Indeed, the production of click sounds during male–male competition has been observed in H. zosterae (Colson et al.,
1998) and in H. reidi in captivity (T. P. R. Oliveira, pers. obs.). In addition to clicking sounds, H. reidi produces MLN0128 cost low-frequency sounds in stress situations when handheld. This is the first study to characterize this sound type. Previous studies mentioned vibration of the seahorse’s body, for example, when taken out of the water, in H. erectus (Anderson, 2009) and in H. hippocampus (Dufossé, 1874). Dufossé (1874) wrote that vibrations were accompanied by ‘drum’-like sounds (tambour) and that they were more frequent and more intense during the breeding season. Based on the overall lack of data, we can only suggest that some seahorse species produce this sound
type in stress situations and perhaps also during courtship. What is the possible role of growling sounds in H. reidi? The functional significance of distress or disturbance sounds has been frequently discussed (Fish & Mowbray, 1970; Ladich, 1997; Bosher, Newton & Fine, 2005; Ladich & Myrberg, 2006) but, due to a lack of appropriate experiments, remains unknown in fish. The assumption is that they serve, similar to other animal taxa, in warning and deterring predators, in attracting secondary predators (which would then attack the first predator) or in alarming conspecifics (Ladich, 1997; Ladich & Myrberg, 2006). Bosher et al. (2005), however, have shown that stridulatory sounds PD0325901 in vivo are ineffective in thwarting predation and have not reduced further attacks by largemouth bass. The low level of H. reidi’s growling sounds probably makes them detectable at only very short distances, thus rendering them unsuitable to function as an alarm call unless individuals are in very close proximity. Alternatively, growls may constitute an additional escape mechanism because sound production is accompanied
by body vibrations, which might startle predators (catfish: Ladich, 1997; Rho weeping lizards: Labra et al., 2013; birds: Conover, 1994). Based on the differences in sound characteristics and on behavioural observations during sound production, clicks and growls are suggested to be produced by two different sound-generating mechanisms. Broadband clicks in seahorses are stridulatory in origin and are produced when a supraoccipital ridge of the neurocranium snaps over the grooved anterior margin of the coronet (Colson et al., 1998). Growls, in contrast, are low-frequency sounds similar to drumming sounds. However, as H. reidi does not possess swim bladder muscles (T. P. R. Oliveira, pers. obs.), we suggest that growl emission results from rapid contraction of other muscles (e.g. lateral trunk muscles). These make the swim bladder and the body vibrate, as also mentioned by Dufossé (1874).