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Size segregation or coincidence?

Sharks do segregate according to size, but whether or not it’s a conscious effort is debatable. At first, the results of Guttridge et al.’s experiment (2009) on juvenile lemon sharks, Negaprion brevirostris, seemed to suggest that these sharks consciously choose to remain in groups of similar size: when juvenile N. brevirostris were introduced to a two-compartment tank containing only other N. brevirostris (except for juvenile sharks between 0-1 years) the lemon sharks spent more time with those similar to themselves in age and size (Guttridge et al., 2009). In this controlled experiment where there are no external stimuli that might cause the lemon sharks to gather, the fact that these juveniles did indicates that there must be a benefit to this behavior.

Not only was size segregation observed in N. brevirostris under experimental conditions, bullsharks, Carcharhinus leucas , also gathered naturally in size-specific groups throughout the Southwest Florida Estuary within region-specific areas: the neonates and juveniles were concentrated mostly in rivers and lakes inland and the larger while full grown adults spread out in deeper, offshore bays (Simpfendorfer et al. 2005). Overall, the best model of size sorting behavior is exhibited by the widespread population of ragged-tooth shark, Carcharias taurus; unlike the bullsharks, whose grouping behavior is confined to a small region in the Southwest Florida Estuary, the C. taurus population is spread out along the coasts of Europe and Africa. With size groups located in distinct off shore areas of both two continents, segregation in these sharks are more apparent: the sub-adult and adult C. taurus were found mostly along the west coast along the tip of southern Africa while the juveniles were found near the east coast of the British Isles. The younger the shark, the further north along the British Isle coastline it was found. Similarly, the older and larger the shark, the further south along the African coast it was found (Dicken, Smale, and Booth 2006). Upon first glance, sharks appear to choose to separate into groups of varying size.

Two lemon sharks.
Two lemon sharks, Negaprion brevirostris, interacting without aggression

According to a study Lowry and Molta did in 2008, a direct relationship was identified between the age and diet of a shark. When they observed feeding efficiency in maturing leopard sharks (Triakis semifasciata) and whitespotted bamboo sharks ( Chiloscyllium plagiosum), they found a positive correlation between suction power and their ontogeny (age). This increase in suction power resulted in dietary shifts as their feeding abilities increased (Lowry&Molta, 2008). Another study on the sevengill shark, Notorynchus cepedianus, also found similar results that correlated ontogeny to prey size: as the shark got older, they moved away from smaller prey they used to feed on in favor or larger, more nutritious game (Edbert, 2002). Thus not only is diet directly related to the age of a shark, so is the shark’s size (Sims et al., 2006b): the older a shark gets, the larger it becomes. This indicates that the diet is also dependent on the size of the shark. Thus what initially appeared to be social interactions between sharks of similar physiques may simply be sharks gathering in an area because they hunt the same prey; size segregation is a direct result of overlapping diets (Wetherbee&Cortes, 2004).

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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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