The fossil history of Grévy's zebra (Equus grevyi) in equatorial East Africa

Aim: Within the last several decades, Grévy's zebra (Equus grevyi) has undergone a massive reduction in geographical range and population size, largely as the result of human impacts. To place its recent decline in a deeper prehistoric context, and to understand the factors mediating its range and abundance over geological time frames, this study examines the fossil history of Grévy's zebra in equatorial East Africa. Location: Equatorial East Africa. Methods: Presence/absence data for ungulates recovered from fossil sites spanning the last c. 400,000 years in Kenya and Ethiopia were compiled from the literature and from previously unreported palaeontological sites. Associations between Grévy's zebra and other taxa were examined using principal coordinates analysis and non-random species pairs were identified using a Bayesian approach. Changes in rainfall were reconstructed using the average hypsodonty index of ungulate species from fossil assemblages. Results: Grévy's zebra was common during dry phases of the Pleistocene and was found to the south and west of its historical range, coinciding with an expansion of arid grasslands. At the onset of the Holocene, Grévy's zebra was extirpated from southern Kenya and almost completely disappeared from the fossil record. Grévy's zebra was associated with several specialized grazers that became extinct by the end of the Pleistocene. These extinctions and the decline of Grévy's zebra from the Pleistocene to the Holocene are explained by increased precipitation and the consequent loss of arid grasslands at the Pleistocene-Holocene transition. Grévy's zebra is never associated with domestic livestock, unlike the widespread plains zebra. Main conclusions: Grévy's zebra thrived in equatorial East Africa during periods of the Pleistocene when environmental conditions favoured an expansion of arid grasslands. Environmental change across the Pleistocene-Holocene transition contributed to decreases in the range size and abundance of Grévy's zebra, setting the stage for the anthropogenic decline observed in recent decades. The spread of pastoralists in the middle Holocene may have additionally contributed to its prehistoric decline. Contemporary climate change warrants further consideration in planning for the long-term survival of Grévy's zebra.