Learning in intimate connections: Conditioned fertility and its role in sexual competition.

BACKGROUND: Studies of sexual conditioning typically focus on the development of conditioned responses to a stimulus that precedes and has become associated with a sexual unconditioned stimulus (US). Such a sexually conditioned stimulus (CS) provides the opportunity for feed-forward regulation of sexual behavior, which improves the efficiency and effectiveness of the sexual activity. OBJECTIVE AND DESIGN: The present experiments were conducted to provide evidence of such feed-forward regulation of sexual behavior in laboratory studies with domesticated quail by measuring how many fertilized eggs were produced by the female after the sexual encounter. During the conditioning phase, male and female quail received a conditioned stimulus paired with the opportunity to copulate with each other. RESULTS: Sexual conditioning increased the number of eggs that were fertilized as a consequence of copulation, especially if both the male and the female were exposed to the sexual CS. This conditioned fertility effect occurred with a range of CS durations and CS types. The conditioned fertility effect also occurred in situations involving sexual competition. When two males copulated with the same female, DNA fingerprinting showed that the male whose sexual encounter was signaled by a sexual CS was responsible for most of the resulting offspring. Sexual conditioning also reduced the first-male disadvantage in fertilization that occurs when two males copulate with the same female separated by several hours. Another significant finding was that sexual conditioning attenuated the usual drop in fertilization rate that occurs when the same male copulates with two females in succession. CONCLUSION: These results show that sexual conditioning increases the number of offspring that are produced in both isolated male-female encounters and in situations that involve two males copulating with the same female or one male copulating with more than one female. By increasing fertilization rates, sexual conditioning can alter genetic transmission across generations and shape evolutionary change.

Learning effects on sperm competition and reproductive fitness.

Learning and other common psychological processes presumably evolved because they contribute to reproductive fitness, but reproductive outcomes are rarely measured in psychology experiments. We examined the effects of Pavlovian conditioning on reproductive fitness in a sperm-competition situation. Typically, two males mating with the same female in immediate succession sire similar numbers of offspring. In a study with domesticated quail (Coturnix japonica), we increased paternity success by presenting a Pavlovian signal that permitted one of two competing males to predict copulatory opportunity. Using microsatellite-based DNA fingerprinting, we found that signaled males sired 72% of the offspring when competing with control males, and this effect was independent of copulation order. In the absence of Pavlovian conditioning, rates of fertilization were not significantly different for two males that copulated with the same female. These findings demonstrate that Pavlovian conditioning contributes to reproductive fitness and suggest that individual past experience can bias genetic transmission and the evolutionary changes that result from sexual competition.