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

Mixed-species grouping is a widespread phenomenon found in arachnids, fish, birds and mammals (Morse 1977; Hodge & Uetz 1992; Bshary & Noë 1997; Herzing & Johnson 1997). Three main reasons have been postulated as to why such heterospecific groups are formed: (1) to improve foraging efficiency, (2) to reduce predation and (3) by chance encounters (Waser 1984; Sakai & Kohda 1995; Noë & Bshary 1997; Chapman & Chapman 2000). Because monospecific grouping can also provide foraging and anti-predator benefits, various advantages of mixed-species groups over single-species associations have been proposed. Most of these advantages stem from the differential sensory capabilities of Correspondence Christina A. D. Semeniuk, School of Resource and Environmental Management, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. E-mail: [email protected]