Previous observations of forced copulation (FC) in captive Mallards (Anas platyrhynchos) showed that most FC attempts were directed at females in prelaying and laying condition and that most FC's occurred in the morning when the females were leaving their nests after egg laying. In order to determine whether or not there is a physiological basis for these observed temporal patterns, sperm competition in captive Mallards was examined using artificial insemination and genetic markers. Results indicated that if a female was inseminated with two competing doses of semen at different time intervals, the proportion of progeny from the first and the second inseminations was not significantly different if these inseminations were simultaneous, 1 h, or 3 h apart. There was a preponderance of progeny (70%) from the second insemination, however, if the inseminations were 6 h apart. Insemination of females less than 1 h after egg laying resulted in 25% of the eggs laid the following morning being fertile. Only 1 of 179 eggs laid the following morning was fertile when the females were inseminated more than 1 h after egg laying.
Our experiment demonstrated that there is an insemination "window," a short period when new sperm are least likely to meet competition from sperm already in the oviduct and from sperm introduced later, and it provided a possible explanation for the observed timing of FC attempts.
A random-bred strain of domestic chickens (Gallus gallus) in single-male and two-male mating groups was used as a model to study strategies of mate guarding and sperm competition. The experiment was designed to examine influences of (a) the presence of a rival male, (b) dominance status of the males in the two-male groups, and (c) the time of day on the frequencies of male courtship and copulations. In the absence of rivalry, male chickens copulated infrequently in the morning and increased the frequency of copulations throughout the day until a maximum was reached in late afternoon. In the two-male groups the dominant males copulated more frequently than the subordinate males. They also competed by altering their diurnal pattern of mounting attempts and increasing their rate of successful copulation per attempt. The subordinate males made frequent attempts to mount the females regardless of whether they were receptive. However, their success rate was low because of interference by the dominant males, especially at that time of the day when a successful insemination would likely fertilize eggs. The experiment showed that not only the relative numbers of copulations by different males, but also the timing and success of copulations by different males may influence the probability of paternity. Furthermore, the interplay of proximate mechanisms and evolutionary factors contributed to variations in mating strategies.
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