Sound production and intraspecific sound signal structure
Males of O. croaticus produced a single type of acoustic signal,
named pulsatile sound, during intersexual (male-female) interactions
conducted within the reproductive season (April – October). Four
resident males (x̄ ± s.d. = 49.1 ± 0.8; range: 48.0 – 50.0
mm LT; 40.9 ± 1.8; range: 38.9 – 42.2 mm
LS; 1.2 ± 0.1; range: 1.1 – 1.4 g W ; 1.7 ± 0.1;
range: 1.5 – 1.9 Fulton’s K ) produced sounds when interacting
with females, while the other four males remained mute and did not
court. We recorded 372 sounds produced by the four males (mean 93.0
sounds per male). These signals were reminiscent of brief felid purrs.
They were produced in a relatively irregular repetition pattern, at a
rate of 4.7 sounds per min, while the maximum recorded rate was 10
sounds per min (7.7 ± 1.4; range: 6 – 10) (Figure 1, Table 1 ).
Structurally, the sounds are short duration signals, lasting around 450
ms (442.0 ±132.6; range: 156.8 – 952.8 ms), and composed of a variable
number of short pulses (14.2 ± 4.0; range: 5 – 32) of around 15 ms
(14.5 ± 1.9; range: 9.7 – 22.9 ms) (Figure 2a-e, Table 1 ). The
individual pulse unit structure differed markedly between sounds,
exhibiting one to three peaks with variable amplitude. Generally, the
amplitude of a sound changed gradually, first increasing and then
steadily decreasing throughout the sound, with the first two or three
pulses being the loudest (Figure 2a-e ). The pulse repetition
rate varied from 26.0 to 38.0 Hz (32.5 ± 1.6 Hz), while the sound pulse
period (PP) averaged 32 ms (31.9 ± 1.4; range: 27.6 – 37.5 ms). PP
changed with water temperature, with higher values occurring at lower
temperatures (Figure 3 ). Fatigue averaged 1.1 ± 0.08 (range:
0.9 – 1.4). The sound peak frequency varied from 89 to 340 Hz (137.4 ±
38.3 Hz), although several higher frequency components were also
present, especially in the range 0.5 - 1.5 kHz. Energy extended from
0.05 to 2 kHz (Figure 2a-e, Table 1 ), with most of the sound
energy within 0.05 – 0.6 kHz. Frequency modulation of the sounds ranged
from 0.7 – 1.1 Hz. Additionally, the calling effort varied between
males from 0.37 to 0.60 (0.49 ± 0.09), indicating that some individuals
emitted sounds more frequently than others (Table 1 ).
Interestingly, sounds were never organised in bursts, which are usually
composed of several consecutive sounds produced with regular inter-sound
interval, as observed in some sand gobies.
Intraspecifically, soniferous O. croaticus males differed
significantly in all acoustic features (Kruskal-Wallis H test,
χ2 = 8.53 – 41.97; d.f. = 3; N = 80; P< 0.05) except for calling effort (Kruskal-Wallis Htest, χ2 = 5.67; d.f. = 3; N = 7; P> 0.05) (Table 1 ). Some acoustic features were
significantly correlated (Spearman rank correlations; rS =
- 0.90 – 0.97; P < 0.05), while the strongest
correlation was observed between NP and DUR (rS = 0.97), PRR with
FMf (rS = 0.7), and PRR with PP (rS = -0.9). Note that for
pairwise correlation between sound variables, we only consideredrS > 0.7 due to the limited sample size. These
relations are expected as DUR results in part from NP, PRR and PP are
redundant metrics, and FMf is the PRR of the initial part of the sound.
In addition, some acoustic features were also correlated with physical
features LS (PF and FMi, rS = -0.53; P< 0.05 for both variables), W (DUR and NP, rS =
0.53 and 0.55; P < 0.05), and water temperature (PRR,
PP, PF and FMi, rS = -0.67 – 0.69; P < 0.05)
(Figure 4 ).