EFFECTOR MECHANISMS 407 



2 msec and having a frequency range of 100-10,000 c/s. The sea-robin 

 Prionotus carolimts produces a modified rhythmic squawk or cackle, with a 

 frequency range of 100-2,500 c/s and a principal frequency of 250 c/s. 

 The toad-fish (Opsanus tau) gives off a series of intermittent boops, each 

 lasting 0-5 sec and having a frequency range of 100-600 c/s; the principal 

 frequency is 200 c/s. All these fish have drumming muscles. A sea-horse 

 {Hippocampus hudsonius) that stridulates and amplifies the sound in the 

 air-bladder gives rise to a series of clicking sounds. The frequency of these 

 clicks lies below 4,800 c/s, with the principal frequency in the region of 

 400-800 c/s. The maximum intensity measured was 22-6 dyn/cm 2 at a 

 distance of 15 cm. 



Although it is certain that many fishes produce noises it is by no means 

 certain what is their significance.lt has been suggested that, in some species, 

 the noises are recognition signals, perhaps to bring about aggregation at 

 breeding time. Sounds made by both sexes of Hippocampus europaeus 

 are most frequent and intense during the breeding season. In some sciae- 

 nids only the males possess a drumming apparatus. The calling of Opsanus 

 appears to be associated with nest-guarding, perhaps as a means of assert- 

 ing territorial rights. In other species sound production may be associated 

 with aggressive behaviour or defensive manoeuvres. 



Under-water recordings made in Chesapeake Bay (Virginia) have 

 revealed interesting changes in the sound-producing activities of croakers 

 (Micropogon undulatus), a species in which both sexes drum. Croakers 

 migrate in large numbers into Chesapeake Bay each spring. The chorus 

 produced by these fish begins in the evening with fading light and continues 

 until midnight, coinciding with the feeding period. The noise level was 

 followed over a period extending from May to July and a seasonal varia- 

 tion was discovered, the noise reaching peak intensity during the first 

 half of June. During this period the group voice of the croakers had fallen 

 about an octave : in early June the main frequency was about 600 c/s, and 

 had changed in early July to 250 c/s. Now the frequency of resonant vibra- 

 tion is inversely proportional to the length of the swim-bladder. It seems 

 that the shift in average frequency reflects a change in the composition of 

 the fish population. As older fish with larger bladders and deeper tones 

 migrated into the Bay, they gradually become the dominant group in the 

 croaker-population (45, 46, 60, 61, 85). 



Toothed Whales (Odontoceti). Porpoises and dolphins are known to 

 produce various kinds of under-water sounds. White whales (Delphinap- 

 terus) at sea are very vocal, producing a variety of high-pitched whistles 

 and ticking noises under water. Recordings made from the bottle-nosed 

 dolphin (Tursiops truncatus) reveal bird-like whistles, approximately 

 0-5 sec in duration, and under-water clicks repeated at rates of 5-100/sec. 

 The whistles have a pitch-range beginning at 7,000 c/s and ending at about 

 15,000 c/s. The dominant frequencies of the clicks are in the sonic range, 

 but there is a strong ultrasonic component, extending to at least 120 kc/s. 



The significance of porpoise phonation has been the subject of some 



