Koen Alonso et al : Food habits of Otaria fiovescens 



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mostly immature individuals whereas female sea 

 lions consumed mature ones. If females search for 

 prey on rocky bottoms, they will catch the mature 

 and larger red octopus. Instead, if male South Amer- 

 ican sea lions feed mostly in the water column 

 near the bottom, they could catch the younger red 

 octopus when they are actively moving on the bottom. 

 Younger octopus could be more vagrant than adults, 

 and in some species, posthatching octopuses do exhibit 

 pelagic behavior (Boletzky, 1977). The difference in 

 the size of red octopus that South American sea lion 

 consumed could be associated with this characteris- 

 tic in prey biology, thus supporting the hyphothesis of 

 different feeding behavior between the sexes. 



In regard to gastrolith function, the hypothesis of 

 buoyancy control has been postulated with the evi- 

 dence of the presence of gastroliths in several living 

 and extinct tetrapods that swim using their limbs in 

 the form of an underwater fly (Taylor, 1993). Taylor 

 (1993), in an extensive comparative study, demon- 

 strated that there is no correlation between the pres- 

 ence of gastroliths and diet, but he found a correlation 

 between gastroliths and underwater flying. Another 

 explanation for the ingestion of stones is that gastro- 

 liths "grind up" parasitic worms that usually infest 

 seals (Riedman, 1990). The role of stomach stones 

 seems to be better explained as "buoyancy control" 

 than as "elimination of stomach parasites" because 

 of the independence between the presence of para- 



sites and gastroliths, and the significant correlations 

 between predator size and gastrolith weight. Gas- 

 troliths found in the South American sea lions could 

 be considered "ballasts" that allow the sea lions to 

 regulate their buoyancy. Moreover, the gastroliths 

 can also be quickly swallowed and vomited, allow- 

 ing sea lions to change buoyancy according to their 

 needs (Harrison and Kooyman, 1968). 



In summary, these stomach content analyses indi- 

 cate that South American sea lions feed primarily 

 on demersal and benthic species and, in general 

 terms, use resources according to their environmen- 

 tal availability. Males and females appear to have 

 different constraints in their feeding behavior and 

 these restrictions could lead females to feed in more 

 coastal and shallower waters than those where males 

 feed. These potential differences in feeding grounds 

 or home ranges, or both, could explain the observed 

 differences in diet between the sexes. 



Acknowledgments 



The authors wish to thank Pablo Mariotti, Barbara 

 Heron Vera, Nancy Mora, Pablo Nepomnaschy and 

 Laura Reyes for their help with the stomach contents 

 analysis; Silvana L. Dans, Pablo Yorio and Guillermo 

 Harris for their critical reading and useful comments 

 on earlier versions of the manuscript; and all of the 



