BOWEN, K.D.; SPENCER, R.J.; JANZEN, F.J.; Iowa State University; Iowa State University; Iowa State University: Quality vs. Quantity of Offspring: Optimal Egg Size Models Break Down in Painted Turtles, Chrysemys picta

In many species of freshwater turtles such as Chrysemys picta, there is a trade-off in reproductive investment by females. As the number of propagules produced (clutch size) increases, the size of the propagules (egg size) decreases. Furthermore, egg size usually varies less than clutch size, presumably because offspring size affects survival and growth. We analyzed clutch size/egg size relationships in a population of Chrysemys picta from northwestern Illinois. Reproductive output was monitored starting with primiparity and continuing for eight years. Over this period we found no relationship between clutch size and egg size. Annual egg production remained constant, but egg mass increased consistently up to the eighth year. Reproductive output therefore appears to be strongly related to body size. Selection may favor quantity over quality early in the reproductive life of the turtle; i.e. maturity is delayed until the animal reaches a body size capable of producing a particular number of offspring. The quality of these offspring is then increased as further gains in maternal body size allow.