182 VISION 



gestation periods as long as two years (Wourms 1977). Representatives of four 

 different families (carcharhinids, sphyrnids, dasyatids, and myliobatids) have 

 evolved yolk sac placentas or placental analogs (trophonemata), increasing 

 energy flow to the embryos by 800 to 5000 % (Wourms 1977). These same 

 families have the most complex neural organization and the highest brain-to- 

 body ratios known for elasmobranchs. Viviparity allows for larger offspring 

 at birth, reducing the numbers of potential predators and competitors and 

 increasing the number of potential prey items. Larger neonates also have 

 greater locomotive and metabolic efficiency (Hutchinson and MacArthur 

 1959, Wourms 1977). 



Endothermy in birds and mammals underlies their high activity levels, but 

 it also imposes a high energy cost— some eightfold more than for ectotherms. 

 The energy cost of endothermy demands that organisms using this strategy 

 be far more efficient in obtaining food than their ectothermic competitors— 

 perhaps one of the reasons for the correlation of large brain size with 

 endothermy. There are few studies of elasmobranch temperature regulation 

 and maintenance, but Isurus and Lamna are known to maintain body tem- 

 peratures well above those of the ambient environment (Carey and Teal 

 1969). These species are high-speed predators feeding on equally fast-moving 

 prey. We need information on the temperature regulatory abilities of other 

 large-brained elasmobranchs— carcharhinids, sphyrnids, and myliobati- 

 forms— to decide whether elevated body temperatures characterize large- 

 brained elasmobranchs or reflect a specific adaptation of lamnid sharks. 



We know virtually nothing about chondrichthian behavioral capacities. 

 Long-term behavioral observations are almost impossible due to the dif- 

 ficulties of maintaining most species in captivity for any length of time. 

 Field studies present equally formidable problems when the animals under 

 study are swift, far-ranging species— many of whom are also efficient preda- 

 tors—in an environment alien to human observers (Nelson 1977). To date, 

 the most detailed observations on the social behavior of chondrichthians are 

 included in one study on bonnethead sharks (Sphyrna tiburo), carried out in 

 a seminatural enclosure by Myrberg and Gruber (1974). These workers 

 recognized 17 separate behavioral units, 8 of which occurred in a social 

 context. Myrberg and Gruber's study does not include observations on court- 

 ship, mating, or feeding. Detailed observations on the behavioral repertoire 

 of a single chondrichthian species do not exist, let alone the interspecific 

 observations needed to recognize correlations and trends in brain-behavioral 

 complexities. 



While some learning studies have used sharks as subjects (see Gruber and 

 Myrberg 1977 for a recent review), the tasks have been limited to simple 

 Y-maze performance or brightness and pattern discrimination. These studies 

 demonstrate that sharks learn at rates comparable to those of birds, mam- 

 mals, and teleosts, but they tell us little about the role of learning in elasmo- 

 branch behavior or about the complexity of problems that elasmobranchs 

 might handle. Evaluation of the role of learning in chondrichthians requires 

 answers to questions that relate learning to the chondrichthian environment: 

 do individuals learn to recognize one another in a social context, do they 



