Everybody loves ducks. Kids going to local ponds to feed ducks with bread it´s a shared picture in our collective hive-mind. Waterfowls are present in our parks and built-up “naturalize” environments in urban areas and they help us to re-connect with nature.
By the way, ducks are dabblers and divers, with scoop-like bills specifically designed to get food by filtering water thanks to a special structure within their beaks called lamellae (very similar to baleen plates in whales, isn’t it cool?), meaning that bread IS BAD FOR THEM[1]. Bread it’s pretty much like junk food for these birds, without any nutritional value. Instead, grapes, corn, grains or bugs are ideal, so load your bags with those products in your next visit to the pond.
Anyway, let’s move on to what really matters about ducks: sex (what else?). Reproduction in the natural world is an amazing fact, from asexual to sexual reproduction, there are countless ways in which different species find their way to increase their population and pass their genetic code to the next generation. Birds are quite diverse and ingenious in this aspect.
Mating usually involves an egg that needs to be fertilized to develop an embryo, that´s the general theory. However, parthenogenesis or the “virgin Mary miracle” can happen in the avian world too. Apparently, Chinese painted quail (Coturnix chinensis) and Commercial turkey (Meleagris gallopavo) are some of the bird species that are able to produce offspring without mating. The downside is that the progeny is usually a male and reduces the genetic pool[2]. That could be the reason why sexual reproduction is the main reproductive driver among the majority of vertebrates, because it triggers evolution. But it also implies that individuals of different gender have to interact through courtship for the ritual of mating, in order to produce viable offspring.
Birds are great in the fine art of seduction. If you don’t believe me, take a look at the birds of paradise, or the so-called “lekking” in which an aggregation of males gather in high-quality resources areas to attract females, a famous case is the one from Lance-tailed Manakins (Chiroxiphia lanceolata). The males literally team-up in order to favour the mating between the alpha male and the female (psss beta males get their chance when alpha is absent or distracted[3]).
In general, we can classify avian mating systems in three, and their corresponding social interactions such as long/short term bonds, mate guarding or parental care:
Monogamy (we all know this one)
Polyandry (many males, one female)
Polygyny (many females, one male)
And what about the ducks? Why are they, somehow, interesting (at least for me)? Well, first of all let’s take a look at the anatomy of some of them, like mallards (Anas platyrhynchos). Males have what’s known as “cloaca protuberance”, or penis in other words (check the video!). Why, on Earth, would they have such a thing? One of the explanations given by McKinney and colleagues (1983)[4] is that they aren’t entirely monogamous. When large aggregations of waterfowls occur, it’s time for party! This party can be translate as polyandry (remember many males, one female), sounds cool, right? An example of females empowering and enjoying their sexuality without any moral restrictions in the natural world?
Well…not so good for females, though. Despite being, generally speaking, monogamous, some species of waterfowls (~50) will go for what’s is known as forced copulation or forced extrapair copulation (males which are already paired, trying to find an extra hook up)[5]. Picture this: few males will chase a single female and will try to mount her in the water, where the intromittent organ comes into play. Females have some alternatives against this aggression.
Run away: few horny and sexually motivated ducks, probably youngsters, and full of energy? Hard.
Resist and fight: They are more, and they will attempt to mount her anyway, which can lead to lethal injuries or drive her underwater, with subsequent drowning. Hard.
Submit: Sounds horrible but this is the strategy that some waterfowl females have chosen. And it explains why males have such a weird-corkscrew looking penis.
This is a classic example of evolutionary arms race [6], in which female ducks develop an adaptation in their bodies to compensate an external input, in this case the “penis” and forced copulation. Females have developed an intricate cloaca system that leads to the vagina, which resembles to a labyrinth. In this case, they will submit to the males, to reduce the effect of the aggression, but their sperm will end up in the “wrong end”, so the egg won’t be fertilized.
But, why are females still resisting forced copulation if they have such a great defensive system? They could avoid all the hassle and troubles by simple submission, right? According to Adler (2009), this could be an example of indirect selection. Females adapt their strategies under a conflict-based mating system, by “selecting” only high-quality males[7]. This system is not failure-proof as the vagina system shows, meaning that they need it in order to avoid low-quality males.
Lesson learn today: don´t be a duck! Roses (or cauliflowers, depends on your taste) and a nice dinner will assure you more mating chances. And if you don´t like the mating system of some waterfowls, well….keep bringing bread to your local pond.
Peace
References [1] Department of Environmental Conservation, New York State https://www.dec.ny.gov/animals/7001.html
[2] Ramachandran, R, & McDaniel, C D. (2018). Parthenogenesis in birds: a review, Reproduction, 155 (6), R245-R257.
[3] DuVal Emily H and Kempenaers Bart (2008). Sexual selection in a lekking bird: the relative opportunity for selection by female choice and male competition. Proc. R. Soc. B.2751995–2003.
[4] McKinney, F., Derrickson, S., & Mineau, P. (1983). Forced Copulation in Waterfowl. Behaviour, 86(3/4), 250-294.
[5] McKinney, F., & Evarts, S. (1998). Sexual Coercion in Waterfowl and Other Birds. Ornithological Monographs, (49), 163-195.
[6] Dawkins Richard and Krebs John Richard 1979Arms races between and within species. Proc. R. Soc. Lond. B.205489–511. [8] Margo Adler, Sexual conflict in waterfowl: why do females resist extrapair copulations?, Behavioral Ecology, Volume 21, Issue 1, January-February 2010, Pages 182–192.