Interference risk and the function of dynamic shifts in calling in the gray treefrog (Hyla versicolor).

Abstract

Male gray treefrogs call to attract females under challenging acoustic conditions. At higher chorus densities, there is considerable background noise and a good chance that a male's calls will often be overlapped by calls of other individuals. Call overlap may reduce the probability of mating because females prefer calls with internal pulse structure that is not obscured. With increases in acoustic stimulation, males lengthen call duration while simultaneously reducing call rate such that "pulse effort" changes little. In our study, we tested the "interference risk hypothesis." This proposes that males change call duration so that, on average, there will be a sufficient number of pulses and interpulse intervals clear of acoustic interference per call to attract a female. However, female choice experiments employing call alternatives of different durations, rates, intensity levels, and degrees of call overlap refuted this hypothesis. Our results leave open the possibilities that the dynamic shift in the two components of pulse effort are related to the problem of call detection in a noisy environment or are responses by males to the perceived threat of competition for females.

Effect of anomalous pulse timing on call discrimination by females of the gray treefrog (Hyla versicolor): behavioral correlates of neurobiology

“…Finally, resettable interval-counting neurons might represent the physiological substrate of the following intriguing finding. Females of H. versicolor often discriminate in favor of longer calls over shorter calls even when the 'pulse effort' (mean call rate ϫ mean pulses per call) of alternative stimuli are equivalent or nearly so (Gerhardt et al, 1996;Klump and Gerhardt, 1987;Schwartz et al, 2001;Schwartz et al, 2008). As discovered by Rose and colleagues, interval-counting neurons differ in the number of intervals required to elicit a response (Edwards et al, 2002) (e.g.…”

Effects of heterospecific and conspecific vibrational signal overlap and signal-to-noise ratio on male responsiveness inNezara viridula(L.)

“…Animals often emit air-borne sounds or substrate vibrations in aggregations of signalling individuals (e.g. Aubin and Jouventin, 2002;Cocroft, 2003;Greenfield, 2005;Miranda, 2006;Schwartz et al, 2008) and/or in the presence of other signalling species (e.g. Cocroft and Rodríguez, 2005;Marshall et al, 2006;Planqué and Slabbekoorn, 2008).…”

“…Reduced responsiveness could therefore result from masking due to spectral and temporal overlap of the signals (Römer et al, 1989;Greenfield, 1993;Marshal et al, 2006) and/or degradation of signal parameters resulting from overlap (e.g. Römer, 1993;Römer and Krusch, 2000;Schwartz et al, 2008) and/or obscuring the species-specific signal repetition time due to alternation of signals (Greenfield, 2005). In addition, whether the amplitude of a compound signal decreases or increases at the position of the receiver depends on the frequency filtering, temporal distortions and oscillations of the amplitude of the signals(s) during the transmission.…”

Calling in gray treefrog choruses: modifications and mysteries

“…For example, Gerhardt et al (1996) showed that females of H. chrysoscelis in Missouri preferred faster calls at slower rates (equal call effort), whereas Ward et al (in review) found that females in Minnesota generally showed no such preference. Some studies of the tetraploid H. versicolor have shown that females prefer longer calls at slower rates when call effort is equal (Klump and Gerhardt, 1987;Gerhardt et al, 1996;Schwartz et al, 2001), whereas other studies of this species have failed to show such preferences (Sullivan and Hinshaw, 1992;Schwartz et al, 2008). Possible reasons for these discrepancies between studies are discussed in more detail by Ward et al (in review).…”

“…Critical to the hypothesis are the observations that, for this species, (a) overlapped calls are less appealing to females than are non-overlapped calls (Schwartz, 1987) and (b) females find very brief calls profoundly unattractive relative to longer calls (Gerhardt et al, 2000). Schwartz et al (2008) tested the IRH using filtered noise or bouts of calling as a source of interference and a range of call alternatives. Tests of female choice refuted the hypothesis unequivocally: under no experimental conditions with such acoustic backgrounds was there an increase in preference strength or significant discrimination for longer relative to shorter calls.…”