Big Fish Cannot Jump? Allometry of Terrestrial Jumping in Cyprinodontiform Fishes

Teleost fishes that emerge onto land must produce effective terrestrial movements to return to the water. Using the Cyprinodontiformes as a model system, we examined a terrestrial behavior termed the tail-flip jump across a size range of individuals representing three species of aquatic killifishes (Gambusia affinis, Poecilia mexicana, and Jordanella floridae) and two species of amphibious killifishes (Kryptolebias marmoratus and Fundulus heteroclitus) to identify potential effects of size (mass) on jumping performance. The ballistic trajectory equation was used to partition the contributions of velocity (determined by acceleration and contact time) and takeoff angle to jump distance. Despite differences in size (over an order of magnitude), all fishes took off from the ground at ∼45°. However, in terms of total displacement, aquatic and amphibious killifish species scaled differently in their ability to perform the tail-flip jump. Aquatic killifishes decrease in total jump distance as mass increases; however, amphibious killifishes increase in total jump distance as mass increases. Aquatic killifishes cannot produce adequate accelerations at larger sizes, but amphibious killifishes produce similar accelerations despite over an order of magnitude size difference. Because of this, amphibious killifish species are able to maintain fast takeoff velocities at large body sizes. Distinct scaling patterns may be generated by differences in body shape. Aquatic killifishes have a fusiform body shape, with most of their body mass in the anterior of the body, while amphibious killifishes have a more uniform body shape that reduces their overall mass present in the anterior body. We hypothesize that reduced mass in the anterior body facilitates raising the head over the tail to prepare for takeoff. In contrast with amphibious species, the negative scaling relationship seen in body size versus displacement in aquatic killifishes implies an upper size limit to producing the tail-flip jump for fish species that infrequently encounter the terrestrial environment.