WURSIG and WURSIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



Captured animals were of both sexes (Wiirsig 

 in press). As well, there were usually only 1 or 2 

 calves or small young within a group of about 15 

 animals, suggesting that mating is not highly 

 polygynous. Given data from captivity (Evans and 

 Bastian 1969 and Caldwell and Caldwell 1972 

 provided reviews) suggesting that promiscuity is a 

 prominent feature of most odontocete cetaceans at 

 least in unnatural circumstances, it is likely that 

 the dusky dolphin social system is promiscuous as 

 well. However, Bateson (1974) suggested rather 

 stable relationships between some spotted, 

 Stenella attenuata, and spinner dolphins for play, 

 mating, and sleep. 



We found that young were bom mainly in the 

 austral summer. If we assume an 11-12 mo gesta- 

 tion period (Sergeant et al. 1973 for bottlenose 

 dolphins), most effective matings took place out- 

 side the winter season. If sexual activity continues 

 throughout the year, then we can assume that 

 there is a physiological change in males or females 

 that allows conception to peak during the spring or 

 summer. Seasonal changes in testis weight have 

 been found for several cetacean species (for exam- 

 ple, Ridgway and Green 1967 for Delphinus del- 

 phis and Lagenorhynchus obliquidens). It is possi- 

 ble that a similar physiological change may exist 

 in dusky dolphins. This might relegate some activ- 

 ity appearing to serve a sexual function to the role 

 of greeting or bond-strengthening ceremonies as 

 has been suggested by Caldwell and Caldwell 

 (1967), Bateson (1974), and others. We suspect, 

 but have no definitive proof, that most mating 

 occurs in large groups after surface feeding. Since 

 this feeding occurs mainly in spring and summer, 

 it correlates well with the summer calving peak. 

 Nevertheless, if this is so, it would not invalidate 

 the possibility of a seasonal physiological cycle, 

 nor of "mating" at times serving a purely social 

 function. 



Groups which had about 10-20 adults and as 

 many calves occurred at times. We saw these nur- 

 sery groups mainly at the periphery of large feed- 

 ing activities. They did not appear to participate in 

 the high-activity level characteristic of large feed- 

 ing bouts and after-feeding. Perhaps, when small 

 groups feed, females with young feed and then 

 split off as activity increases. This can be of adap- 

 tive value. Young may in this way avoid possible 

 aggression and competition within the large feed- 

 ing aggregation, and they may avoid possible 

 predation by killer whales and sharks attracted to 

 the activity. We saw large (3-5 m) unidentified 



sharks moving in dolphin feeding activity on four 

 separate occasions, but they did not appear to 

 bother the adult dolphins engaged in feeding. 



The relationship of dusky dolphins and 

 bottlenose dolphins was in some ways puzzling. 

 Dusky dolphins moved in generally deeper water 

 than bottlenose dolphins, but the two at times 

 probably came into acoustic range of each other. 

 Yet they did not appear to take notice of each 

 other, although both species independently sought 

 contact with southern right whales, sea lions, and 

 the boat. Dusky dolphins were found in shallow 

 water in the morning, but bottlenose dolphins 

 were in even shallower water in the morning, then 

 moved into intermediately deep water around 

 noon, then into shallow water once again. It has 

 been suggested (Wiirsig and Wiirsig 1979) that 

 bottlenose dolphins may have been feeding on 

 southern anchovy, the same food as that of dusky 

 dolphins in these intermediate waters. At any 

 rate, by that time of day, dusky dolphins were 

 more often found in deeper water, and as a result, 

 their food niches did not appear to overlap. As 

 well, the two species were found with approxi- 

 mately opposite frequency within sight of the 

 study area at different times of year. This suggests 

 that one or both species may at times have actively 

 avoided the other, although alternative expfana- 

 tions such as different ecological requirements 

 may be more important. 



Bottlenose dolphins moved in small groups close 

 to shore, dusky dolphins moved in small groups 

 while not feeding, but in larger groups around 

 feeding time. Bottlenose dolphins appeared to 

 spend most of their nearshore time feeding indi- 

 vidually or perhaps in groups of two and three on 

 large solitary fishes inhabiting nearshore rocks. 

 Large groups £ire possibly not of advantage in 

 exploiting a presumably scattered food resource. 

 On the other hand, dusky dolphins appeared to 

 hunt in small groups spread out over a large area, 

 thus increasing their food-finding efficiency for a 

 patchily distributed food resource. When food was 

 found, they rapidly coalesced, and appeared to 

 herd prey cooperatively, allowing more efficient 

 feeding. 



Dusky dolphins fed near the surface in deeper 

 water in the afternoon, and moved slowly and with 

 little activity in early morning. We suggested that 

 the surface-feeding pattern may be associated 

 with availability of fish at different times of day 

 and in different areas of water. A similar change 

 in area from nonfeeding to feeding was found for 



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