WURSIG and WURSIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



distEince animals are actively echolocating on the 

 fish school, and thus some behavior of the feeding 

 dolphins is implicated. Perhaps underwater vocal- 

 izations serve as cues. The possibility of different 

 sound emissions by dolphins feeding and not feed- 

 ing has not yet been investigated in the present 

 study, although Tyack ( 1976) found such a varia- 

 tion in wild bottlenose dolphins. The incidence of 

 noisy — omnidirectional sound source — leaps in- 

 creases before and during feeding, and these may 

 provide cues to nearby dolphins. Norris and Dohl 

 (1980) discussed the likelihood of leap sounds serv- 

 ing a communication function. We believe that the 

 likelihood of such a function specifically in dusky 

 dolphin feeding is great. If noisy leaps serve to 

 attract or inform nearby schoolmates, and are at 

 least in part designed to do so, this recruitment 

 behavior may be analogous to the "drumming" 

 recruitment of African chimpanzees, described by 

 Reynolds and Reynolds (1965) and others. 

 Saayman and Tayler (1979) suggested that re- 

 cruitment may occur in Sousa sp. ofT South Africa 

 as well, but they did not link it to the possibility of 

 aerial behavior in their population. 



How do groups orient towEirds feeding locations 

 from a distance of more than several kilometers? 

 Our underwater recordings of sounds created by 

 leaps suggests that they probably do not propagate 

 much over 1 km. Norris and Dohl ( 1980 ) also found 

 rapid attenuation for underwater Hawaiian spin- 

 ner dolphin leap sounds. But, we observed dol- 

 phins that were leaping towards a large feeding 

 bout from as far as 8 km. One possible explanation 

 is that at least one dolphin swam to the distant 

 group with information about the feeding bout, a 

 feat not unknown in the animal kingdom (von 

 Frisch 1967, on honey bees). We have no evidence 

 that such messenger service may take place, and 

 suggest a possible alternative. (Although 

 Eberhard and Evans 1962, Evans and Dreher 

 1962, and Dreher and Evans 1964 reported that 

 individuals of the Pacific bottlenose dolphin, Tur- 

 siops gilli, have been seen detaching from a group, 

 moving to "investigate" something, and then 

 going back to the group. Their interpretation of 

 scouting behavior, however, is open to specula- 

 tion.) When we saw dolphins swimming towards a 

 feeding bout from more than a few kilometers, we 

 saw individuals leaping out of the water in high 

 forward leaps, clearing the water by as much as 

 three times their own length, and thus leaping as 

 high as 4-5 m. This leaping became lower and 

 finally subsided altogether as the animals came 



closer to the activity. It seems possible that dol- 

 phins are using in-air vision to orient to the feed- 

 ing bout, taking the birds flying above the activity 

 and the leaping dolphins of the activity as a cue. 

 We present this as a tentative hypothesis because 

 many investigators do not believe that dolphins 

 have a high degree of long-range in-air visual 

 acuity. Dral (1975), Herman et al. (1975), and 

 Rivamonte ( 1976), however, believe that Tursiops 

 sp. may have good in-air vision at infinity. 

 Perhaps dolphins gain information about the feed- 

 ing bout in some other manner, and are leaping 

 that high and often simply as part of their rapid 

 movement (although such high leaps are not seen 

 during after-feeding rapid movement). The high 

 leaps may decline when the dolphins get near the 

 activity because they are tiring. If it should prove, 

 however, that dolphins are capable of long-range 

 vision, and use it in this manner, it would mean 

 that the birds associated with dolphin feeding — up 

 to now assumed to represent a parasitic or neutral 

 role as they scavenge on the dolphins' herding 

 efforts — may serve as a signal to other dolphins. 

 Dolphin leaps would assume a similar in-air sig- 

 naling function. To observers, the number of birds 

 above an activity was a sign of the feeding activi- 

 ty's "success," and if dolphins can see these birds, 

 there is no reason to assume that they could not as 

 well gage such activity level. 



Various different types of leaps and aerial dis- 

 plays are associated with different stages of sur- 

 face feeding. What function could these leaps 

 serve? To answer this question we will attempt to 

 reconstruct a typical feeding bout in detail: Before 

 surface feeding, dolphins move rapidly, and dive 

 for long periods, indicating that they are covering 

 a large distance and are looking for fish deeper 

 than a few meters below the surface. Immediately 

 before and during feeding, forward movement 

 stops and long dives continue, interspersed with 

 clean, noiseless leaps. During these leaps, animals 

 reenter the water headfirst and rapidly swim 

 down. We therefore believe that the clean leaps 

 allow dolphins to breathe rapidly, and then force- 

 fully and efficiently return to the depths. The 

 humping variant of this leaping type appears 

 similar to the headfirst surface dive employed by 

 experienced skin divers. 



As long dives decrease, a tightly bunched fish 

 school, usually numbering several thousand fish 

 in an area 3-5 m in diameter, is first seen at the 

 surface. It thus appears that dolphins actively 

 herd fish towards the surface, probably to use the 



885 



