Aggressive Interactions during Basking among Four Species of Emydid Turtles

I observed 123 instances of aggression in 100.7 h among basking emydid turtles (Trachemys scripta, Pseudemys concinna, Graptemys pseudogeographica, and G. ouachitensis). No consistent pattern occurred in heterospecific interactions with regard to outcomes. Larger turtles "won" (i.e., caused or resisted displacement) in 70% of all interactions, and initiators won 80%; in all, 61% were won by larger, initiating turtles. Larger turtles displaced smaller turtles into the water more often than the reverse. Larger turtles also elicited avoidance behaviors from smaller turtles that were not elicited from larger turtles by smaller turtles. The hypothesis that smaller turtles assess and avoid larger turtles was suggested by (1) the lower rate of initiation by smaller turtles, (2) the turning away response that was observed only in smaller turtles, and (3) the fact that smaller turtles sometimes voluntarily displaced themselves when larger turtles emerged from the water or advanced toward them on a substrate, but the reverse was not observed. Alternatively, inherent aggressiveness may be a function of body size in these species.