NORMS and DOHL: BEHAVIOR OF THE HAWAIIAN SPINNER DOLPHIN 



more difficult to disturb the animals sufficiently to 

 cause them to flee the bay. Even persistent at- 

 tempts to enter their ranks merely caused avoid- 

 ance and often a transitory flurry of aerial activity 

 (Figure 13). 



Defecation was a common feature of arriving 

 schools, prior to subsidence into rest. From the 

 underwater viewing vehicle the olive colored 

 trails of semiliquid fecal material were often seen 

 streaming from the dolphins. Three or four ani- 

 mals sometimes defecated simultaneously within 

 the field of view of the vehicle. A rate for one 

 40-animal school was calculated at one defecation 

 every 15 s. Presumably this rather short defeca- 

 tion period is related to nighttime feeding and 

 early morning digestion. The trailing animals of a 

 school swam through the dispersed clouds of feces 

 with no evident reaction. 



Subsidence Into Rest 



Once a spinner dolphin school arrives deep in 

 Kealakekua Bay, it normally subsides slowly into 



rest, a process sometimes requiring 2 h or more. 

 This process is so gradual and so affected by fea- 

 tures such as school size and the time of day, that 

 its precise onset was difficult to assign (Figure 13), 

 and an arbitrary definition was necessary. Be- 

 cause rest involves the cessation of aerial behavior 

 by all school members, we defined rest as occur- 

 ring when a 10-min observation of the school re- 

 vealed no aerial behavior. Occasionally, even this 

 criterion was confounded because aerial behavior 

 is, to some extent, "infectious," and a school sub- 

 siding into rest may sometimes exhibit 10-min 

 periods without aerial behavior followed by 

 periods in which some aerial activity occurs in 

 several animals. But, generally, once a school was 

 quiet for 10 min, little or no aerial activity oc- 

 curred until arousal. Typically arriving schools 

 were in ranked form, with group structure evi- 

 dent. Such schools often moved quickly (5-8 km/h) 

 and swam resolutely, with considerable aerial be- 

 havior. Little time was spent below the surface. 

 Dives were brief (Figure 14). Once such a school 

 arrived at the back of the bay, under the lava cliffs. 



0800 0900 1000 1100 



1200 1300 1400 

 Time of Day 



1500 1600 1700 1800 



Figure 13.— Aerial behavior per 10-min interval for a spinner dolphin school of approximately 40 animals, 30 June 1971, 

 Kealakekua Bay, Hawaii. Synchronous dives define the rest period, broken briefly by the speedboat. See text. 



839 



