rowe et al.: Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology. Evolution Volume 69, Issue 4, pages 10441052, April 2015
Melissah Rowe et al.: Extra-pair paternity, sperm competition and their evolutionary consequences in the Maluridae. EMU Austral Ornithology, 113, 218231.
Melissah Rowe et al.: Evolution of sperm structure and energetics in passerine birds. Proceedings of the Royal Society, Biological Sciences, Published 2 January 2013. DOI: 10.1098/rspb.2012.2616
Matt Kaplan: The sex wars of Ducks. Nature, 23.12 2009
Patricia Brennan: Why I study Duck Genitalia. Slate.com, APRIL 2 2013
Ryan Jacobs: In Defense of Studying the Duck Penis. Pacific Standard, 5. mars 2014
Yngve Vogt: Fuglenes sædceller konkurrerer i et evig race. Apollon og forskning.no, 6. mai 2014