Ducks give larger eggs when playing with preferred drakes

If you are an attractive duck, you have something to quack about. Not only will the females prefer you over rivals, but new research has also shown the females will go on to produce larger eggs for you, ensuring a better start for your offspring.

The females don't produce more eggs, they produce better, larger eggs for the preferred males. The extra effort gives the ducklings a head start compared to ducklings sired by lesser males.

This unusual finding comes from research by Dr Emma Cunningham and Dr Andrew Russell, both formerly of the University of Sheffield and now with the department of zoology at the University of Cambridge. They report their findings in the current issue of Nature.

Their study involved mallard (Anas platyrhynchos). "We found that individual females laid significantly larger eggs when paired to more attractive males. Egg size is a critical trait influencing fitness in birds," the authors explained. "This difference in egg investment resulted in females producing significantly heavier chicks when they were assigned to preferred males than when assigned to less preferred males."

So how do you know if you are a preferred male? The researchers took two groups of 20 males and 20 females and let them mingle. Over the subsequent pairing period, males were ranked for attractiveness to females by recording the number of females that directed pairing displays toward them. The researchers described this as a "reliable measure", with results that could readily be duplicated. They then began the laborious task of allowing pairings and then making off with the eggs that resulted, comparing them with controls to ensure statistical equivalence. Pairings with the lower-ranking drakes resulted in smaller eggs, whereas the higher-ranking drakes caused the ducks to produce larger eggs. The team controlled for any contribution from the males, for example arising from having comparatively better genes.

"This is the first study to our knowledge to show that female investment, in terms of egg volume, may vary according to male attractiveness, even though males appear to provide no direct material benefits to females," the researchers noted.

"The question then is why do some females lay eggs of different sizes when assigned to preferred and less preferred males. Females may alter their egg investment in response to male attractiveness because attractive males are more likely to sire successful offspring."

Unfortunately, the researchers were unable to determine what it was in particular that impressed the females when they considered prospective mates. Naturally they were unable to say, but would they have even if they could?