Good moms: dependent young and their mothers cope better than others with longer dry season in plains zebras.

Abstract

In large herbivores, the timing of births often coincides with the seasonal peak of food resources availability, likely to improve juvenile survival and reduce reproduction costs. Some species, however, breed year-round, even in seasonal environments. Demographic processes, such as to what extent being born during the lean season reduces survival of juveniles and reproductive females, remain understudied in large mammals inhabiting tropical ecosystems. We investigated survival rates in 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 early juvenile survival (φ = 0.458 ± 0.044 SE, < 6 m.o.), whereas late juvenile and yearling survivals were higher and decreased with increasing length of the dry season (from 0.850 ± 0.095 SE to 0.480 ± 0.120 SE). Female survival was high (> 0.703 ± 0.057 SE and up to 0.995 ± 0.006 SE) but decreased with exposure to the dry season in non-reproductive females. The probability of females becoming reproductive in the following year was not affected by the length of the dry season (0.423 and 0.420 for reproductive and non-reproductive females, respectively). 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.