NORRIS and DOHL: BEHAVIOR OF THE HAWAIIAN SPINNER DOLPHIN 



this behavior took 2.75 h. Typically zigzag swim- 

 ming ended with fast swimming that took the 

 school beyond the confines of the bay altogether. It 

 was as if one had been rocking a blob of mercury in 

 a bowl, and a final strong motion sent the blob 

 flying, completely free of the bowl. 



If the animals left the bay early in the day, the 

 school usually traveled close to the shoreline, 

 either to the north or south, and later in the day, 

 turned toward deep water offshore. When a school 

 left the bay near dusk, it usually headed directly 

 out to sea. 



As exodus from the rest location began, schools 

 traveled either as ranked schools (wider than 

 long) or in straggling lines arranged in more or 

 less linear fashion. As the school moved offshore, it 

 spread. It sometimes coalesced with other schools 

 moving in the same general direction. By the time 

 feeding grounds were reached, usually near dusk, 

 a school that, during rest, had formed a 25 m 

 diameter disc might have expanded until its 

 groups were scattered over a kilometer or more of 

 sea. As noted earlier, we have estimated some 

 feeding schools as 3 km in breadth. 



Social behavior, including mating, aerial be- 

 havior, sexual play, and aggressive chases, be- 

 comes especially evident in spinner dolphin 

 schools moving toward the feeding grounds. Once 

 there subsynchronus feeding dives begin. 



Dive Patterns 



An example of the daily cycle of dive patterns is 

 shown in Figure 14. As animals entered the rest 

 area, the pattern was one of short dives, with most 

 time spent on the surface. Then this pattern 

 gradually shifted as dives became longer and sur- 

 face times shorter. When the school was near or 

 over the shallow area where rest occurs, dives 

 become synchronous, or neairly so. During arrival 

 the groups of a school, especially if it was a large 

 one, often dive out of synchrony with one another. 

 During rest, as shown in Figure 14, the duration of 

 dives continued to increase until the longest dives 

 were approximately 3.5 min duration; surfacings 

 at that time were brief, between 10 and 30 s dura- 

 tion. Throughout the rest period, the school, if it is 

 small or moderate in size, dives in synchrony. Dur- 

 ing the arousal period, surface times gradually 

 increase while dives tend to become much more 

 variable in length than during rest. Finally, as the 

 school travels out to sea, individualism reaches its 

 peak, with animals scattered in pairs or small 



subgroups, or even alone, within the envelope of 

 the school as a whole. Synchronous diving is lost as 

 movement is at or close to the surface and directed 

 into horizontal travel. Then, on the feeding 

 grounds, when the school is at its most dispersed, 

 the scattered school slows and begins syn- 

 chronized diving again, presumably to feed. Inter- 

 nal factors, such as the return to equilibrium after 

 a dive might play an important role in determin- 

 ing diving patterns. As for mediating signals, the 

 cessation of aerial behavior in an area of the school 

 that has dived could signal adjacent school seg- 

 ments that diving is occurring; or, vocal signals 

 could mediate it, and thus a wave of information 

 about a dive in progress could travel across the 

 school. The high incidence of aerial behavior in 

 feeding schools and the lack of precise synchrony 

 in feeding dives support such a speculation. 



Social Behavior 



Social behavior in wild spinner dolphin schools 

 has thus far proved all but impossible to observe in 

 an orderly fashion. Glimpses of individual ani- 

 mals or subgroups are fleeting, and the opportun- 

 ity to identify individuals or their sex is sporadic. 

 Hence, the observations that follow are highly 

 fragmentary. 



Mother- Young Behavior 



Very small calves are always seen in the com- 

 pany of adults. However, young spinner dolphins 

 of quite small size (about 1.2-1.7 m TL) may form 

 groups within a school with no evident adults in 

 close attendance. Newborn calves with adults 

 have been seen at all seasons of the year (Figure 

 16), as have groups of unattended larger calves. 



• NEWBORN WITH ADULT 

 A INDEPENDENT YOUNG 



JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 



Figure 16. — Annual occurrence of newborn in spinner dolphin 

 schools off Hawaii (1968-72). Only sightings of newborn with 

 evident fetal folds are included. 



843 



