Reproductive output of ornate box turtles (Terrapene ornata)in Illinois, USA

Reproductive traits in turtles are often positively correlated to body size, with larger females producing more eggs. At northern latitudes, breeding frequency typically decreases, whereas clutch size increases. Given such patterns of variation in reproductive output, population-level data on clutch size and frequency are needed to predict the persistence of remaining Ornate Box Turtle (Terrapene ornata) populations. We radiographed and measured female Ornate Box Turtles in Illinois to learn if clutch size is positively correlated to body size and, if so, what is the best measurement to use to approximate body size. We then compared mean clutch size from published accounts across the species range. Of 106 turtles radiographed in Illinois, 36 had visible eggs. Clutch size ranged from 1 to 6 eggs, with a mean of 2.64 and 4.55 at our two most intensively studied sites. We found a modest fecundity advantage in clutch size with female body size. Body size accounted for 38% of clutch size variation. Our results highlight the need for additional studies on Ornate Box Turtle growth and reproduction to guide conservation decisions and better understand life-history constraints.