Reproductive Responses to Rainfall in the Namaqualand Speckled Tortoise

Although many tortoise species inhabit drought-prone regions with potentially limiting resources, these species have long, iteroparous lives. To assess reproductive responses to variation in rainfall, and to understand interactions among egg size, body size, body condition, and rainfall, we studied egg production in the Namaqualand Speckled Padloper, Homopus signatus signatus, in Springbok, South Africa, in five consecutive spring seasons. Annual rainfall was low and varied substantially (131–226 mm). The percentage of females that were gravid (36–75%) differed among years and correlated with the amount of rain in the months prior to nesting. Gravid females had a higher body condition than that of non-gravid females, presumably because individual variation in resource acquisition caused some females to forfeit reproduction. The body condition of gravid and non-gravid females differed among years and was lowest in the year of lowest rainfall. In most years, egg size correlated to female size, but neither female size nor egg size differed among years. Egg size did not significantly correlate to maternal body size in dry years, when other determinants, such as body condition, seemed to outweigh the effect of body size. Egg volume represented up to 11.9% of female shell volume. The adult shell is somewhat flexible dorso-ventrally, which may help females accommodate the large egg, as indicated by the larger shell height and volume of gravid compared to non-gravid females. Large eggs may be advantageous for H. s. signatus, as larger hatchlings may survive better in arid environments. Since the range of H. s. signatus is threatened with aridification, the effects of drought on egg production may seriously challenge the long-term survival of populations.