Current Projects

Please click on the below links to learn more about each project.

Grevy's Zebra movements and population dynamics

Project start: February 2007

Grevy's zebra are critically endangered, numbering less than 2,000 in the wild. Over much of their range in Kenya, livestock competition and lion predation are the main hypothesized threats to their long-term survival. In this project we are studying Grevy's zebra movements and population dynamics. This study will unravel the interactions among Grevy's zebra, domestic animals, and lions. Our results will provide recommendations on how to better conserve Grevy's zebra.

Numbering under 2,000 globally, Grevy’s zebra have their last stronghold in the semi-arid savannahs of Kenya’s Laikipia-Samburu ecosystem. The Laikipia-Samburu ecosystem spans approximately 25,000 square kilometers and is defined by the Ewaso Ngiro river watershed. Ninety-five percent of the global Grevy's zebra population is found here.

There are two main hypothesized causes of Grevy's zebra population declines in this region: 1) competition with livestock and 2) predation by lions. Changes in movements provide an important behavioral mechanism through which Grevy's zebra respond to livestock and predators.

Our past research shows that Grevy's zebra avoid using areas close to active cattle corrals. We have also found that Grevy's zebra alter their daily foraging and drinking cycles when livestock dominate the landscape. In certain protected parks or tourist conservancies, it is hypothesized that unusually high lion densities are depressing Grevy's zebra survival and reproduction. These unusually high lion densities may be a consequence of tourist demand. By contrast, in unprotected areas, lions are often killed by people who are protecting their livestock. Animal husbandry and predators may combine to have stronger impact on Grevy's zebra. In areas where people block off waterholes during the day for the exclusive use of their livestock, Grevy's zebra are forced to either abandon the area or to drink at night. We expect that drinking at night places Grevy's zebra at higher risk of lion predation.

In this project, our goal is twofold. First, we seek to characterize how variation in movement patterns of Grevy's zebra relates to variation in human land use intensity and lion density. Second, we ask how variation in movements impacts population distribution and vital rates.

We characterize movements by tracking Grevy's zebra equipped with GPS radiocollars. In June 2007, we deployed seven radiocollars on Grevy's zebra in central Laikipia. We will monitor forage resources using satellite images. We collect data on survival and reproduction by conducting periodic surveys of the study area. Grevy's zebra individuals have unique stripe patterns. This allows us to monitor individuals over time. We will use a capture-recapture modeling framework to estimate population size and vital rates.

We intend this project to provide the basis for specific management recommendations to stakeholders about actions they can take to preserve Grevy's zebra while meeting human needs. We may be able to suggest livestock corral placement that least conflict with Grevy's zebra. We may determine levels of lion density that do not constrain Grevy's zebra population growth. If Grevy's zebra benefit from use of specific habitat types to escape from lions, then our study will identify habitat modification that promote Grevy's zebra.

Collective equid movements

Project start: April 2007

Zebra herds are often seen making coordinated movements. These movements determine where populations are found, and their relationships to competitors and predators. We are examining how group movements emerge from the interactions among neighbors within the groups. Through behavioral observations and GPS radiocollar data, we are investigating how variation in individual interactions influences the speed and destinations of plains zebra and Grevy's zebra herds. 

One often sees zebra herds making what appear to be highly coordinated movements, in groups ranging from a few individuals to thousands. They may walk in columns toward distant targets, or form a slow, wavelike front while grazing. How do these herds achieve such coordinated behavior, when individuals can only respond to the behavior of their nearby neighbors? How do all individuals in the group meet their needs for food and water?

Zebra herds typically contain individuals who vary in their resource needs due to differences in their reproductive state. In a landscape where grass and water are found in different places, variation in needs of individuals may result in their having different preferences about where to go. The herd movement patterns that emerge in such a heterogeneous group will depend on the interactions among individuals.

We are examining how movements of plains zebras are shaped by individual interactions. Through direct behavioral observations, we aquantify how individual interactions give rise to group movements. Herd movements begin by one individual walking. For several harems and bachelor groups within the herd, we record every time the group makes a move. We record which individual is leading the group, how far the group goes, and what they do at the end of their move -- grazing, resting, socializing. Others in the herd may choose whether to follow or not. Responses by other groups in the herd typically occur within several minutes.

From these data, we can determine how individuals’ movements vary depending on changes in their needs and the social and environmental circumstances. For example, behavior may vary with the individual’s reproductive states, the identity of its neighbors, the time of day, habitat, or the presence of predators or livestock. We can test whether the identity or reproductive class of leaders predicts the distance or direction of a group move. By periodically mapping the configuration of all individuals in the group, we can ask how individual responses to neighbors depend on their physical proximity.

For both plains zebra and Grevy's zebra, we are also using radiocollar data to investigate how variation in individual movements depend on the composition of their herd and the movements of neighbor zebras. In June 2007, we deployed GPS radiocollars on two plains zebra and seven Grevy's zebra. Later this year we will be putting out two more collars on Grevy's zebra and four more on plains zebra. The hourly locations we gather from these collars will allow us to measure the movement properties of zebra herds, including speed, turning angle, and preferred locations for forage or water.

This project will provide new insights into how collective movements emerge in social species like plains zebra and Grevy's zebra. It is these herd movements that shape the distribution of populations on the landscape, and their encounters with competitors or predators.