ELASMOBRANCH BRAIN ORGANIZATION 181 



the pallial amygdala in other vertebrates. This suggestion is based primarily 

 on the relationships exhibited in Notorynchus (Figure 10). However, this 

 region is more complicated in most other elasmobranchs, and this condition 

 is likely due to a subdivision of nucleus a and/or the migration of additional 

 pallial elements into this region in advanced elasmobranchs. 



DISCUSSION 



Brain— Behavioral Correlates 



Fifteen years ago it was easy to characterize chondrichthians as primitive 

 fishes with small, simple brains and limited behavioral repertoires. Renewed 

 interest in all aspects of elasmobranch biology and the emergence of experi- 

 mental neurobiological studies are forcing a reevaluation of these conclu- 

 sions. Cartilaginous fishes possess large brains whose brain-to-body ratios fall 

 within the ranges for birds and mammals. Their brains are not merely large; 

 the relative development of their major divisions closely parallels that of 

 birds and mammals (Bauchot et al. 1976, Ebbesson and Northcutt 1976, 

 Northcutt 1977 b). 



The discovery that elasmobranchs possess high brain-to-body ratios was 

 noted much earlier (Quiring 1941), but was reported as merely a curious 

 finding that did not conform to Quiring's "scale of being" in which verte- 

 brate brain size increased with supposed phylogenetic level. It would be 

 equally fallacious simply to assume that elasmobranchs and mammals pos- 

 sessing comparable brain sizes also possess comparable neural capacities. We 

 know too little about the neural basis of vertebrate behavior in general— and 

 even less about that of elasmobranchs— to make such a bald assumption. 

 However, one task of comparative neurobiology is to compare, and the 

 number of rapidly accumulating similarities among large-brained verte- 

 brates—restricted olfactory projections, differentiation of telencephalic 

 pallial areas receiving multiple sensory inputs, long descending pallial ef- 

 ferents, and expanded cerebellar cortices— warrant comparisons in a search 

 for possible common selective pressures that may underlie the evolution of 

 large-brained vertebrates. 



Pair bonding, parental care, endothermy, increased dependence on learned 

 behaviors, a large number of species-specific behaviors, high levels of explora- 

 tory behavior, and extensive manipulation of food characterize birds and 

 mammals. Clearly, these traits are not exclusive to birds and mammals, bony 

 fishes and reptiles frequently exhibit one or more of these traits, but they 

 are not seen in concert except in birds and mammals. However, there is a 

 direct correlation between large brain size and complex behavior in bony 

 fishes (Northcutt and Braford 1977) and reptiles (Platel 1976, Northcutt 

 1977c). 



Pair bonding and parental care confer on offspring a number of advant- 

 ages, including protection and extended maturation periods. These qualities, 

 as such, are unknown in elasmobranchs (Gruber and Myrberg 1977). How- 

 ever, 22 of the 31 chondrichthian families are solely viviparous, with 



