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

Preference functions, which quantify preference strength relative to variat ion in male traits or signals, are central to understanding mechanisms and consequences of female choice. Female tree frogs (Hyla versicolor) choose m ates on the basis of advertisement calls and prefer long calls to short cal ls. Here we show, in two experimental designs, that preference strength inc reased 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 ex periments simulating costly choice (unequal playback levels), non-linear ef fects were more pronounced than in the conventional design (equal playback levels). Repeated estimates of preference strength using the unequal-playba ck design revealed significant among-female variation. These patterns of pr eference 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 pot entially costly choice, could reflect differences in the net benefits to fe males of mating with males producing calls of different duration.