Female preference functions based on call duration in the gray tree frog (Hyla versicolor)

Preference functions, which quantify preference strength relative to variation in male traits or signals, are central to understanding mechanisms and consequences of female choice. Female tree frogs (Hyla versicolor) choose mates on the basis of advertisement calls and prefer long calls to short calls. Here we show, in two experimental designs, that preference strength increased significantly as the difference in call duration was increased only if the absolute durations of alternative stimuli were below average. Hence preference strength was a non-linear function of duration, and females did not base preferences solely on the percentage difference in duration. In experiments simulating costly choice (unequal playback levels), non-linear effects were more pronounced than in the conventional design (equal playback levels). Repeated estimates of preference strength using the unequal-playback design revealed significant among-female variation. These patterns of preference suggest that selection by female choice for males producing calls of average duration over males producing very short calls is stronger than selection for males producing very long calls over males producing calls of average duration. Female preferences, especially in tests simulating a potentially costly choice, could reflect differences in the net benefits to females of mating with males producing calls of different duration.