Egg and offspring size, as well as other morphological and behavioral performance traits, play important roles in life-history models yet they vary greatly both within and among populations and taxa and temporally (Tucker et al., 1998, J. Herpetol.; Bowden et al., 2004, Funct. Ecol.; Schwanz et al., 2009, Ecology; Janzen and Warner, 2009, J. Evol. Biol.). We have investigated physiological mechanisms underlying this variation in snapping turtles (Janzen et al., 1990, J. Exp. Zool.) and in painted turtles (Harms et al., 2005, Physiol. Biochem. Zool.), differences in size variation between sympatric populations of map turtles (Graptemys) (Janzen et al., 1995, Funct. Ecol.), the heritability of shell shape variation in red-eared sliders (Myers et al., 2006, Evolution), the relationship of this variation to offspring performance in smooth softshell turtles (Apalone mutica) (Janzen, 1993, Physiol. Zool.; Ashmore and Janzen, 2003, Oecologia; Mullins and Janzen 2006, Herpetologica), painted turtles (Colbert et al. 2009, Funct. Ecol.), and red-eared sliders (Myers et al., 2007, Funct. Ecology), and the impact of this whole process on offspring fitness during a crucial stage of the life history of snapping turtles (Janzen, 1993, Ecology; Kolbe and Janzen, 2001, Funct. Ecol.), painted turtles (Paitz et al. 2007, Biol. Let.), and red-eared slider turtles (Janzen et al., 2000, Ecology and J. Evol. Biol.; Filoramo and Janzen, 2002, Herpetologica; Janzen et al., 2007, Funct. Ecol.; Tucker et al., 2008, Behav. Ecol.). We are actively continuing studies in various species to further elucidate the relationships between developmental environment, hatchling morphology, performance, and ultimately fitness (Tucker et al., 1999, Am. Midl. Nat.; Spencer et al., 2006, Ecology). Additional experiments have been or will be conducted to evaluate the causes of natural selection on offspring size using both size-manipulated hatchling Trachemys and soft plasticine replicas that "record" predation attempts.