Locomotor compensation in the sea: body size affects escape gait in parrotfish

There has been surprisingly little attention to adaptive variation in the locomotor speed and gaits used in antipredator behaviour. We investigated the relationship between body size and the use of two alternative gaits by three species of parrotfishes (princess, Scarus taeniopterus; queen, Scarusvetula; stoplight, Sparisoma viride) escaping an approaching snorkeller in their natural fringing reef habitat in Barbados. As body size increased from about 7 to 58cm, the proportion of fish using an energetically more costly but relatively faster escape (body and caudal fin swimming) rather than a less costly and relatively slower escape (paired fin swimming) decreased from 100% to 0%. In contrast, the study confirmed previous research showing that larger fish fled at greater distances from the snorkellers, behaviour which would have increased safety but incurred higher opportunity costs. We conclude that small fish require a more expensive gait to attain an adequate escape speed. Thus, the gait used for escaping shows a compensatory relationship with body size because small individuals with lower swimming capacity use a higher proportion of that capacity. On the other hand, flight initiation distance shows cospecialization with body size because larger fish with higher swimming capacity further reduce their risk by fleeing sooner. ► Gaits reveal how fast an animal moves relative to its potential speed. ► We studied the gaits that parrotfish use to escape an approaching snorkeller. ► Small fish swam away by undulating their tails (fast, expensive gait). ► Large fish flapped their pectoral fins (slower, cheaper gait). ► Small fish thus compensate for size-limited swimming capacity by a faster gait.