1.1 Sharks: solitary or group animals?  (Page 8/10)

Thermal niche-fecundity and social factors hypotheses

However, despite the clear association between sexual dimorphism (due to viviparous and ovoviviparous reproduction) and sexual segregation, none of the hypotheses presented above are able to explain the presence of sexual segregation within the catshark, Sychliorhinus canicula (Wearmouth&Sims, 2008). Since catsharks oviparous , the females do need larger physique for bearing live young. As a consequence, S. canicula is one of the few monomorphic species of elasmobranches, making it an ideal model of study for environmentally and socially instigated sexual selection (Sims, 2003).

The thermal niche-fecundity and social factors hypotheses are very different theories: where the thermal niche-fecundity hypothesis suggests that sharks maximize reproductive success by responding to changes in environmental temperatures, the social factors hypothesis proposes that the sharks consciously segregate themselves by sexual preference and other social factors (Wearmouth&Sims, 2008).

Sychliorhinus canicual practices both spatial and habitat segregation despite being monomorphic: the female catsharks are found to spend most of their time in small tunnels in tidal loughs while the males are found to spend most of their time in deep waters offshore (Sims et al, 2001). However the cause of segregation in the catshark , S. canicula , is very different from the previous model: the two sexes are segregated for different reasons. While the female catsharks may gain benefits from the warm temperature of the shallows, these benefits are speculative at best; their behaviors are better explained as a social response to male violence during copulation. On the other hand, the males segregate themselves from the females solely because of an abiotic factor. Since all physical traits and prey preferences are the same, the differing reasons for segregation boil down to one thing: reproductive benefits.

Sims et al speculated in 2001 that female S. canicula lounge in the warm shallows, rather than the colder water found offshore, in order to better facilitate egg development, as observed in similar behaviorisms exhibited by many other elasmobranch species, such Triakis semifasciata , the leopard sharks . Refuging system behavior is often observed in adult female leopard sharks shortly after early summer mating (Hight&Lowe, 2007). The leopard sharks would migrate in groups to the warmest areas in the shallows, occasionally shifting position in order to remain in the warmest spot at all times, to better facilitate digestion, somatic growth, and embryonic development (Hight&Lowe, 2007). However, despite helping with growth and digestion, embryonic development is the most important proximate cause that facilitates this refuging behavior since reproduction is the only factor that directly increases the fitness of the organism. This is further supported by the fact that only mature female T. semifasciata exhibit this basking behavior during August, a time period associated with early embryonic development (Hight&Lowe, 2007). If this behavior was solely for facilitating growth, then juvenile females and males would also be spotted.