To us humans, ducks are a symbol of tranquility, and one of a select group of avian species that cohabitates with humans in urban areas. Yet the adorableness of the duck family you might observe at the park belies their bizarre sexual behavior, which truly put the human sexual imagination to shame.
Indeed, ducks are not merely one of the few species to have mastered land, water and air, but they also have some very weirdly-shaped genitals, which have co-evolved through a process called sexual conflict.
Sexual conflict occurs when there is a difference in the mating strategies benefitting the male and the female of a species. These different reproductive goals in ducks and other waterfowl have ultimately led to the coevolution of complex and very weird genital structures – long, anticlockwise corkscrew phalluses, and vaginas that have dead ends and a clockwise corkscrew goings in the opposite direction. Ouch.
How did ducks evolve such odd genitals?
Ducks are mostly socially monogamous, forming a bond with a single partner per breeding season. These bonds are formed by female choice, while the males compete by giving vocal and visual ritual displays in which they show off their colorful plumage to prove their "quality."
Yet while ducks have a reputation for being sweet, innocent birds, forced copulation outside of the chosen mating pair is pretty common, and is observed in one-third of all waterfowl species — including mallards, pintails and gadwalls.
So why do certain species of ducks perform this horrible forced copulation? Dr. Patricia Brennan, a biology professor at Mt. Holyoke who is an expert in bird genitalia, says it may stem from the sex ratio among duck populations.
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"There are more males than females in most duck populations," Brennan says. "Since most ducks are socially monogamous, there are typically many males that are left without a mate. Those males sometimes turn to forced copulations as a way of gaining some paternity any given year. Males who had a partner also can perform forced copulations after their partner is incubating the eggs."
In other words, the male wants to maximize his reproductive success and the number of offspring that he sires, which is why extra pair copulation — meaning, cheating by mating with someone else besides his monogamous partner, then ditching her — is useful to him. The female only wants to mate with the male she chose as her mating partner, wanting her offspring to inherit the genetic benefits of that male only; thus, she doesn't want to extra-pair copulate.
This "forced extra-pair copulation," as it's known, is a fairly rare strategy across birds as a whole. So why do so many waterfowl species do this? It's because they can – they have an organ that lets them.
Big duck energy
Ducks, among other waterfowl, are one of only 3% of bird species that even have a penis at all. Waterfowl are therefore quite different compared to other birds. Their penises can range between 1.5 centimeters to 40 centimeters— sometimes longer than the bird himself — and vary hugely in terms of elaboration.
Ducks' evolution of a penis is thought to have facilitated males being able to force copulations to gain control of fertilization. In fact, the length of the male penis has been shown to be significantly correlated with the frequency of forced extra-pair copulation seen in that species.
This is crucial to the coevolution of the male and female genitals, as the female has coevolved defenses against the males' advances to gain back control.
Not only do waterfowl have very, very long penises, but they also have an anticlockwise corkscrew shape. Dr Brennan thinks that the spiral penis shape is an adaptation to increase the male's chance of successful fertilization.
"Female birds only have one active oviduct on the left side, so the penis has to bend to the left in order for it to go in the oviduct", she says. "To make the penis bend left, one part is shorter and one part is longer, so as the penis grows, there is a spiral."
The male appears to have the upper hand in term of fertilization objectives. But, the female does not just let this happen.
The fempire strikes quack
Firstly, females always resist all forced extra-pair copulation, at large direct costs to themselves. Female ducks can actually make their injuries worse by struggling, but do it nearly without fail in order to reduce the success of the extra-pair male.
The female duck has also evolved a vagina structure to combat males: it corkscrews in up to eight 360-degree spirals in the opposite direction to the male's penis, and has up to three dead-end pockets near to the cloaca, which is a catch-all organ in avians for both reproduction and waste excretion. The spirals are thought serve as barrier to entry without the female's cooperation, preventing the penis from entering or at least fully everting, thus forcing him to deposit his sperm much lower in her reproductive tract.
The duck vagina also has several pockets, thought to impede sperm deposition further inside.
"Females benefit by preventing these unwanted males from siring any of her young. That means her chosen partner gets most of the paternity, and therefore she gains the benefit through her offspring," says Dr Brennan.
The females choose their mates for their perceived quality; therefore, her male offspring will benefit by gaining his mother's intended mate's genes – his sons will be more likely to be chosen as a mate too. In mallards, while forced extra-pair copulation can constitute up to 40% of all sexual encounters, only 2-5% of offspring are actually sired in the act of forced copulation. This indicates that the female's adaptations do indeed lower the extra-pair male's chances of successfully fertilising her.
Additionally, in mallards, offspring viability has been experimentally shown to be lower when females are prevented from reproducing with the males that they prefer, further supporting the main benefit of her funky vaginal shape in giving her her choice of partners.
So, the weird and wacky genitals of ducks are a result of millennia of coevolution, with the males and females both racing to one-up the other in terms of reproductive outcome. This sexual conflict-driven genital evolution can be seen in other species across the animal kingdom, from bedbugs' traumatic insemination (the male stabs the female in the abdomen with his penis) to flatworm hermaphrodite penis swordfights (really). Humans may pride ourselves for our sexual inventiveness, but compared to the animal world, we are real vanilla.