Grevy's zebra (Equus grevyi)

Mohamed bin Zayed Species project number 10051129

Low foal survival is believed to be one of the key reasons why Grevy's zebra populations are declining. In this project, we quantified Grevy's zebra foal survival for the first time, and then investigated the factors contributing to the patterns of survival we observed. Additionally, we investigated how differences in grass nutrient content impact the way lactating females and their foals use the landscape in a key conservation area for the species. Equids such as zebra are believed to make foraging decisions based on the quantity of food rather than its quality. However, lactating females have higher energetic and nutritional demands compared to other zebra and therefore face increased stress during the dry season when both the quantity and quality of resources decreases. We therefore analyzed the landscape use patterns of these females to determine if the quality of resources influences how they select which areas of the landscape to use frequently.

 

 

Part 1: Foal survival

For the endangered Grevy's zebra, low foal survival has been implicated as a major factor driving population declines. In order to investigate patterns of foal survival, the Lewa Wildlife Conservancy (LWC) in central Kenya has been using cheap, non-invasive sight-resight methodology to regularly survey Grevy's zebra foals since 2003. We used the resulting data to create statistical models to investigate how survival changes as foals age, as well as how it is impacted by rainfall and resource availability. We found that first year foal survival is less than 30%. The lowest survival rates occur within the first three months after birth and again at the time of weaning (approximately 7-8 months). Further, we showed that rainfall influences foal survival, where low rainfall months result in lower foal survival. Despite often being cited as a reason for low foal survival, we found that predation is less important in driving patterns of foal survival than is resource availability.

Our estimates of foal survival were lower than those reported in any previous study of equid populations and therefore raise serious conservation concerns for this species. For the future, we intend to evaluate how foal survival impacts population trends relative to other demographic parameters such as adult survival and fecundity. Collecting data on these other parameters will also allow us to more reliably model population dynamics and project population growth. 

 

Part 2: Landscape use patterns of lactating females and their foals

The dry season is a critical period for African ungulate survival, as resources decrease substantially in quantity and quality. For equids such as zebra, grass quantity is typically considered more important than quality in determining foraging decisions and resulting spatial patterns of grazing. However, pregnant and lactating females have higher energetic and nutritional demands and therefore may be more sensitive to differences in forage quality. In this study, we used monthly survey data to identify areas of LWC frequently used by lactating female Grevy's zebra and their foals during multiple dry seasons. We then collected grass in these and control areas to determine if landscape use patterns are driven by spatial variation in grass nutrient content. The results of this study support the hypothesis that lactating Grevy's zebra selectively forage in areas of high grass quality during the dry season. Levels of potassium (K), sodium (Na), phosphorus (P), and crude protein (related to percent N), as well as the ratio of calcium to phosphorus (Ca:P) appear to be particularly important in influencing these landscape use patterns.