Function of the tail in myliobatid rays: role in controlling body stability.

Eagle rays, cownose rays and manta rays are the only batoid families exhibiting oscillatory locomotion, and are characterized by long, slender tails. This study investigates whether tail length influences body stability when the pectoral fins are held in an extended, static position. We measured relative tail lengths across the four families (Rhinopteridae, Myliobatidae, Aetobatidae and Mobulidae), finding that spotted eagle rays have the longest tails (greater than 4× body length (BL)), while giant manta rays have the shortest (approx. 0.9× BL). To test the effects of tail length on posture and stability, we used three-dimensional-printed models based on a myliobatid body and a NACA 0012 foil in a flow tank across increasing speeds. Pitch, roll and overall dynamic body acceleration (ODBA) were recorded using embedded accelerometers. Models without tails showed increased roll and ODBA, while models with tails greater than or equal to 0.9× BL, which was the minimum length found in measured animals, maintained a steadier position. This result indicates that tails enhance passive stability by providing drag-based damping and a restoring torque that helps the ray models resist and recover from disturbances. As longer tails did not further improve stability, tails exceeding 0.9× BL may serve additional roles, such as communication, mating or sensing.