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Predation-risk, forage selection, or activity budget hypothesis?

The main difference that distinguishes these three hypotheses is their different emphases on the role food plays in maximizing sharks’ reproductive success. The predation-risk hypothesis considers food to be less important than predation avoidance while both forage selection and activity budget hypotheses emphasis that it’s because of differences in diet that sexual segregation occurs (Wearmouth&Sims, 2008). However, the forage selection hypothesis and activity budget hypothesis differ in that the latter states that energy cost differences causes group conflict that lowers fitness while the former emphasizes that there was no prior obligation to remain as a collective group, thus division of the sexes occurred independent of a mixed social group (Wearmouth&Sims, 2008).

Sphyrna lewini , the scalloped hammerhead shark, is a great example of a socially segregated elasmobranch species. Both males and females remain in the nursery after birth and successfully coexist as a size-segregated group until they reach adolescence (Klimely, 1987). As juveniles, the female hammerheads depart for the shallows to feed on larger pelagic prey while the males remain behind to continue feeding in the nursery.

According to the predation-risk hypothesis, this behavior is a result of increased predation on females (Wearmouth&Sims, 2008). However, at this time, no sexual dimorphism is observed in the juveniles: both the males and females are physically the same. Thus nothing distinguishes one sex from the other as preferable prey (Klimely, 1987) – the females aren’t smaller or weaker than males, making them easier to feed upon. In fact, no biotic or abiotic condition differed for the two sexes during separation (Klimely, 1987). The absence of sexual dimorphism in juveniles of elasmobranches isn’t confined to only scalloped hammerhead sharks; many juveniles of sexually-segregated elasmobranch species are commonly observed to form groups containing both sexes, such as the oceanic whitetip sharks, Carcharhinus longimanus (Coelho et al, 2009). Although they are often found in segregated groups as adults, juvenile whitetips caught in the Gulf of Mexico are found in groups composed of both females and males of similar sizes (Coelho et al, 2009). Hence size differences fail to account for why the sexes separate upon maturity: the predation-risk hypothesis remains largely unsupported.

Similarly, the activity budget hypothesis appears just as implausible. Since this hypothesis centers on the differences in energy consumption between sexually dimorphic organisms, the lack of difference in the physique of the two sexes at the time of sexual segregation indicate that energy expenditure isn’t the source of the behavior (Klimely, 1987). Also, juvenile sharks are smaller than mature specimens of the same species, thus they would gain fitness by practicing protective segregation in the nursery (Heupel&Heuter, 2002)(Heupel&Simpfendorfer, 2005). Rather, if they separate according to sex while physically immature, the loss of protective cooperation would result in mutual lose of fitness. By this reasoning, the proposal that energy usage by males and females are different enough to incur loss of fitness is unlikely (Wearmouth&Sims, 2008). Thus the suggestion that sexual segregation resulted due to energy cost conflict between sexually dimorphic sexes remains largely unsupported.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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