FISHERY BULLETIN: VOL 77. NO 



ing morning has been reported for Hawaiian spin- 

 ner dolphins by Norris ( 1974), Norris and Dohl l in 

 press), and Norris and Dohl (see footnote 3) and for 

 dusky dolphins by Wiirsig and Wiirsig (see foot- 

 note 4). Norris and Prescott ( 1961) also mentioned 

 that T. gilli off California appears more active in 

 the late afternoon than during the early part of the 

 day. Why this period of rest should be concen- 

 trated in the forenoon in at least three different 

 coastal species is not known. It contrasts in the 

 present population with a greater amount of feed- 

 ing activity in the afternoon, and it may be that 

 schooling prey is more available in the deeper 

 offshore waters in which these porpoises feed in 

 the afternoon. As a result, they rest more fre- 

 cjuently when prey is not .available. 



During the afternoon, activity level increased. 

 Aerial displays were generally performed singly, 

 however, and were often spaced in time, with only 

 a few leaps or noseouts per hour. This amount of 

 aerial displaying was less than for the spinner 

 dolphins which spread out over large distances 

 and for which the omnidirectional splashing 

 sounds attendant to most aerial behavior is 

 thought to serve a possible communication func- 

 tion (Norris 1974). Such communication would be 

 most important when the animals are not close 

 together as a tightly knit unit, which the present 

 individuals were at almost all times. Neverthe- 

 less, it is still possible that noisy leaps, e.g., served 

 to attract the attention of the rest of the subgroup 

 in a highly efficient manner. The exact meaning, 

 however, of these leaps is not clear. 



Noseouts, belly-ups, kelp tossing, and clean 

 leaps make little noise. These also occurred with 

 higher frequency in the afternoon. Clean leaps, 

 with individuals reentering the water headfirst, 

 as has been mentioned previously, appeared to 

 precede steep dives in intermediate and deep wa- 

 ters. They may be correlated with feeding on or 

 near the bottom. Noseouts, belly-ups, and kelp 

 tossing occurred when individuals were close to- 

 gether, often touching, and may be associated with 

 "play" and copulatory activity. Especially during 

 times when individuals moved upside down 

 (belly-up) for 50 m or more, they were attended by 

 one or more individuals rubbing along their flanks 

 and dorsum. These close interpersonal associa- 

 tions need not necessarily indicate copulatory be- 

 havior, however. They were also performed on 

 several occasions by adults and their small calves, 

 and may represent a form of nonsexual social 

 communication as has been proposed by several 



other works (Caldwell and Caldwell 1967; Bate 

 son''; and others). Especially significant for thi> 

 hypothesis may be the fact that we observed more 

 of these behaviors when two subgroups which had 

 been separated for several hours or longer re- 

 joined. Rubbing behavior and attendant aerial 

 displays may at least in part serve a gi-eeting func- 

 tion, where individuals renew and strengthen so- j 

 cial bonds in a manner analogous to many social 

 terrestrial mammals (for a review, see Wilson 

 1975). , 



Belly-up movement was described by Leather- 

 wood ( 1975) for T. trunaitiis , and by Saayman and 

 Tayler (in press) for Sausa sp. as being performed 

 by individuals while pursuing fish near the sur- 

 face of the water. Although we saw belly-up be- 

 havior only in conjunction with other behavior 

 which we assumed to be social, it may also occur 

 for feeding in the present population. 



A final form of aerial behavior which also made 

 a loud sound was tailslapping. It was performed at 

 any time that the group may have been disturbed, 

 such as upon the approach of our boat. We there- 

 fore concur with other researchers (Norris and 

 Pre.scott 1961; D. K. Caldwell and M. C. Caldwell 

 1972) who believe that tailslapping by dolphins in 

 general serves as a warning signal or fright reac- 

 tion. It was performed with highest frequency by a | 

 large adult who was part of a "core" of individuals 

 present throughout the 21-mo study. We suggest 

 that this individual may have been a "leader" of 

 the subgroup of animals, posssibly dominant over 

 other individuals. This suggestion is based only on 

 this one behavioral pattern, however, and must 

 therefore be treated with caution. 



Dolphins associated with whales by riding on 

 the waves created by the larger cetaceans, and 

 rode on wind-driven waves and the pressure wave of 

 the boat. This type of behavior has been seen in 

 many species, and was described for dolphins rid- 

 ing near whales by McBride and K ritzier (1951), j 

 for dolphins riding wind-driven waves by Wood- 

 cock and McBride ( 1951 ), and for dolphins riding 

 boat bow waves by Matthews ( 1 948 ) and Woodcock 

 (1948). Especially insightful analyses of this be- 

 havior have been provided by Scholander (1959) 

 and Norris and Prescott ( 1961 ). They showed that 

 dolphins could travel with less muscle movement, 

 and therefore presumably less expenditure of 



"Bateson, G. The cetacean community in Whaler's Cove - Sea 

 Life Park. Unpuhl. manuscr., 16 p. Center for Coastal Marine 

 Studies, University of California, Santa Cruz. 



410 



