Bimodal nesting season in Karoo dwarf tortoises (Chersobius boulengeri)

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.