Unveiling Rainforest Mysteries

Researchers study how Costa Rican bats adapt to climate change

By Lynn Davis

It’s nearly midnight in one of the lushest places on earth, the Neotropical montane forests of southern Costa Rica. Thick clouds deliver a steady downpour, obscuring moonlight and stars. Coniferous and broad-leafed trees—some stretching over 300 feet high—along with a dense undergrowth of ferns and grasses, muffle the sound of the rain. Earlier, the clicking, buzzing, and screeching sounds of millions of cicadas was loud, powerful, and electric. Now, the forest sounds are soft, quiet.

It’s February in a region that receives more than 100 inches of rainfall each year. Unlike September and October, when it pretty much rains throughout the day, February in the southern mountains of Costa Rica is considered a “dry month.” Generally, the rain starts around dusk—as bat researcher Paula Iturralde makes her way into the forest—and dries up in the waning hours of night, after she has returned to the comforts of a field station.

Tonight, sheltered under a metal canopy, Iturralde trains her headlamp on a small bat she gently holds in her gloved hand. She’s been in the forest since dusk, when the sounds of the cicadas swell and insectivorous bats begin to feed on the forest’s multitude of moths, mosquitoes, and beetles. Hundreds of bats continue to dart through the trees, navigating by echolocation, finding and devouring their prey.

Most nights, Iturralde will be out until at least 2 a.m., going through a precise routine to record important information about how bats currently adapt to increased temperatures in order to help forecast how bats may adapt as the world warms.

The perfect research location

Costa Rica is one of the best places to study evolutionary adaptation, which is how plants and animals adjust in their environment to improve their chances of survival. Its location, in fact, offers a grand-scale history of adaptation that goes back 3 million to 5 million years when a bridge formed and connected the North and South American continents, setting up conditions for flora and fauna to mix and adapt.

Today, Costa Rica has an extraordinary amount of biodiversity due in part to its varied terrain. The country’s two coastlines (the Caribbean Sea to the east and the Pacific Ocean to the west), grassy lowlands, rugged mountains of tropical rainforests, dry forests, and cloud forests support more than 500,000 species, including insects and marine life. The region accounts for an estimated 4 to 5% of all species in the world, and attracts international scientists of varied disciplines.

When Dr. Amanda M. Adams, BCI’s conservation research program manager, read Iturralde’s proposal to study bats in Costa Rica, she was intrigued and supportive. Not only did Iturralde’s research proposal fit nicely with BCI’s mission to end bat extinctions, Dr. Adams also connected on a personal level.

Prior to joining BCI, Dr. Adams conducted field work in Costa Rica, and she knew the Central American nation was paradise for a bat researcher, containing more than double the number of bat species that live in all of North America. More than 15 years ago, Dr. Adams witnessed bats moving from the lowlands to cooler forests at higher elevations.

“Bats are sensitive to temperature changes,” Dr. Adams says. “So the science of thermal tolerance—figuring out how bats survive cold and heat—is crucial to species survival. Bats tend to adapt, but if they can’t adapt, they move. Typically, around the world, the hotter it gets, the more likely bats will move to cooler elevations, to higher places on the mountain. But here’s the warning, worldwide: The mountain extends only so high.”

Mist netting and an ‘acoustic flight tent’

Enter Paula Iturralde, born in Ecuador and currently a Ph.D. candidate at the Universidad de Costa Rica. Iturralde proposed to meticulously record the echolocation sounds of diverse bat species in the rain-slick montane forests near the Panama border and simulate how predicted increases in climate temperatures might affect bats.

Iturralde set up her research at the Las Cruces Research Station in an area with at least 2,000 plant species, 800 butterfly species, 400 bird species, and 113 mammal species, including 60 species of bats. The research station, owned by the Organization of Tropical Studies, offered Iturralde comfy quarters, flavorful meals, interesting conversations with other visiting researchers, and a covered platform in the forest from which to conduct her research.

Working at an elevation of approximately 5,000 feet, Iturralde hauled in her equipment and set up an “acoustic flight tent” on the open-sided platform under a metal roof. The generously sized tent (approximately 19 feet long, 8 feet wide, and 8 feet tall) was covered in a synthetic fabric to keep air inside the tent from mixing with the ambient air outside. Inside, the tent was lined with non-reflective material to prevent acoustic echoes, and outfitted with an ultrasound detector and four ultrasound microphones, specifically designed for recording wildlife. All were placed in precise positions within the tent to ensure decipherable and comparable recordings. An electric heater and temperature-humidity data logger completed the tent set-up.

Outside the tent, Iturralde established her “desk” on a sturdy plastic tub, and affixed an external microphone nearby to record the sounds of each night. At dusk, she strung up ultra-thin mist nets. Made of barely perceptible, horizontally strung nylon threads, and commonly used by bat researchers and ornithologists, mist nets safely capture bats and birds by forming a loose baggy pocket when the bat or bird encounters the net. On most nights, Iturralde captured, tested, and recorded comprehensive data on an average of seven bats. It was a time-consuming process.

Turning up the heat

With each captured bat, Iturralde first conducted a visual assessment, noting species, sex, and health by the light of her headlamp. She then placed each bat, alone, in the tent for a couple of minutes to establish a baseline recording of echolocation—a seemingly simple process made difficult by the fact that most bat echolocation sounds cannot be heard by the human ear. Iturralde relied on an ultrasound detector and a research volunteer, stretched out on the floor of the dark tent, to confirm bat flight and sound. She says finding willing volunteers to lie quietly for hours and observe the bats in the tent was never a problem.

Throughout the night, Iturralde adjusted humidity levels and cranked up the heat by 1.5 degrees and then 3 degrees—modeling what many climate scientists predict may happen if the climate continues warming. With each change in humidity or temperature, Iturralde placed the bat back in the tent to record every variable.

“We know bats change the characteristics of their calls between rainy and dry seasons, probably as an adaptation to climatic variation, so simulating different weather conditions is necessary,” Iturralde says. “Adding data on how bats respond to environmental conditions is vital to understanding if increased temperatures affect how bats navigate and find their prey.”

After two weeks in the field, Iturralde was armed with a baseline database. She is currently comparing her collected data and analyzing differences in signal emission from each experimental trial.

Iturralde is one of 32 recipients over the last three years to receive a BCI Student Scholarship from a fund dedicated to assisting global conservation research.

“Paula’s in-depth project is vital to the work of bat conservation around the world,” Dr. Adams says. “Right now, we’re seeing such climate extremes, so it’s important we maintain fresh and insightful data as we look for solutions.”