Three ants crawl on a vine. The ants and the vine are seen in silhouette with a yellow sunset behind them.

Ants are a bedrock of forest ecosystems, and they might not be adjusting well to warming temperatures.

In newly published research, scientists found that foraging ants preferred to gather food placed at specific temperatures but did not avoid food that was too hot or too cold. Long-term exposure to these hot, but sublethal, temperatures could be changing the ants’ food and energy usage, harming colonies and broader forest ecosystems.

Hotter temperatures force ants to use more energy to survive, said lead researcher Elsa Youngsteadt, an insect ecologist at North Carolina State University in Raleigh. “The question is now, Is there enough energy for ants to keep them going in this hotter situation?”

Warmer Climate, Warmer Ants

More than 20 quadrillion ants live on Earth (that’s 2.5 million ants for every human). These tiny invertebrates form the foundation of many ecosystems. They are soil aerators, seed dispersers, predators, and scavengers, Youngsteadt said. “They keep nutrients cycling through the system.” If we didn’t have ants in the forest, processes would slow down a lot, she explained.

But despite their importance to so many environments, there has not been much research into how ants might evolve physically or behaviorally to the warmer temperatures expected because of climate change. Ants are ectotherms; their environment regulates their body temperature. But an individual ant’s life is too short to notice temperatures creeping up because of climate change. One generation of ants wouldn’t experience enough of a temperature shift to change its behavior. Some species might physically evolve for hotter climes over several generations, but scientists haven’t been measuring ants’ heat tolerance for that long.

A red ant is posed on the end of a thermal probe that hangs over the edge of a lab bench.
Researchers posed dead ants, like the chestnut carpenter ant above, on the end of an ant thermometer to measure how the ants’ bodies respond to ambient temperatures. Credit: Elsa Youngsteadt, North Carolina State University

Urban heat islands, which have a steep temperature gradient between their centers and their outer reaches, provide a nice proxy for a warming climate, Youngsteadt said. Ants in forested areas in and near the city live in warmer conditions than their comrades farther out. By studying individuals across the temperature gradient created by an urban heat island, scientists can start to understand how ants might respond to global warming over time.

The researchers collected specimens of five different ant species that live in the forests near Raleigh, a midsized urban area.

They placed those ants in laboratory enclosures that were heated to different temperatures. By placing food evenly along the temperature gradient, the team saw that different ant species preferred to forage at particular temperatures. The team also measured the hottest temperature the ants could handle and posed dead ants on the end of an ant thermometer to measure how the ants’ bodies respond to ambient temperatures.

Hotter sites had hotter ants.

They then studied the ants at 16 forest sites near Raleigh. Forest sites closer to the city were hotter on average than those farther away, but temperatures varied within sites—some spots were sunnier and hotter or cooler in the shade. The team distributed food baits containing canned tuna, strawberry jam, and cookie crumbs throughout each site to examine whether ants gathered food only from baits placed at their preferred temperature and avoided food in places that were too hot or too cold.

The field observations showed that the ants avoided extremely hot and cold temperatures, but few of them avoided baits that were placed in hotter-than-preferred temperatures. The team found that ants avoided hotter baits only slightly more often than random.

“They didn’t adjust their behavior,” Youngsteadt said. “They didn’t become less active during the day or less active in the hottest parts of the site. They had a fixed routine, and they were sticking to it no matter what.” The result: Hotter sites had hotter ants.

Live Warm, Die Young

Youngsteadt noted that none of the field sites reached temperatures that would be lethal. “At some point, I expect that ants would eventually change their behavior,” she said. “But in this sublethal temperature range where the main consequence is that their metabolism is amping up—they might live faster, die younger—they’re certainly not doing things to avoid that kind of warming.”

Black ants swarm around a shallow white plate containing different foods. The bait is sitting on a leafy forest floor.
Researchers placed food baits at 16 forested sites to lure ants and study their behavioral responses to warm temperatures. Credit: Sara Prado

David Vasseur, an evolutionary biologist at Yale University in New Haven, Conn., said that this research was “very well conceived” and that “studies like [this] are critical for linking the large body of laboratory-based knowledge we currently have on thermal impacts [on insects] to the natural arena where climate change is happening.” Vasseur, who was not involved with the research, added that ants’ behavior seems to be tuned to the average temperature of a location and that they don’t respond to quick changes in temperature that they might experience as they travel across a site. If ants could recognize too-hot temperatures more easily, they would spend less time exposed to dangerous conditions, he said.

Entomologists don’t know what the long-term consequences might be if ants don’t adjust their behaviors or evolve to warming environments. “That giant mass of life out there is now metabolizing faster or is hungrier. Energy is flowing differently,” Youngsteadt said. She and her colleagues are planning to study how ant colonies, which are more insulated than the foraging worker ants used in this study, respond to warmer temperatures. This research “raises a lot of questions about what [heat exposure] is doing for the colony economy of ants and what it’s doing for energy flow and ecosystems.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2023), Ants aren’t adapting to warmer temperatures, Eos, 104, Published on 17 February 2023.
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