Reproduction of two species of freshwater turtle, Chelodina rugosa and Elseya dentata, from the wet–dry tropics of northern Australia

Abstract

Chelodina rugosa is a fast-growing, carnivorous turtle that occupies seasonally ephemeral habitats, whereas Elseya dentata is relatively slow-growing, largely herbivorous and confined to permanent water. Ovarian cycles followed an annual cycle of enlargement beginning in December (E. dentata) and January (C. rugosa), followed by ovulation and then follicular regression. Nesting season was determined from presence of gravid females, and for C. rugosa spans some 8 months from the late wet season until mid dry season, during which the turtle lays multiple clutches of eggs in underwater nests. Nesting may continue for longer when waterholes dry later following prolonged wet season rains. Nesting by E. dentata also begins in the late wet season but is complete by May and only one clutch is laid. Females of both species reproduce each year and clutch size and mass are related to female size. Annual spermatogenic cycles are similar in timing to those reported for temperate-zone chelids but would be classified as pre-nuptial because spermatogenesis begins before breeding. Gonial proliferation begins at the start of the wet season and spermiation coincides with the onset of ovulation. Mating was not observed but in C. rugosa probably occurs during December and January soon after turtles emerge from aestivation. Post-mating storage of sperm in the epididymes did not occur in E. denatata but did appear to occur in some C. rugosa. The high reproductive output and extended nesting season of C. rugosa reflects its occupation of highly productive but unpredictable habitats and a reproductive strategy which ensures that some hatchlings meet optimal conditions for emergence. In contrast, the comparatively low fecundity and short nesting season of E. dentata, despite year-round warm temperatures and access to permanent water, suggests that its reproductive output may be energetically limited by a largely herbivorous protein-poor diet.