Edwards: Duration of unassisted swimming by Stenello oltenuata 



131 



only, because Fish's (1993) analysis does 

 not include metabolic efficiency. Assum- 

 ing the same metabolic efficiency of 0.2 

 for the Tursiops in Fish's study as used 

 in the present model, an estimate of total 

 power by his Tursiops swimming 6 m/s is 

 115 W/kg (i.e., 23x(l/0.2)), compared to 

 the model estimate of about 125 W/kg for 

 the adult spotted dolphin. 



Mass-specific cost of swimming 



300 



250 - 



200 



E 



5' 150 



100 



50 



Model estimates of mass-specific power 

 requirements (watts per kilogram muscle, 

 W/kg,„) for unassisted swimming by spot- 

 ted dolphins in the ETP increase quickly 

 with velocity, regardless of dolphin size, 

 but increase much more quickly for smaller 

 dolphins (Fig.l). Model results indicated 

 that a neonate (85 cm) spotted dolphin 

 must produce 3.6 times more power per 

 kilogram of muscle than an adult swim- 

 ming at the same speed. The factors are 

 3.5, 3.3, 2.8, 2.4, 1.8, and 1.4 times adult 

 power for 87-, 90-, 98-, 110-, 129-. and 

 154-cm spotted dolphins. The factors do 

 not vary with velocity because the relative differences 

 between body dimensions in dolphins of different sizes 

 remain constant regardless of swimming speed. 



For example, at speeds typical of ETP dolphins at- 

 tempting to evade speedboats during an impending 

 tuna-purse-seine set or during escape from the net 

 after release (about 3 m/s) (Chivers and Scott^), the 

 model predicts that an 85-cm neonate swimming un- 

 assisted by its mother would require a muscle power 

 of about 105 W/kg„, versus 30 W/kg„, for a 190-cm 

 adult. The difference is still relatively marked even for 

 two-year-old calves (154 cm), which would require an 

 estimated 42 W/kg,,^ to maintain a steady submerged 

 unassisted swim speed of 3 m/s, i.e., 1.4 times adult 

 power requirements. 



Velocity duration limits 



Velocity duration limits for adults, drawn from litera- 

 ture sources, appear to range from a few seconds for 

 burst speeds above about 5 m/s, to several minutes for 

 maximum speeds of about 4 m/s, to hours at prolonged 

 speeds below about 3.5 m/s (Table 1). 



Estimated mass-specific muscle power required of 

 adult spotted dolphins to achieve these speeds ranges 

 from 208 W/kg,,, for burst speeds of 6 m/s greater, to 67 

 W/kg,,, for maximum sustainable speeds around 4 m/s, 

 and 4-10 W/kg„, for prolonged duration speeds between 

 1 and 2 m/s (Fig.l). 



If one assumes that these power-duration limits apply 

 also to dolphin calf muscle, neonate dolphins (85 cm) 

 for example, could generate a burst duration power of 

 about 200 W/kg,„ for a few seconds at unassisted swim- 

 ming speeds of about 3.0 m/s, maximum duration power 



Estimated duration limits 



-Seconds 



-Minutes 



-Hours 



00 



m/s 



Figure 1 



Estimated swimming duration limits for eastern tropical Pacific Ocean 

 spotted dolphins iStenella attenuata) of various sizes (total length, tip 

 of rostrum to fluke notch), swimming at various speeds. 



of about 70 W/kg,,, for some minutes at about 2.2 m/s, 

 and prolonged duration power of about 5-10 W/kg„, for 

 hours at about 1.0 m/s (Fig.l). For two-year-old spotted 

 dolphins (154 cm), estimated swimming duration limits 

 were about 5 m/s for burst effort at 200 W/kg„,, about 

 3.5 m/s for maximum effort at 70 W/kg„,, and about 2 

 m/s for prolonged effort at 5-10 W/kg,„. Intermediate 

 ages produced intermediate results. 



Discussion 



Model results demonstrated clearly that swimming capa- 

 bility of dolphin calves is much lower than that of adults. 

 The especially pronounced difference between neonates 

 and adults appears a likely basis for development of the 

 ubiquitous drafting behavior observed in both captive 

 and wild neonates. Given the large difference in swim- 

 ming capacities, it is difficult to imagine how small 

 dolphin calves could maintain any long-term association 

 with the mother without the hydrodynamic benefits of 

 drafting, even under normal circumstances. 



During the relatively fast-paced evasion swimming 

 associated with tuna purse-seine sets in the ETP, it 

 appears that spotted dolphin calves swimming unas- 

 sisted are likely to have serious difficulty maintaining 

 the same speed as adults — difficulties being especially 

 pronounced for younger dolphins. It is also likely that 

 the differences in swimming capacity presented in this 

 study are underestimates because they do not include 

 effects such as positive bouyancy (Cockroft and Ross, 

 1990; Mann and Smuts, 1999) and soft, flexible fins 

 and flukes that are present for several hours after birth 

 (McBride and Kritzler, 1951; Tavolga and Essapian, 



