Good Moms: Dependent-Young and Their Mothers Cope Better Than Others with Longer Dry Season in Plains Zebras

In large herbivores, the timing of births often matches the seasonal peak of availability of food resources, likely to improve juvenile survival and reduce reproduction costs. Some species however breed year round, even in seasonal environments. To what extent being born during the lean season reduces juveniles and reproductive females survival remains mostly unknown. This holds particularly true in tropical ecosystems where the demographic processes underlying population dynamics are often poorly studied. We investigate the variability of survival for plains zebras ( Equus quagga ) in Hwange National Park (Zimbabwe), a highly seasonal savanna ecosystem. We used capture-recapture models to analyse long-term demographic data (2008-2019). We investigated the effect of seasonality as a categorical (wet versus dry season) and continuous (duration of the dry season) variable on survival. We found little variability in the survival of younger foals (< 6 m.o.), whereas older foals and yearlings’ survival, which was higher on average, decreased with the length of the dry season. Female survival was high but decreased with increasing exposure to the dry season for the non-reproductive ones only. The probability of females becoming reproductive the next year was not affected by the length of the dry season. Together, our results highlight the importance of individual quality in reproductive performance, as reproductive females seem to buffer the effect of environmental variability on their own survival and that of their foal. Timing of birth in this population might be the result of a trade off between minimising mother’s reproductive costs and maximising juvenile survival.