Aggressive Interactions during Basking among Four Species of Emydid Turtles

Abstract

-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 (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. l erpetology, Vol. 3 , No. 2, p . 214-219, 1999 1999 Society for the Study of Amphibians and Reptiles i e I teractions during Basking among Four Species of tles Aquatic emydid turtles are noted for basking on logs and branches that project from the water (Boyer, 1965). Basking behavior is primarily a thermoregulatory behavior but may also serve to rid turtles of skin parasites and epizootic algae and to promote vitamin D synthesis (Boyer, 1965; Moll and Legler, 1971; Vogt, 1979). Aggressive competition for space may occur among basking turtles if basking substrates are 3 Present Address: Division of Biological Sciences and Related Technologies, Madisonville Community College, 2000 College Drive, Madisonville, Kentucky 42431 USA. E-mail: LPeterO@pop.uky.edu limiting. While many authors have commented on the seeming lack of aggression among basking emydid turtles (Boyer, 1965; Ernst, 1971; Waters, 1974; Vogt, 1980), several studies have documented aggressive behavior between conspecifics, including open-mouth gestures, biting, and various forms of pushing (Bury and Wolfheim, 1973; Auth, 1975; Bury et al., 1979; Lovich, 1988; Pluto and Bellis, 1986). Limited data show that similar aggressive interactions also occur between basking heterospecifics (Auth, 1975; Lovich, 1988; Lindeman, 1996). Body size may play a role in determining the outcome of aggressive interactions. Larger Grap214 This content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 05:42:45 UTC All use subject to http://about.jstor.org/terms AGGRESSION AMONG BASKING EMYDIDS temys geographica displaced smaller conspecifics more often than the converse (Pluto and Bellis, 1986), and in three cases of interspecific aggression, the larger turtle displaced the smaller turtle (Auth, 1975). Avoidance behavior in Clemmys marmorata was also size dependent (Bury and Wolfheim, 1973). I observed interactions of basking turtles in a mixed-species assemblage in a riverine reservoir in western Kentucky. The objectives of my study were to determine whether (1) some species have a consistent advantage over other species in competition for basking space; (2) relative body size predicts the outcome of interactions in a mixed-species assemblage; and (3) initiators tend to win interactions (i.e., cause or resist displacement) more frequently than recipients of interactions. MATERIALS AND METHODS During 1992-1994, I used a spotting scope with 22-60x zoom magnification to observe basking behavior of turtles using single basking substrates (logs and branches) or clusters of 14 substrates in three coves on Kentucky Lake, an impoundment of the lower Tennessee River: Nickell Cove (on 15 d), Jones Creek Cove (1 d), and a cove on the canal that connects Kentucky Lake with Lake Barkley (1 d). I observed turtles from early morning until late afternoon but terminated observation earlier for inclement weather or heavy recreational use of the area. The primary species observed were the emydids Trachemys scripta, Pseudemys concinna, Graptemys pseudogeographica, and G. ouachitensis. Notes on instances of contact or aggression among turtles were recorded using a hand-held tape recorder. I identified species involved and recorded the relative sizes of the two turtles if there was a discernible difference, with details about their interaction and its outcome. Individuals were classified as initiators or recipients of interactions.