A URSIG and WURSIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



the rapidly moving dolphin group. Dolphins 

 tended to stay together for up to several hours once 

 groups had converged to feed, and feeding activity 

 usually started again in these larger groups. 

 When it did so, it lasted longer than when fewer 

 dolphins were feeding, and therefore larger groups 

 were more often seen in association with feeding 

 bouts (Table 2). 



Group Organization and 

 Calving Periodicity 



As mentioned previously, the most common 

 nonfeeding group size was about 6-15 animals. In 

 general, these groups were composed of adults, 

 and at times included juveniles and calves. We 

 were not able to determine sex of individuals by 

 observing them from boat or shore, but captures of 

 individuals prior to radio tagging (Wiirsig in 

 press) demonstrated that males and females usu- 

 ally travelled together in these small groups. We 

 saw small calves (about equal to or less than one- 

 third adult size) from November through Feb- 

 ruary. Furthermore, we saw "juveniles" (about 

 one-half to two-thirds of the size of adults) or 

 young under 1 yr of age during April-May and 

 August-September (Table 5). The sample size was 

 too small, however, to say definitely that young 

 were born only during the summer, and births 

 may have been more spread out over the year. 

 Nevertheless, the data suggest a summer calving 

 peak. 



Although we saw small calves and young in 

 small groups, we also saw groups of 8-20 adults 

 and as many calves on six separate occasions. We 

 called these groupings "nursery" groups, on the 

 assumption that the adults may have been 

 females, and the calves their young. During all six 

 sightings of nursery groups, most or all other ani- 

 mals in the vicinity were engaged in feeding activ- 

 ity and aerial behavior 0.5 to several l^ilometers 



Table 5. — Sightings of calves and juvenile dusky dolphins. 

 Incidence of calves during November to February was sig- 

 nificantly higher than in the rest of the year if- 0.001, Raleigh 

 test, Greenwood and Durand 1955). 



distant from the nursery group. During 12 other 

 sightings of calves and adults, they were found in 

 small groups in the ratio of approximately 1 calf to 

 10 adults and were not engaged in large-group 

 feeding activity (although on 3 of the 12 occasions, 

 we saw calves in small groups that were feeding). 

 We suspect from these observations that young 

 normally travel with adults in small groups, but 

 when many groups coalesce to feed and socialize 

 (see below), calves and certain adults split off at 

 some time and form temporary nursery groups. 



If there is a calving peak during the year, then 

 most successful matings are probably also carried 

 out in a relatively restricted time period. Most 

 apparent copulations, consisting of rapid belly- 

 to-belly swimming and frequent pelvic thrusts by 

 one or both animals, appeared to take place in 

 large groups during and after surface feeding. 

 Most of these large groups were found in summer 

 (Figure 11). However, it was difficult to approach 

 small, nonsurface feeding groups, and we have few 

 data on their underwater behavior and possible 

 mating attempts. Although we saw some apparent 

 mating in groups of all sizes and at all times of the 

 year, we were not able to quantify these observa- 

 tions. 



When we saw small groups of 6-15 animals, we 

 usually saw many of them, up to about 30 such 

 groups, in an area approximately 10 km in diame- 

 ter. However, we were able to count these groups 

 only under the best conditions, on a calm sea. 

 When these groups converged to surface feed, the 

 upper limit of group size estimate was 300 ani- 

 mals, and this estimate — made by different obser- 

 vers and at different times of year — did not vary 

 appreciably. It thus appears that small groups 

 made up part of a larger school or herd of animals. 

 We do not know how stable small groups were over 

 time, although evidence has been presented by 

 Wiirsig (in press) which suggested that at least 

 some groups remained stable over a period of at 

 least several days, and appeared to remain to- 

 gether in "subgroups" of a large group during and 

 after feeding bouts. 



Interspecific Interactions 



Dolphins associated with the boat at times by 

 rapidly moving towards a moving boat from as far 

 as 2-3 km. They would then ride the bow and stern 

 pressure waves of the boat in characteristic dol- 

 phin fashion. This activity took place mainly when 

 the dolphins had been surface feeding for a long 



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