Cost/benefit analysis of group and solitary resting in the cowtail stingray, Pastinachus sephen

Unless a safe refuge is found where predation threats are negligible, resting poses risks for many animals, necessitating risk management strategies. The adult cowtail stingray (Pastinachus sephen) of Shark Bay, Western Australia, is a solitarily foraging animal that facultatively groups when resting on shallow, inshore sand flats. We hypothesized that environmental conditions influence the propensity of cowtails to group due to the limited ability to detect predators visually in certain conditions. We then explored the possible benefits of grouping, such as bodily protection, early warning, and predator confusion, in conjunction with potential grouping costs, such as increased interference when initiating flight and decreased escape speeds. Our study revealed that in poor underwater visibility (due to turbidity and/or low ambient light levels), cowtails primarily rest in small groups (three rays). Tests of flight initiation distance to a mock predator demonstrated that solitary cowtail escape distances are significantly shorter in poor than in good underwater visibility conditions. As to grouping benefits, filmed boat transects revealed that cowtails most often arrange themselves in a rosette position, possibly as a means to protect their bodies and expose their tails. The first cowtail in a group initiates flight to a mock predator at a significantly greater distance than a solitary cowtail, and grouped cowtails escape an approaching boat in a significantly more cohesive manner than a simulated group of escaping individual rays. Grouped cowtails exhibit behaviors that would impede immediate flight after detection. As a result, grouped rays escape a boat at significantly slower speeds than solitary cowtails. Results from this study demonstrate that the interplay between costs and benefits of grouped and solitary resting under differing environmental conditions is driven by differences in perceived predation risk and ultimately reflected in the facultative grouping behavior of this species.