Abstract
Complex social networks and behaviors are difficult to observe for free-living marine species, especially those that move great distances. Using implanted acoustic transceivers to study the inter- and intraspecific interactions of sand tiger sharks Carcharias taurus, we observed group behavior that has historically been associated with higher order mammals. We found evidence strongly suggestive of fission-fusion behavior, or changes in group size and composition of sand tigers, related to five behavioral modes (summering, south migration, community bottleneck, dispersal, north migration). Our study shows sexually dimorphic behavior during migration, in addition to presenting evidence of a potential solitary phase for these typically gregarious sharks. Sand tigers spent up to 95 consecutive and 335 cumulative hours together, with the strongest relationships occurring between males. Species that exhibit fission-fusion group dynamics pose a particularly challenging issue for conservation and management because changes in group size and composition affect population estimates and amplify anthropogenic impacts.
Highlights
While many sharks are solitary predators, some are known to live in groups and are suspected of engaging in complex social behaviors[1], others aggregate due to similar habitat, food, or mating requirements
Sand tigers have high brain to body mass ratios when compared to other Chondrichthyes[16], and may have the ability to maintain complex social structures and social behaviors[17,18] such as coordinated group feeding behaviors[19] similar to those observed in marine mammals[20]
ST1 and ST2 were implanted with the VMTs on the 24 August 2012, and were recaptured within 16 days of each other in 2013 (26 July and 11 August respectively) and 9 km apart in the Delaware Bay. These sand tigers were tagged with Pop-up Satellite Archival Tags (PSATs) to potentially add a spatial context to detection events, little useable spatial information was recovered from the PSATs deployed
Summary
While many sharks are solitary predators, some are known to live in groups and are suspected of engaging in complex social behaviors[1], others aggregate due to similar habitat, food, or mating requirements. Studies have observed fission-fusion behavior in response to changes in prey availability[3,4,5], risk of predation[6,7], and territorial competition[8], but this behavior has been primarily documented in mammals, including humans[9], non-human primates[3,8,10], bats[11,12], and cetaceans[4,5], in addition to reptile[13,14] and fish species[15], but not in elasmobranchs. Understanding changes in the composition and size of sand tiger groups may be useful for effective conservation and management strategies because these behavioral dynamics allow perturbations to disproportionately affect different life history stages
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