Bimodal nesting season in Karoo dwarf tortoises ( <i>Chersobius boulengeri</i> )

Tortoises in arid, unpredictable regions may use a bet‐hedging strategy of regularly laying small clutches regardless of actual environmental conditions, so that some hatchlings will emerge when conditions are favourable. Karoo dwarf tortoises ( Chersobius boulengeri ) are endangered in their arid, unpredictable range in South Africa, yet studies of reproduction are lacking. I used radiography and captive breeding to study reproduction in wild (n = 22, 2018–2022) and captive (n = 2, 2020–2023) individuals, expecting to find bet‐hedging characteristics. My expectation was that females would regularly lay eggs in summer, when most rain falls. In addition, I expected that females would produce consistently large eggs and hatchlings capable of surviving their harsh environment. Indeed, females produced multiple clutches of large single eggs per year, some even during drought. However, the egg‐laying season had a unique bimodal pattern in autumn and spring, possibly dispersing hatching events throughout the unpredictable rainfall season. Fecundity was low, approximately 5 eggs per year when resources were unlimited. The large eggs were usually wider than pelvic width, but egg size was nevertheless constrained by female size. Hatchling size correlated positively with egg size, confirming a potential survival benefit. Surprisingly, egg size decreased in successive clutches in a season, apparently to increase offspring frequency while resources were limited. Reproduction of Karoo dwarf tortoises is unlikely to offset current predation levels that contribute to their endangered status, thus predation reduction should be a conservation priority. Projected regional aridification due to climate change may pose a previously underestimated threat, because females lay fewer eggs during drought, females that lay eggs during drought may suffer increased mortality when body reserves cannot be restored, and hatchlings (particularly from late clutches) may have insufficient reserves for survival under aridified conditions. Conservation plans might focus on sites least affected by aridification.