Parallel female preferences for call duration in a diploid ancestor of an allotetraploid treefrog

Abstract

The gray treefrog species complex (Hyla chrysoscelis and H. versicolor) comprises a single allotetraploid species (H. versicolor) that arose multiple times from hybrid matings between an extant diploid species (H. chrysoscelis) and at least two other extinct diploid treefrogs. While previous studies have investigated female preferences for call duration in the tetraploid, we know little about these preferences in its putative diploid anscestors. Here, I report results from two-choice phonotaxis experiments investigating call duration preferences in H. chrysoscelis. Females preferred an average-length call over shorter-than-average calls (0.5-2.0 standard deviations [SD] below average), and they preferred longer-than-average calls over average or shorter-than-average calls if the difference in pulse number was at least 2.0 SD. When the amplitude of the longer alternative was attenuated by 6 dB, females still preferred an average-length call over a shorter-than-average call, but there was no preference for longer-than-average calls over an average call. In the presence of chorus noise, female preferences for both average and longer-than-average calls over shorter alternatives were weakened or reversed. Together, the results from this study reveal patterns of female preferences for call duration that are strikingly similar among two members of a species complex with a novel evolutionary history. In both species, female preferences are directional, nonlinear, and limited by environmental noise. Furthermore, these results also highlight the need for caution in studies of sexual selection when extrapolating from patterns of female preference obtained under ideal laboratory conditions to conclusions about how those preferences are expressed in the real world.

Fig. 1

Oscillograms showing (a) a single synthetic H. chrysoscelis pulse and (b–d) examples of synthetic calls with (b) 24 pulses, (c) 32 pulses (i.e., the average call), and (d) 40 pulses. The scale bar in b also applies to panels c and d.

Fig. 2

Histograms showing the distribution of pulse number in the calls of H. chrysoscelis in Minnesota. (a) Distribution of individual means from 16 males. (b) Distribution of 320 calls recorded from 16 males (20 calls/male). In both (a) and (b) a normal curve is fitted to each distribution. Values along the x-axis depict inclusive upper bounds.

Fig. 3

Female preference functions for call duration showing the number of females that chose each alternative as a function of pulse number in two-choice discrimination tests. In each graph, the normal curve depicts the curve fitted to the distribution of average pulse number and is redrawn from Fig. 2a. Lines join the two alternatives in each test; solid lines indicate a significant difference (two-tailed binomial P < 0.05) and dashed lines indicate no significant difference (two-tailed binomial P > 0.05). Closed symbols depict results from equal-amplitude tests; open symbols depict results from tests with the longer alternative attenuated by 6 dB. (a) Results when both alternatives were broadcast at 85 dB SPL with no noise. (b) Results when the amplitude of the longer call was attenuated by 6 dB (79 dB SPL) relative to the shorter call (85 dB SPL). (c) Results when signals were broadcast in the presence of a chorus-shaped noise.