Tiny acorn ants reveal secrets of urban evolution

By Mary Lauterbur

This acorn ant is only the size of the head of a pin! Photo credit: Ryan Martin

An acorn ant’s whole world is a hollowed-out acorn she and her ninety-nine sisters call home. These pinhead-sized ants are common wherever oak and hickory trees drop their nuts, but they don’t stray far from their small colonies. You can find them from urban parks and backyards to forests all across the eastern and mid-western United States. There’s a good chance one is living in your backyard right now.

An acorn ant colony in a hickory nut, one of their favorite places to call home. Photo credit: Ryan Martin

Despite this, cities cause problems for acorn ants: they are hot. We have all experienced it, buildings and roads absorb the sun’s heat, making cities hotter than surrounding rural areas. But if your city becomes sweltering this summer, your backyard acorn ant can’t move to the country or borrow your air conditioning. She has to deal with that heat right where she is.

Dr. Sarah Diamond and Dr. Ryan Martin of Case Western Reserve University in Cleveland, Ohio study if animals are capable of evolving fast enough to keep up with urban warming. With how fast people are changing the landscape (urban heat islands have developed in the last 100 years) this is important to understand. Acorn ants are perfect for this – 100 years is barely 20 acorn ant generations. In evolutionary terms, this is a very short time.

To find out if acorn ants are adapting to urban heat islands, Dr. Diamond, an entomologist, and Dr. Martin, an evolutionary ecologist, compared the heat tolerance of urban acorn ants to that of their rural cousins. These “very friendly, gentle critters” are ideal for this work, according to Dr. Diamond. They are easy to find and live happily in the laboratory. And they provide opportunities to get non-scientists involved. After all, our little acorn ant isn’t half a world away; she’s right outside your door.

Dr. Diamond takes students around campus to find acorn ants, cracking open hollow acorns to look for colonies. But it can be hit or miss. On one campus expedition with her impressionable new students, she couldn’t find a trace of these common ants. Dr. Martin, who had never studied ants before, decided to try his hand. To the delight of the students, the first acorn he opened was home to a colony. “It was only the second acorn I ever opened,” Dr. Martin laughs. (He adds that even squirrels can tell when an acorn has a colony.)

Student Yu Huang prepares to open a nut. Will it contain a colony? Photo credit: Lacy Chick

First they had to see if urban ants can in fact tolerate higher temperatures than rural ants. In the lab, they compared the maximum and minimum survivable temperatures between urban and rural ants. They placed ten ants from each colony, one at a time, into an incubator at room temperature. They then gradually increased the temperature until the ant stopped moving, indicating the ant’s maximum survivable temperature. They did the same with another ten from each colony, this time lowering the temperature to get the minimum. Not surprisingly, urban ants could survive hotter temperatures, and rural ants could survive colder temperatures.

It’s not enough to show that urban ants tolerate higher temperatures than rural ants. After all, a person who gets used to the heat of summer is not evolving. The challenge is to show that the ants have inherited differences in heat tolerance.

To test if evolution is occurring, Dr. Diamond and Dr. Martin had to find out if ants from urban or rural areas have different heat tolerances even when they’re raised in the same conditions. This tests whether the tolerance was inherited, or if they merely got used to the temperature where they were raised. When the daughters of urban and rural ants are raised in the same temperatures, do daughters of urban ants still survive higher temperatures than rural daughters do?

To answer this, they enlisted Dr. Diamond’s lab members (including post-doctoral associates Lacy Chick and Stephanie Strickler, and PhD student Abe Perez), high school students, and undergraduate students to collect colonies from urban and rural locations on and off campus. They brought the colonies, acorn and all, back to their lab at Case Western to count every ant. Then the researchers administered their test: They kept some colonies at a temperature that replicated their home location, and exposed others to temperatures from unfamiliar locations, and waited for new ants to be born and grow up in these colonies.

This experiment made it possible to see if heat tolerance was a result of the temperature new worker ants were raised at, or evolution. Ten ants, born and raised in the lab, were randomly chosen from each colony to test the maximum survivable temperature, and ten randomly chosen to test the minimum.

Urban and rural ant colonies, in their acorns, were kept at the same temperature until new ants were hatched that had never experienced outside temperatures. Photo credit: Lacy Chick

When they looked at the results from their lab-reared ants, they were amazed. Those with urban ancestry that grew up in the lab could survive temperatures almost two degrees Fahrenheit higher than those with rural ancestry, no matter the temperature they were raised at. Urban ants had evolved!

This adaptation is “super rapid,” says Dr. Diamond. That two degree difference is more than the global average temperature has risen since 1900. That this change happened in 20 generations or less was shocking. After all, evolution is supposed to be slow – most of our examples of evolution took place over hundreds or thousands of generations.

Dr. Diamond and Dr. Martin aren’t stopping here. Next, they will transplant entire ant colonies, urban to rural and rural to urban, to see if the pattern they found in the lab is the same in the wild. (Since the ants don’t go far from their acorn, this is easy.) They are also looking at ants across different cities. Some places, like Knoxville, Tennessee, are warmer to begin with, so ants that live there are already used to the heat. If ants have a limit to their evolution, they won’t be able to adapt as much in cities that are already warmer.

As urban areas expand and climate change continues, this research shows that humans are driving evolutionary change. And scientists such as Dr. Diamond and Dr. Martin can study it right in your backyard. City ant or country ant, next time one wanders across your path, remember how these tiny critters are helping us to understand how the world around us is changing.

Transplanted urban ant colonies in their new rural homes. Each transplanted nut is surrounded by a metal fence to keep the ants from wandering off. Photo credit: Lacy Chick
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