INFLUENCE OF THE HYDRIC ENVIRONMENT AND CLUTCH ON EGGS AND EMBRYOS OF2 SYMPATRIC MAP TURTLES

1. Substrate moisture exerts substantial yet variable effects on the p hysiological ecology of reptilian eggs and embryos. Although previous studies have focused on one species at a time, simultaneous comparativ e examinations can lend greater insight into the functional significan ce of this variation by allowing direct quantitative, as opposed to in ferred, comparisons. 2. We concurrently incubated eggs of two syntopic , congeneric turtles (Graptemys ouachitensis and G. pseudogeographica) at 29 degrees C on 'wet' (-150kPa) and 'dry' (-950kPa) substrates to assess the effects of substrate moisture on growth and survivorship of embryos. 3. Water potential did not influence embryonic mortality or hatchling abnormalities but small eggs were less likely to hatch than large ones. 4. Eggs of both taxa gained mass, took longer to hatch and produced larger hatchlings when incubated on the wet substrate compar ed to the dry substrate. These physiological traits were also influenc ed by clutch and taxon, suggesting possible genetic variation. 5. Eggs of G. ouachitensis gained relatively more mass at -150kPa, lost relat ively less mass at -950kPa and hatched sooner than did eggs of G. pseu dogeographica. Hatchling G. ouachitensis were relatively larger than h atchling G. pseudogeographica and this taxon effect was greater than t he influence of the hydric environment. 6. Significant quantitative di fferences in the physiological responses of the two taxa to substrate moisture support recent taxonomic work suggesting that they are separa te species. Detection of and confidence in these differences were only possible because eggs were incubated simultaneously.