Phonotaxis scores as a function of frequency separation (ΔF) in a test of sound source segregation.

<p>(<i>a</i>) The absolute carrier frequencies of the distractor pulses (F) shown in relation to the magnitudes of frequency separation (in semitones) for the two target signals with carrier frequencies of 1.3 kHz (ΔF_{1.3 kHz}) and 2.6 kHz (ΔF_{2.6 kHz}). Note that for the ΔF of 3 semitones, we tested values of absolute frequency that were 3 semitones above and below each signal frequency; we designate these as ΔFs of ±3 semitones, with the positive designation corresponding to the direction of frequency change (either higher or lower) of the other distractor frequencies tested. (<i>b</i>) Mean (± SE) phonotaxis scores as a function of ΔF (n = 40). Asterisks indicate significant differences (p<0.05) in planned contrasts comparing the indicated value of ΔF to ΔF = 0. (<i>c</i>) Mean (± SE) phonotaxis scores as a function of ΔF shown separately for subjects tested with target signals having a carrier frequency of 1.3 kHz (circles and solid line; n = 20) or 2.6 kHz (squares and dashed line; n = 20). (<i>d</i>) Phonotaxis scores from (<i>c</i>) plotted as a function of the absolute carrier frequency of the distractor pulses.</p

The acoustic scene of a mixed-species breeding chorus.

<p>Spectrograms (top traces) show frequency as a function of time (amplitude shown as color intensity) and oscillograms (bottom traces) show amplitude as a function of time. In Minnesota, U.S.A., where our study was conducted, three heterospecific species that form mixed-species choruses with gray treefrogs are boreal chorus frogs, American toads, and northern leopard frogs. (<i>a</i>) The advertisement call of a male gray treefrog (<i>Hyla chrysoscelis</i>) (see text for description). (<i>b</i>) American toads (<i>Bufo americanus</i>) produce a long (≈5–50 s), trilled call (≈35–45 pulses s⁻¹) with a single spectral component (≈1.7–2.0 kHz) that falls between the two spectral components of the gray treefrog call <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021191#pone.0021191-Howard1" target="_blank">[53]</a>; a 2-s segment of a longer call is shown here. (<i>c</i>) Boreal chorus frogs (<i>Pseudacris maculata</i>) produce a pulsed advertisement call of approximately 750–950 ms in duration (≈13–18 pulses s⁻¹) and with a bimodal frequency spectrum having peaks at about 1.9 kHz (−8 to −22 dB) and 3.8 kHz (0 dB) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021191#pone.0021191-Bee8" target="_blank">[54]</a>. (<i>d</i>) Northern leopard frogs (<i>Rana pipiens</i>) also produce a relatively long (≈2–5 s), trilled call (termed a “snore”) that is fairly broadband (≈0.5–2.0 kHz), with dominant frequencies ranging from about 0.9 to 1.5 kHz <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021191#pone.0021191-Larson1" target="_blank">[55]</a>. (<i>e</i>) A mixed-species chorus in Minnesota comprising calls by all four species depicted in (<i>a</i>–<i>d</i>). All recordings were made with Sennheiser microphones (ME66 or ME67) and a Marantz PMD670 recorder. Spectrograms were generated using an FFT window size of 1024 points with Blackman-Harris windows.</p

Experimental stimuli for testing the role of ΔF in source segregation.

<p>Shown here are examples of (<i>a</i>) the waveform of the pulsed target signal; (<i>b</i>) the waveform of a 2-s segment of the continuous train of distractor pulses; (<i>c</i>) a waveform showing the interleaved target signal and distractor pulses; and (<i>d</i>) a spectrogram showing an interleaved target signal (2.6 kHz) and distractor pulses (1.093 kHz) separated by a ΔF of 15 semitones.</p

Results of two-choice discrimination tests for pulse rate selectivity.

<p>Females were given a choice between two alternating stimuli that differed in pulse rate (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021191#s4" target="_blank">Materials and Methods</a>). Results are shown for tests in which both alternatives had carrier frequencies of (<i>a</i>) 1.3 kHz (−9 dB) and 2.6 kHz (0 dB), (<i>b</i>) 1.3 kHz, or (<i>c</i>) 2.6 kHz. Each line connects two points that show the proportions of females (n = 12 per test) choosing the alternative with a conspecific pulse rate (45.5 pulses s⁻¹) and a call with either a slower (23 pulses s⁻¹; solid line) or faster (91 pulses s⁻¹; dashed line) pulse rate. Insets depict the power spectrum (based on the 45.5 pulses s⁻¹ call) for the alternatives in each corresponding two-choice test showing relative amplitude (from 0 dB to −36 dB in 6-dB steps; y-axis) as a function of frequency (from 0 to 4 kHz, 0.5-kHz steps; x-axis). In all tests, females chose the alternative with a conspecific pulse rate significantly more often than expected by chance (two-tailed binomial ps<0.05). These results confirmed that females were selective for conspecific pulse rates with unimodal calls having carrier frequencies of either 1.3 kHz or 2.6 kHz.</p

Responses to the distractor stimuli.

<p>Each plot shows the distribution of response angles (dots) and the angle and length of the mean vector (arrow) corresponding to the angles at which subjects (maximum possible n = 20 per plot) first touched the wall of the circular test arena relative to the playback speaker positioned at 0°. The text insets show the proportion of subjects that met the response criterion of touching the arena wall during 5 min (k), the length of the mean vector (r), and the results of a Rayleigh test (Z and p) of the null hypothesis that the data are uniformly distributed. Data are shown for the three reference trials tested at the beginning, middle, and end of a sequence of test trials (top row; 1 dot  = 2 subjects), for each of the distractor stimuli (middle rows; 1.093 kHz through 3.092 kHz; 1 dot  = 1 subject), and for subjects tested in the main source segregation experiment in response to the unimodal target stimuli with carrier frequencies of 1.3 kHz or 2.6 kHz presented alone with no distractors (bottom row; 1 dot  = 1 subject).</p

Results from no-choice tests of audibility.

<p>Depicted are the proportions of subjects that responded to unimodal calls presented at 67 dB SPL with carrier frequencies as indicated along the x-axis. Insets depict the power spectra of three selected stimuli showing relative amplitude (from 0 dB to −36 dB in 6-dB steps; y axis) as a function of frequency (from 0 to 4 kHz, 0.5-kHz steps; x-axis). The sample size for each bar was n = 12 for all stimuli except that at 4.0 kHz, for which the sample size was n = 11. Asterisks indicate significant differences (p<0.05) in one-tailed binomial tests of the hypothesis that the represented proportion exceeded the expected null proportion of  = 0.2 (dashed line).</p

The vocal repertoire of <i>Pseudophilautus kani</i>, a shrub frog (Anura: Rhacophoridae) from the Western Ghats of India

<div><p>Taxonomic descriptions of new anuran species are published with high frequency in the recent systematics literature. In contrast, there are fewer detailed quantitative descriptions of the vocalizations of these new species. Here, we describe the vocalizations of a recently described shrub frog, <i>Pseudophilautus kani</i> (Anura: Rhacophoridae; Biju and Bossuyt 2009) endemic to the Western Ghats region of India. We recorded two distinct, pulsatile call types that could be distinguished by their temporal and spectral properties as well as their mode of production. Type 1 calls (<i>creek</i>) were short ( ≈ 85 ms), consisted of about six or seven pulses (92 pulses/s) and had a spectrum with a single frequency peak at about 3.5 kHz. By comparison, Type 2 calls (<i>ta-ta-ta-ta</i>) were typically longer ( ≈ 320 ms), contained fewer pulses (4–5) produced at much lower rates (12 pulses/s) and had broadband spectra with multiple frequency peaks, two of which were consistently present at about 2.3 and 4.6 kHz. Video analyses indicated that males produced pulses in Type 1 calls using a single, continuous contraction of the trunk musculature, whereas pulses of Type 2 calls were produced with separate muscle contractions. In both call types, spectral properties were significantly negatively related with body length and body mass. The pulse rate and pulse period of Type 2 calls were also related to body size. No call properties were correlated with body condition. Patterns of individual variation in call properties were generally consistent with those described for other anurans.</p> </div

Territories of males that appeared in the study pond on August 22, 2008.

<p>There are three stones aside the pond (black areas). A solid black line encloses the estimated territory of each male. Labels represent the ID of a male and the number of egg clutches laid in the territory on that night. Egg clutches laid on that night are indicated by *. Gray areas represent aquatic vegetation measured as part of this study.</p

The mating success rate of undisturbed pairs and disturbed pairs.

<p>Mating success was defined as successfully completing oviposition before breaking amplexus. The mating success rates were different significantly (χ² = 133.1, df = 3, <i>P</i> < 0.001) between undisturbed pairs (100%, <i>N</i> = 114) and disturbed pairs (16%, <i>N</i> = 49).</p

Short Amplexus Duration in a Territorial Anuran: A Possible Adaptation in Response to Male-Male Competition

<div><p>Mating duration is a reproductive behaviour that can impact fertilization efficiency and offspring number. Previous studies of factors influencing the evolution of mating duration have focused on the potential role of internal sperm competition as an underlying source of selection; most of these studies have been on invertebrates. For vertebrates with external fertilization, such as fishes and frogs, the sources of selection acting on mating duration remain largely unknown due, in part, to the difficulty of observing complete mating behaviours in natural conditions. In this field study, we monitored breeding activity in a population of the territorial olive frog, <i>Rana adenopleura</i>, to identify factors that affect the duration of amplexus. Compared with most other frogs, amplexus was short, lasting less than 11 min on average, which included about 8 min of pre-oviposition activity followed by 3 min of oviposition. We evaluated the relationship between amplexus duration and seven variables: male body size, male condition, operational sex ratio (OSR), population size, clutch size, territory size, and the coverage of submerged vegetation in a male’s territory. We also investigated the influence of these same variables, along with amplexus duration, on fertilization rate. Amplexus duration was positively related with clutch size and the degree of male-bias in the nightly OSR. Fertilization rate was directly related to male body size and inversely related to amplexus duration. Agonistic interactions between males in amplexus and intruding, unpaired males were frequent. These interactions often resulted in mating failure, prolonged amplexus duration, and reduced fertilization rates. Together, the pattern of our findings indicates short amplexus duration in this species may be an adaptive reproductive strategy whereby males attempt to reduce the risks of mating and fertilization failures and territory loss resulting from male-male competition.</p> </div