Table L — School .'iize. depth, .shark length, and sex competition of schools of scalloped hammerhead 

 .sharks at Las Arenitas Rocks, El Bajo Espiritu .Santo. Isla Las Animas, and all locations in the Gulf of 

 California. Numbers in parentheses refer to numbers of ob.servations. 



sharks extended to depths beyond our vision. 



that day, only 5 were reobserved 2 d later, despite 

 the fact that an equivalent amount of searching 

 was conducted. The possibility of such migratory 

 or dispersal movements was supported by the 

 occasional observation of single scalloped ham- 

 merhead moving slowly at the surface over deep 

 water between islands. 



The mean depth of the schooling individuals 

 was 10.4 m with sharks as shallow as 0.6 m and as 

 deep as 22.7 m (Table 1). Depths were estimated 

 with the aid of wrist-worn depth gages. Small 

 groups of sharks were seen swimming close to the 

 sandy or rocky substrate and at times appeared to 

 explore rocky crevices. 



The scalloped hammerhead schools were com- 

 posed of sharks of variable size; both sexes were 

 present in a school, but females were more com- 

 mon (Table 1). The largest individuals, which 

 appeared to be females, often remained at the top 

 of the schools. The sharks at El Bajo were larger 

 than those at Las Arenitas Rocks, but no differ- 

 ence existed between those at El Bajo and Isla Las 

 Animas and at Las Arenitas Rocks and Isla Las 

 Animas. Females were more prevalent at all three 

 sites with male:female ratios ranging from 1:3.8 at 

 El Bajo to 1:1.6 at Las Arenitas Rocks (Table 1). 



Within the groups, sharks performed a number 

 of behavior patterns, including the following (in 

 order of decreasing frequency): 1) tilting the body 

 laterally; 2) accelerated swimming with head- 

 shaking (Figure 2C); 3) accelerated swimming, 

 dorsal or ventral surface upward, while thrusting 

 the midsection; 4) accelerated corkscrew swim- 

 ming, rotating 360° around the longitudinal axis; 

 5) hitting of conspecific with the snout; 6) jaw 

 opening; and 7) clasper flexion. Headshaking has 



been observed previously in wild hammerheads by 

 Herald^; hitting, jaw opening, and clasper flexion 

 resemble behaviors observed in captive bonnet- 

 head, S. tiburo (Myrberg and Gruber 1974). 



Sharks of all sizes and both sexes performed 

 tilting. This generally occurred when a diver was 

 swimming above or behind the shark, perhaps 

 enabling the shark to see the diver better. In one 

 instance, while tilting, a shark flexed his clasper 

 in the direction of an adjacent conspecific. Six 

 males and three females varying from 1.5 to 1.8 m 

 long were observed headshaking. Medium-sized 

 sharks also performed corkscrewing; the sex of the 

 single shark identified was male. Although jaw 

 opening, apparently directed at divers, occurred in 

 sharks outside of groups, headshaking, midsec- 

 tion thrusting, and corkscrewing usually occurred 

 within the group, often at long distances from the 

 diver. On two occasions sharks followed head- 

 shaking and corkscrewing by hitting their snouts 

 against adjacent individuals, lateral or anterior 

 to the first dorsal fin. 



Many sharks bore abrasions (Figure 2D). These 

 were small, recently inflicted, whitish patches 

 (estimated diameter 4-8 cm), or partially healed 

 black patches. In 21 out of 27 sharks, these were 

 located lateral or anterior to the first dorsal fin, 

 and if two patches were present, at times appeared 

 bilaterally symmetrical. Possessors were predom- 

 inately female (23 out of 27) and ranged from 1.2 

 to 2.1 m long (x = 1.7 m). These scars may be 

 inflicted by males during mating. Captive, small 



'Herald. E. The shimmy behavior of the hammerhead shark. 

 Unpubl. manu.scr., 4 p. Steinhart Aquarium, San Francisco, 

 Calif, available from authors. 



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