Anti-Predator Benefits of Mixed-Species Groups of Cowtail Stingrays (Pastinachus sephen) and Whiprays (Himantura uarnak) at Rest

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Heterospecific grouping can sometimes provide greater antipredator benefits to individuals than grouping with conspecifics. We explored the potential benefits of mixed-species group resting in the cowtail stingray, Pastinachus sephen, and the reticulate whipray, Himantura uarnak, in Shark Bay, Western Australia. From focal follow data on individual resting choice, we first ascertained that cowtails preferred to rest with heterospecifics, as they chose to settle next to whiprays more often than to pass them (with the opposite trend observed for conspecifics). In addition, we determined from filmed boat transects that cowtails formed larger hetero- than monospecific groups despite the low density of whiprays. Possible benefits accrued by the cowtail were investigated in terms of predator protection. Whiprays responded earlier than cowtails to a mock predator (boat), and were most frequently the first to respond when in a mixed group. Thus, cowtails may benefit from grouping with heterospecifics by receiving earlier warning of a predator's approach. A decoy experiment using model whiprays demonstrated that cowtails were more willing to rest with models with relatively longer tails (controlled for body size). Ray tails, which are equipped with a mechanoreceptor capable of detecting predators, may constitute an important secondary means of predator detection aside from early warning. This contention is supported by the observation that stingrays mainly form resting groups when their visual ability is likely to be impaired by environmental conditions, and that tail length is negatively allometric with body size, suggesting its importance in vulnerable early life stages. If the efficacy of the mechanoreceptor increases with tail length, then cowtails may have further improved their likelihood of detecting predators by grouping with longer-tailed heterospecifics.