BEHAVIOUR OF THE AMAZON DOLPHIN 



69 



c) Swimming, blowing, diving and leaping 



I estimated the speed of swimming at about 2-3 knots although in flight the 

 animal swims much faster. Layne (1958) gives 2 knots as the normal speed and 

 10 knots during flight. In Marineland, I was able to observe the animais swimming 

 in a vertical circle, three or four times, one circle after the other. This is not a 

 stéréotype (in my opinion stéréotypes do not exist by Ceteceans) but corresponds 

 to similar behaviour seen by fishermen and hunters in Beni. Contrary to the opinion 

 of Mohr (1964), I noted that the Inia does not surface to breathe as often as sait 

 water dolphins e.g. Delphinus delphis, Stenella styx. I clocked periods of up to a 

 minute between spouts. The spout is noisy and even by températures over 30°C, 

 very obvious. It rises 1 meter above the surface of the water. The animal is almost 

 horizontal when it surfaces (fig. 2, 11, 12, 21, 22, 23). Most of the time only the 

 melon and the blow-hole appear out of the water. The beak and the eyes are seldom 

 seen. Sometimes, following a blow, the head is stretched out of the water so that 

 the long beak slants over the water (fig. 16). This movement is often followed by 

 a shaking of the head. Perhaps the animal is trying to see over the water. The eyes 

 of the Inia, which are pea sized, lie on the side of the head (fig. 18, 30). It is interest- 

 ing to note that the trochlearis and abducens nerves are absent, whereas the 

 oculomotorius is threadlike. The optic nerve is also relatively narrow (Pilleri and 

 Gihr 1968). The eye of the Inia therefore seldom seems to be limited in its move- 

 ments and it is possible that the animal is forced to move its whole head in an 

 attempt to make up for the lack of movement of the eye. 



Deep diving follows in a characteristic manner. The head and throat sink, 

 the dorsal fin rises above the water, the animal bends at the latter part of the 

 body and then dives although the tail does not appear above the water. At the 

 last stage of the dive before the animal disappears entirely, the parts of the body 

 out of the water have the shape of a triangle (fig. 2, 10, 11, 12, 13, 14, 15). I saw 

 only one leap out of the water. It was a large animal about 3 meters long. It 

 shot quite unexpectedly completely out of the water. Dr. Barbara Hegner-von 

 Stockar reported that she has often seen Inia leaping quite high out of the wate 

 (Iquitos, Peru). 



d) Tumbling in shallow water 



A form of behaviour which I saw repeatedly in the Ibaré and the Mamoré 

 rivers was the tumbling and rolling of the animais in shallow water, mostly at the 

 edge of the reeds (fig. 9). The Inia lay in the water on its side with quite a large part 

 of the body above the surface. It rolled about its own axis and paddled with its tail 

 and the flippers in the water. It could be cleaning itself although the exact reason 

 for this behaviour remains to be investigated. 



