Full Description

"We show that males and females produce two distinct sound types: ‘clicks’ (main energy between 50 and 800 Hz) during feeding and courtship, and previously undescribed ‘growls’ (main energy concen- trated below 200 Hz). The latter consists of series of sound pulses uttered in stress situations when the animals were handheld. Growls were accompanied by body vibrations, and may constitute an additional escape mechanism in seahorses, which might startle predators. During reproductive behaviour, clicks were most abundant on the third (last) day of courtship; they were particularly associated with the males’ pouch-pumping behaviour, suggesting synchronization between sound production and courtship behaviour."

"Hippocampus reidi produced two distinct sounds (Table 1) in different behavioural contexts: click sounds – single pulses, recorded during feeding and courtship, frequently audible to the observer during trials; and growling sounds – a series of sound pulses emitted only when handheld and never during intraspecific interactions."Feeding clicks were produced during prey capture and con- sisted of short broadband sounds that were typically uttered singly (mean duration: 16.1 ms), with the main energy ranging from 50 to 800 Hz. The mean SPL (LLFP re: 2 cm) of feeding clicks was 119.8 dB re 1 Pa (see Table 1). They were pro- duced in all feeding events recorded (Fig. 1a). Click duration [r = _0.297, n = 13, not significant (NS)], dominant frequency (r = 0.279, n = 13, NS) and SPL (rS = _0.022, n = 13, NS) were not significantly correlated to seahorse height. Moreover, no differences were found between males and females in these sound characteristics (t-test; P > 0.05 in all cases; see Table 1)."

"These comprised short single pulses with mean duration of 19.4 ms (Table 1; Fig. 1b), main energy between 100 and 800 Hz and SPL (re: 2 cm) of 105.0 dB. The SPLs of courtship clicks were lower than of feeding clicks but did not differ from distress growls (see next topic) (Kruskal–Wallis test: H = 12.9; n = 20, P < 0.01; Dunn’s post hoc: P < 0.001). Male courtship clicks had higher SPLs than clicks of females [t-test: t = 5.10, degrees of freedom (d.f.) = 11, P < 0.01; Table 1], but with similar duration and dominant frequency (t-tests, P > 0.05 in both cases; Table 1). Sex differences in SPL were not due to differences in size as the sound characteristics of courtship clicks were not correlated to body height (SPL: r = 0.490; duration: r = 0.276; dominant frequency: r = 0.468; in all cases, n = 10, P > 0.05). Hippocampus reidi’s courtship lasted for 3 days and encom- passed a sequence of behaviours accompanied by sound pro- duction (Table 2). The behaviour pouch pumping was the most frequently associated with clicks (63.6%), followed by prom- enading (25.0%), brightening (9.1%) and raising (2.3%), whereas no sounds were emitted during circling, quivering, pointing, attempting to copulate and copulating. Both males and females produced click sounds during courtship, and sound production mostly (86.1%) took place when the distances between the fishes were <15 cm. The number of sounds produced by males and females during courtship did not differ significantly from one another, neither considering all days together (U-test: U = 6.0, n = 10, NS) nor separately (U-tests, n = 10 in all cases: day 1, U = 10.0, NS; day 2, U = 9.5, NS; day 3, U = 7.0, NS). A considerable increase in click production was observed on the last (third) day of courtship (Friedman test: _2 = 15.2, d.f. = 2, P < 0.001, Dunn’s post hoc: P < 0.05; Fig. 2)."

"Seahorses emitted low-frequency growling sounds while hand- held. Growls consisted of series of sound pulses often lasting as long as the fish were handheld (maximum duration: 260 s) (Fig. 1c). The mean duration of sound pulses was 35.6 ms, with a mean pulse period of 111.2 ms. The mean SPL was 115.0 dB and the main energy was concentrated below 200 Hz (see Table 1). During production of growling sounds the seahorse’s body vibrated. Growls could not be recorded during feeding or courtship. The acoustic characteristics of growling sounds were neither related to seahorse height (Pearson’s correlation, P > 0.05 in all cases) nor differed between sexes (t-test, NS in all cases; Table 1). The dominant frequency of growling sounds was significantly lower than that of both feeding and courtship clicks (Kruskal–Wallis test: H = 11.9; n = 20, P < 0.01; Dunn’s post hoc: P < 0.05)."

"Clicks emitted during courtship were lower in level than those emitted during feeding. This may indicate that feeding and courtship clicks are addressed to different receivers or to receivers at different distances."

"Accordingly, an increase in male click production may communicate to females a readiness to mate."

"Our results also revealed that male H. reidi produced court- ship clicks of higher SPL than females. Louder sounds may indicate higher fitness and higher success rates during com- petitive interactions."

"Growls, in con- trast, are low-frequency sounds similar to drumming sounds."

Observation Environment Quotes

"Males (n = 10; body height: 10.9–17.3 cm) and females (n = 11; 11.6–17.0 cm) used in this study were kept separately in two bare bottom tanks (100 _ 50 _ 50 cm) filled with arti- ficial sea water (salinity 35; Reef Crystals, Aquarium Systems Inc., Sarrebourg, France) and kept under a 12:12 h L:D cycle at 25±1°C."

Behaviour Description Quotes

"We show that males and females produce two distinct sound types: ‘clicks’ (main energy between 50 and 800 Hz) during feeding and courtship, and previously undescribed ‘growls’ (main energy concen- trated below 200 Hz). The latter consists of series of sound pulses uttered in stress situations when the animals were handheld. Growls were accompanied by body vibrations, and may constitute an additional escape mechanism in seahorses, which might startle predators. During reproductive behaviour, clicks were most abundant on the third (last) day of courtship; they were particularly associated with the males’ pouch-pumping behaviour, suggesting synchronization between sound production and courtship behaviour."

Sound Name Quotes

"We show that males and females produce two distinct sound types: ‘clicks’ (main energy between 50 and 800 Hz) during feeding and courtship, and previously undescribed ‘growls’ (main energy concen- trated below 200 Hz). The latter consists of series of sound pulses uttered in stress situations when the animals were handheld. Growls were accompanied by body vibrations, and may constitute an additional escape mechanism in seahorses, which might startle predators. During reproductive behaviour, clicks were most abundant on the third (last) day of courtship; they were particularly associated with the males’ pouch-pumping behaviour, suggesting synchronization between sound production and courtship behaviour. "

"Pulse duration was analysed in all behavioural contexts."