ELASMOBRANCH BRAIN ORGANIZATION 

 ICOOO- 1 — —  — ■— - 1 — >- — •- - 1 - 



143 



0.001 



0.1 1 10 100 1000 



BODY WEIGHT IN KILOGRAMS 



10,000 100,000 



Figure 18 Brain and body weights for four vertebrate classes expressed as minimum 

 convex polygons, after Jerison (1973). Stippled polygon encloses elasmobranch brain-to- 

 body ratios and overlaps polygons for bony fishes, birds, and mammals. (After Northcutt 

 1977.) 



rajiforms are characterized by low brain: body ratios and the more advanced 

 myliobatiforms by the highest brain: body ratios known for elasmobranchs. 



Numerical estimates of these brain size differences can be obtained by 

 calculating encephalization quotients {EQ) for the species listed in Table 2. 

 The encephalization quotient is the ratio of actual brain size to expected 

 brain size, defined by the allometric equation for brain: body relations. The 

 expected brain size is an "average" for members of a group and controls for 

 body size. Thus, calculating an EQ allows comparison of different species 

 regardless of body size. 



The EQs in Table 2 were calculated by the equation EQ = E/k p a , where 

 E and P are brain and body weights, respectively, and k and o: are respectively 

 intercept and coefficient of allometry determined from the sample shown in 

 Figure 19. Values of 0.012 (k) and 0.75 (a) for sharks and 0.002 (k) and 

 1.04 (a) for batoids were used in the calculations, rather than the mean 

 coefficient and intercept for the entire sample. This seemed preferable, as 

 the coefficients of allometry for sharks and batoids are very different; how- 

 ever, it precludes direct comparisons between sharks and batoids, as the 

 batoid EQs would be higher if the calculations were based on a mean a 

 and k. 



