Annual and local variation in reproduction in the red-eared slider, Trachemys scripta elegans

We studied variation in reproductive traits among 789 red-eared sliders (Tr achemys scripta elegans] from three sites in west-central Illinois from 199 4 to 1996. Overall, mean clutch size ranged between 12.5 and 15.1 eggs, mea n egg mass per clutch ranged between 10.03 and 11.68 gr and mean relative c lutch mass (clutch mass/spent body mass) ranged between 0.079 and 0.097 for nine site-year cohorts. Maternal body size was the most important source o f variation in all measures of reproduction because larger females laid lar ger eggs and larger clutches than smaller females. Reproductive output inde pendent of body size was greater in 1995 than in 1994 or 1996. Clutch size and egg size varied similarly among years at each site. Overall, clutch mas s in 1995 was 15.8% and 17.8% greater than dutch mass in 1994 and 1996, res pectively. This variation in reproductive output was primarily due to chang es in egg number rather than changes in egg size, consistent with theories of egg optimality. Turtles at one site produced relatively small but relati vely more eggs per clutch than turtles from the other two sites. This patte rn persisted despite varying amounts of energy devoted to reproduction from year to year. However, an egg size-clutch size trade-off could only be wea kly demonstrated among nine site-year cohorts. Geographic variation was con firmed for clutch size, which decreased with decreasing latitude, but not f or egg mass and plastron length.