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Great Migrations: Running With the Herd

Wildebeests traverse the Serengeti’s plains each year in numbers nearly too vast to count. But research shows that by forming enormous herds, they manage to make themselves scarce to free-roaming predators. By Ford Cochran The largest programming event in the ten-year history of the National Geographic Channel, Great Migrations premieres in the U.S. beginning at 8 p.m. EST/PST Sunday, November 7, 2010, with related coverage in National Geographic magazine and an official companion book.

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John Fryxell of the University of Guelph in Ontario, Canada, studies the migrations of herbivores, including the nearly two million wildebeests and zebras that loop the Serengeti plains from Tanzania to Kenya and back each year. Fryxell’s work, and the great Serengeti migration, feature prominently in National Geographic’s multi-night Great Migrations special.

I spoke with Fryxell about the wildebeests’ long and sometimes-harrowing journey. Why do the wildebeests migrate, and what has your research revealed about their herding behavior?

We’ve been working on migration, one way or another, for almost 30 years now. I was first drawn to think about migration because of work I did for my Ph.D. in the Sudan, where we were the first scientists to go in and look at an area called the Boma National Park. One of the most striking aspects of Boma is the migration of white-eared kob.

In more recent years, I’ve shifted my attention to Serengeti, which is an area where we’ve learned much over the years, but nonetheless continually come up with new surprises.

Some of the aspects of migration that we’ve been looking at relate to trying to understand the tension that always exists for migratory herbivores in these large African ecosystems. They’re faced with a paradox: They’re surrounded by grass, but grass is often very poor as a food. So one of the important themes that we’ve worked on over the years is to try to tease apart the delicate balance that an animal has to achieve in meeting its nutritional requirements.

Migratory herbivores achieve this in part by moving across the ecosystem in such a way that they continually take advantage of grasses and new herbs that are at a relatively modest stage of maturation. What they’re looking for is a wave of green growth where plants are tall enough to make it efficient to graze on them, but not so tall that they’ve built up lots of the structural compounds that make them almost inedible.

It’s like being caught between the devil and the deep blue sea: Herbivores need to move continually to stay ahead of the maturation process. By being mobile, migrants are able to achieve that much more easily than animals that remain resident for a long time.

There are lots of other features of migration that we’ve been interested in. One of those features has been simply understanding how the process of social aggregation and information that individuals share with other members of the population might play into the process of migration and movement. In recent years, we’ve been interested in how that process of social formation actually translates into the aggregation of animals across the landscape, and the continual ebb and flow of large herbivores across much of the Serengeti ecosystem.

Part of the reason we think that social process is important is that by grouping together, herbivores are able to make themselves scarce to carnivores that are spread right across the landscape. The carnivores don’t have the luxury of moving continually to keep track of the herd.

By joining together, the herbivores can make sure that they aren’t in any one location for a long time, but more importantly make sure that individuals aren’t spread evenly where a predator walking at random can knock off the nearest available food item when it finds one. Instead, large carnivores such as lions and hyenas have to search over broad areas where they don’t encounter any prey. Every once in awhile, if they’re lucky, they run into a herd and that leads to a successful attack. But overall, there’s a benefit for the herbivores. It reduces the risk for all the members of the herd if they’re grouped together.

So one might know the iconic images of massive herds of wildebeests crossing the Serengeti plain, but in fact those herds are surrounded by sprawling regions of open grassland, and predators have to prowl an enormous area in hopes of encountering their prey?

That’s right. In a sense, it creates open space that makes it much harder for the carnivores to make a living. You’ve been able to observe the distribution of these prey animals in nature, and then to model mathematically random interactions with predators wondering solo on the plains. Those models help illustrate why herding is in fact an optimal behavior for prey species, correct?

Correct. We use mathematics and computer simulation because it helps us to test our gut impressions about what might be guiding the behavior, and to make sure the conclusions that we think might be reasonable actually hold up to strong logical tests.

More importantly, however, building these kinds of models helps us to piece together all the different elements of the ecosystem to understand how the function of the entire ecosystem is influenced by the process that we’re interested in. We’re able to knit together all the different species of the Serengeti into a larger migratory model to help understand what the impact of predation would be on many different prey species–not only the migrants, but also those species that aren’t fortunate enough to move around. And in turn, to understand what aspects of ecosystem processes are actually key to maintaining all the players in the system. Obviously, that’s a key element in conservation. You also study the migrations of grazing animals in North America in regions and climates that may look different from the Serengeti, but support creatures that survive in similar ways. Do you see similar patterns of behavior among those species?

One of the recurrent things we see is that many mobile species have this social aggregation tendency. If one looks at the barren-ground caribou, for example, you see the same sort of formation of massive herds. Even species that have much less dramatic migrations, such as the woodland caribou or elk in Yellowstone, show a strong tendency for individuals to group together and to use shared information to help guide, not only their habitat choices, but also (we think) to make themselves scarce to predators in the process of moving about the landscape. So, safety in numbers?

Right. First benefit, there’s the selfish herd, where all the individuals in the herd want to be in the center because if there’s a predator attack, the individuals on the edge will be most vulnerable. That dilutes the overall risk for individuals–but not all individuals can be at the center of the herd. And second benefit, the process of simply reducing the probability that the herd itself gets encountered by a predator. That’s one of our major contributions. We’d like to develop this further and see how extensive it might be right across the animal kingdom.

One of the things we ideally would like to do is predict day-by-day the behavior of both carnivores and herbivores. The carnivores are also shaped by natural selection to try to do the best they can in the face of these incredible evolutionary strategies developed by the herbivores. We’re quite interested in that minuet of decision-making that happens between predators and prey. We hope that over the next decade, one of the major achievements we help contribute to will be understanding the daily dance of death that happens between mobile herbivores with their survival strategies and the predators that can anticipate and possibly circumvent those strategies.

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  • Watch Great Migrations on the National Geographic Channel beginning at 8 p.m. EST/PST Sunday, November 7, in the U.S. (Check local listings for air dates and times outside the U.S.)

  • Help conserve the landscapes of migratory species with National Geographic’s Global Action Atlas.

Photo of wildebeests on the move by Anup Shah; book cover courtesy National Geographic Books

Ford Cochran directs Mission Programs online for National Geographic. He has written for National Geographic magazine and NG Books, and edits BlogWild–a digest of Society exploration, research, and events–and the Ocean Now blog. Ford studied English literature at the College of William and Mary and biogeochemistry at Harvard and Yale, with a focus on volcanoes, forests, and long-term controls on atmospheric CO2. He was an assistant professor of geology and environmental science at the University of Kentucky before joining the National Geographic staff.

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More posts by Ford Cochran

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