The social network: Dynamic lives of Grevy’s zebras and wild asses explored

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A side-by-side comparison of the activity of Grevy's Zebras and Onagers. Graphic: Tanya Berger-Wolf, Chayant Tanitpathananandh, Daniel Rubenstein and Siva Sundaresan
A side-by-side comparison of the activity of Grevy’s zebras and onagers. Graphic: Tanya Berger-Wolf, Chayant Tanitpathananandh, Daniel Rubenstein and Siva Sundaresan

Africa’s Grevy’s zebra and the closely related Asian wild ass, the onager, have radically different social behaviors and community structures, researchers have found.

The study team made the discovery using a new, dynamic computer-based social-networking analysis tool.

Learning more about the inner workings of any community – human or animal – is no easy task.

Communities are defined by flux: friendships that form and break, loyalties that shift, and visitors that pass through.

Such dynamic interactions cannot be represented in static maps of social networks. Diagrams with lines drawn between individuals to show their ties represent only a snapshot based on data collected over time. They cannot tell the whole story.

Zoologists face challenges when trying to analyze the community organization of social animals. The bonding behaviors of related species may seem similar, even though the environments that shaped the animals’ group-behavior are quite different.

University of Illinois at Chicago computational ecologist Tanya Berger-Wolf led a multi-disciplinary team that created CommDy, a dynamic network computational framework, to better understand group behavior and communities.

The tool was applied to learn more about the community dynamics of onagers and Grevy’s zebras, with the findings published in the peer-reviewed open-access journal, PLOS ONE.

Both Grevy’s zebra and Asian wild asses form what are known as fission/fusion communities, said Berger-Wolf, who is associate professor of computer science at the university. In fission/fusion communities, individuals meet and spend time with others in different groups at different times.

The two animals’ communities look similar, using a traditionally static social-network analysis. But the zebras are fewer in number, limited in range, and menaced by large predators such as lions. They often lack access to water.

Onagers, in contrast, are relatively abundant and widespread, with no major predators and reliable access to water. One would expect the two species to have evolved very different social structures to cope with their very different circumstances.

To observe the daily interactions within each of the two animal communities, three co-authors of the study from the Mpala Research Centre in Kenya – ecologists Daniel Rubenstein of Princeton University, Siva Sundaresan of the Jackson Hole Conservation Alliance in Wyoming, and Ilya Fischoff of the University Corporation for Atmospheric Research in Washington DC – drove repeatedly along the same route through the animals’ territory and recorded the size, duration and membership of different groups. The new software allowed the researchers to contextualize the observed interactions.

“We were looking for the latent community structure – loyalties, changes in affiliation, visiting with other groups – and the social cost of change,” Berger-Wolf said.

Some interactions may have a negative impact for an individual, she said, by increasing stress or inviting harassment. Other social contacts may be positive, by increasing status or access to resources.

“The dynamic communities that resulted from that computational analysis were actually strikingly different,” Berger-Wolf said. The Grevy’s zebra lived in large, stable groups, with loyalty rewarded and visiting with other groups discouraged. Onagers formed smaller, less cohesive groups, with individuals able to change circles with little social cost.

Berger-Wolf said these newly revealed differences make sense, given the different adaptation each species needed to survive. And for the first time, scientists were able to quantify the differences, using the new computational tool.

Facing a constant threat from predators, the Grevy’s zebras find strength in numbers, forming large groups of loyal individuals. The large, stable herds are also able to share found resources, such as water. The onager, in contrast, can form smaller, more transient groups, being less dependent on the herd for protection or finding scarce resources.

“Dynamic community analysis can be an important tool for testing ideas about the selective ecological and evolutionary forces that generate behaviors, revealing their adaptive value and significance,” Berger-Wolf said.

Berger-Wolf co-authored the paper with Chayant Tantipathananandh, of Google Inc.

The work was supported in part by Princeton University, McMaster University, the Smithsonian Institution, the Denver Zoological Foundation, the Wildlife Conservation Society, UIC, a Microsoft award, and several National Science Foundation grants.

Rubenstein DI, Sundaresan SR, Fischhoff IR, Tantipathananandh C, Berger-Wolf TY (2015) Similar but Different: Dynamic Social Network Analysis Highlights Fundamental Differences between the Fission-Fusion Societies of Two Equid Species, the Onager and Grevy’s Zebra. PLoS ONE 10(10): e0138645. doi:10.1371/journal.pone.0138645
The full study can be read here

The study was published under a Creative Commons License

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