Female ducks are choosy when it comes to mating. Some male mallards (Anas platyrhynchos) are much more attractive to females than others, and females that mate with these ‘preferred’ males seem to raise more chicks to adulthood. It has generally been assumed that this bias reflects a genetic advantage conferred by the father.

Credit: SCOTT NIELSEN

Elsewhere in this issue (Nature 404, 74–77; 2000), Emma Cunningham and Andrew Russell propose a different explanation. They have compared the clutches of eggs produced by females after mating with more attractive males with those produced by females mated with less attractive males. They find that pairings with preferred males result in bigger eggs being laid.

Chicks that hatch from large eggs are more likely to survive the critical first few days after hatching. So the higher viability of the offspring of attractive males may have nothing to do with the genetic legacy of the father — instead, it may result from increased maternal investment in the eggs. When Cunningham and Russell controlled for egg size, the attractiveness of the father made no difference to the health of the chicks.

So why do female ducks invest more energy in eggs that are fathered by more attractive males? It is likely that male attractiveness is linked to some characteristic that makes their offspring more successful in the long run. Females would then invest more in these eggs to further their own breeding success.

Male mallards are not good fathers — females do all the work of rearing chicks. So males cannot be selected by females for their paternal qualities. But they do defend feeding areas around their mates during the breeding season, so attractiveness may be linked to the ability of a male to fend off other mallards. This ties in with the fact that females prefer males from early-hatching clutches, who tend to be bigger and stronger.

Cunningham and Russell point out that researchers need to be careful, when studying the influence of male attractiveness or dominance on the viability of offspring, to allow for the effects of differential maternal investment rather than attributing all differences to paternal genetics.