1.1 Sharks: solitary or group animals?  (Page 7/10)

While the predation-risk and activity budget hypotheses both focused on external differences that may inhibit possible fitness benefits, the forage selection hypothesis takes into account both the physical and physiological changes experienced by the developing juvenile scalloped hammerhead sharks (Wearmouth&Sims, 2008). As recounted, there are no differences in the physique of the juvenile male and juvenile female S. lewini , and other juvenile sharks of sexually dimorphic species, thus their underdeveloped states would indicate that they suffer from equal amounts of predation and should have continued their practice of protection segregation (Klimely, 1987). However, the fact that females deviated from such a behavior indicate that they suffer to lose rather than gain fitness if this behavioral pattern persists.

Since female fitness relies heavily on maximizing reproductive success, physiological needs compel them to leave the safety of the nursery (Klimely, 1987). Since scalloped hammerhead sharks are viviparous , females require larger body sizes to accommodate the young (Sims, 2003). If the females were to grow at the same rate as males, they would be at a reproductive disadvantage because there’s less strain on the males when reproducing; males invest fewer resources (the sperm) compared to the females who have to house and feed the developing offspring. Thus, in order to match the reproductive output of similarly-aged males, the females need to reach reproductive maturity earlier than the males, resulting in sexually dimorphic adults. To initiate their earlier development, the female S. lewini leave the nursery ahead of the developing males to feed on larger, more nutritious prey in deep waters. This difference in prey preference leads to the sexual segregation observed in this shark species, as well as many other elasmobranches (Klimely, 1987)(Sims, 2003).

When comparing the predation-risk, forage selection, and activity budget hypotheses, the model, S. lewini , clearly indicates that sexual segregation isn’t an aggregative behavior that occurs due to physical needs but rather an act of lowering reproductive losses. While all three hypotheses cite sexual dimorphism as a factor of sexual grouping behavior, only the forage selection hypothesis indicates that sexual dimorphism is a result of physiological needs to increase reproductive output rather than the cause of segregation (Sims, 2003). Thus it’s not surprising that females of elasmobranch species that are viviparous and ovoviviparous practice sexual segregation. This accounts for the fact that sexual dimorphism is only observed after sharks have evolved, as seen with both S. lewini and Carcharhinus longimanus, the oceanic whitetip sharks. On the other hand, both the predation-risk hypothesis and the activity budget hypothesis focused on how size differences in the adult specimens force different needs on the sharks rather than the ultimate cause. They both fail to note that sexual segregation occurs before sexual dimorphism does.