If females are mating with
multiple males, as I hope to establish in my field work, then sperm competition
may be important in shaping their mating strategies. Sperm competition
occurs whenever sperm from more than one male are in the female reproductive
tract at the same time. In birds, the last male to mate with a female
fertilizes more eggs than earlier males. This phenomenon (known as last
male sperm precedence) might occur because sperm are gradually lost from
the reproductive tract after mating. By the time a second male mates
with a female, sperm from the first male have gone down in numbers.
Because sperm are like lottery tickets - the more the better - the chances
of fatherhood by the second male are greater.
sperm stained with a fluorescent dye, to facilitate counting and determine the rate of sperm loss
This proposed mechanism for last male sperm precedence is expressed mathematically as the passive sperm loss model, and I am testing this model in captive wild-type Muscovy ducks. During summer 2000 I did an experiment to learn what the rate of sperm loss is for female Muscovies. During summer 2001 I will run mating trials in which a female mates with two males at a specified time interval and then lays a clutch of eggs. If I know the rate of sperm loss and the time interval between matings, I can predict the number of offspring that each male will fertilize (assuming they inseminate the same number of sperm - an assumption which needs to be tested!). If the predicted proportion of offspring sired by each male matches the observed proportions, then the passive sperm loss model can be considered a viable explanation for last male sperm precedence in Muscovy ducks.
Once the mechanism of sperm
competition is understood, it can be used to make predictions about behavioral
strategies in wild birds. Testing these predictions will then contribute
to a better understanding of the mating system.
wild-type Muscovy pair in outdoor pen
female Muscovy leaving nest box