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A Day in the Life: Prof’s Adventures in Costa Rica

By Sean O'Donnell
Professor, Department of Biodiversity, Earth and Environmental Science

August 1, 2013


Abby Mudd, Kaitlin Baudier, Tessa Erickson, Siobhan O’Donnell in the cloud forest

The crew in the cloud forest L-R: Abby Mudd, Kaitlin Baudier, Tessa Erickson, Siobhan O’Donnell

The drive up the mountain to my field site in Monteverde, Costa Rica never fails to amaze. The way is long and jarring, unpaved and rocky, but magical to a tropical ecologist—a succession of forest types (dry, montane, cloud) replace each other along the way. Short distances bring dramatic elevation changes in this steep terrain.

As we climb from the sultry lowlands, new bird songs drift in the open car windows, more orchids and bromeliads appear on roadside trees, then comes mist and refreshingly cooler air… which is what brought us here. I traveled to Costa Rica with PhD student Kaitlin Baudier, recent Drexel biology graduate Tessa Ericson, and STAR fellow Abby Mudd to study the thermal ecology of top predators of the tropical forest realm: army ants.

Army ants are among the most complex animal societies known. Colonies of hundreds of thousands of workers and a single mother queen hunt for food and migrate to new nest sites in massive coordinated swarms. Their temporary nests, known as bivouacs, are made up of the interlocked bodies of the workers, a living building. Our goal on this trip is to find out if the building is heated.

Army ants emigrating to a new nest site at night

Army ants emigrating to a new nest site at night. The workers form living bridges to ease the way for their colony mates

The work is challenging but fun. The crew hikes long distances searching for colonies. The beauty of the mountain forests can almost make you forget that, as the locals say, it is uphill both ways in Monteverde. We pass sweeping vistas; we cut trails through deep forest with our machetes; and we marvel at the rich tropical life that surrounds us. But always we keep any eye to the ground, searching for our army ant subjects.

The special draw of one army ant species, Eciton burchellii, is that it ranges across elevations, from sea level to high up in the mountains. This ant occurs more than a mile above sea level at our field site. In lowland (warmer) rain forests, Eciton bivouacs can thermoregulate to keep their broods of 50,000 larvae at a constant temperature at night. But nights are much cooler in the mountains. Can the ants keep their brood as warm in the high, cool cloud forest? Or do they have to lower the thermostat?

Sunrise over the active Volcano Arenal, seen from San Gerardo field research station

Sunrise over the active Volcano Arenal, seen from San Gerardo field research station

Kaitlin Baudier intends to find out using small thermal probes called iButtons. She has become skilled at carefully inserting the probes into the ants’ nests—no small feat when confronted with thousands of suicidal stinging workers and soldiers. Early results look promising: iButtons inside the first bivouacs we tested recorded higher temperatures relative to probes placed nearby.

The challenge now is to find bivouacs at the highest, coolest, elevations. Powered by ice cream from the local dairy plant, and carrying an array of activated iButtons, the crew heads off to climb into the cloud forest.

All photos courtesy of Sean O’Donnell

Dr. Sean O’Donnell is a professor in the Department of Biodiversity, Earth and Environmental Science. His research interests focus on community ecology and biological diversity of tropical social insects, particularly army ants. He received his Ph.D. in zoology and entomology from the University of Wisconsin-Madison. To learn more about O’Donnell, visit his faculty profile.