Just like humans, many animals get more aggressive in the heat

Humans aren’t the only animals with hot tempers. 

In 2016, ecologist Kristen Cecala and a colleague watched black-bellied salamanders (Desmognathus amphileucus) from Appalachian streams lunge at one another inside a lab incubator. The little animals — barely a hand’s length — can be fiercely territorial, thrashing to bite their opponents or send them fleeing, says Cecala, of the University of the South in Sewanee, Tenn. 

“Salamander fight club,” as Cecala calls the experiment, was set up to test how rising temperatures would affect the amphibians’ behavior. Black-bellied salamanders, it turned out, were nearly four times as likely to act aggressively at 25° Celsius — much warmer than their stream habitats — compared with more natural conditions at 15° or 20° C. 

And salamanders aren’t alone in their raucous behavior. Studies show that many animals —monkeys, rats, mice, fish, ants — tend to get more aggressive at higher temperatures.

As the planet warms due to climate change, rising temperatures could subtly affect some species’ social structures and ecosystems. But the findings may also tell us something deeper about how heat affects animals physiologically — and potentially reveal clues to increased violence and crime among humans in hot weather. 

“We’re all [products] of the environment and our biology,” says Clas Linnman, a neuroscientist at Massachusetts General Hospital in Boston who is studying the heat-aggression link in humans. 

Not every animal responds to heat with aggression. In Cecala’s experiment, only black-bellied salamanders — and not seal or Ocoee salamanders —showed this trend. But among those that do, many are ectotherms, cold-blooded creatures that use environmental conditions to regulate their body temperature. 

In one 2024 study, freshwater biologist Erin Francispillai and her colleagues placed bluntnose minnows (Pimephales notatus) in tanks where temperatures fluctuated from 18° to 24° C within a day — a change that mimics similar conditions documented in streams that have lost shade due to deforestation. At higher temperatures, the small fish behaved more aggressively toward their shoalmates compared with fish kept at constant temperatures. “They were just like nipping at each other a lot,” says Francispillai, of McGill University in Montreal.

And in a 2023 study spanning the European Alps, ecologist Patrick Krapf and his colleagues reported that Tetramorium alpestre ants inhabiting warmer alpine regions tend to be more hostile. When two worker ants from different colonies are placed in a little glass “arena,” they’re often combative toward one another. Warmth-accustomed ants tended to be especially aggressive, baring their mandibles or pulling at opponents’ limbs. “They start really fighting, like grappling,” says Krapf, of the University of Innsbruck in Austria. 

One possible explanation for this heat-aggression link in ectotherms is that warmth dials up the animals’ metabolisms, using up more of their energy. This requires more calories, making animals more territorial and aggressive to secure food.

For salamanders, Cecala says, “they’re less willing to let other individuals, perhaps, come into [their] territories in a situation where they’re feeling a little bit calorie-limited.” For ants, Krapf suspects that simpler explanations are possible: The insects’ food — honeydew excreted by aphids — is more abundant at warmer temperatures, giving them more energy to spend on behaviors like aggression. 

By contrast, warm-blooded endotherms like mammals may be less sensitive to these effects, Francispillai says, because they can cool their bodies through sweating or panting, for instance. But the heat-aggression association has been documented in some monkeys, rats, mice and, according to Linnman’s research, even dogs.

Even for endotherms, warmth boosts metabolic rates, and calorie loss may be exacerbated by the energy needed to cool their bodies, Francispillai says. When the priority is getting more calories, less energy may go toward maintaining social behaviors and regulating aggression, she speculates. Yet Linnman suggests that increased aggression could also arise from the discomfort that warm-blooded animals feel in hot weather. 

In humans, scientists debate the extent to which the aggression-heat relationship is due to biological effects of heat on behavior versus increased outdoor activities on hot days. But “as the heat-aggression correlation is consistent across multiple species, it suggests that simple ‘sociological explanations’ … are not sufficient,” Linnman says.

There is indeed some evidence that heat can affect brain chemistry and connectivity, potentially leading to aggressive behavior, says environmental neuroscientist Kim Meidenbauer of Washington State University in Spokane. “Uncomfortable, stressful heat affects a number of psychological and physiological processes that can lead to increased aggression,” she says.

While scientists investigate the underlying mechanisms — and important questions such as whether aggression disappears once animals get used to heat — they’re also thinking about possible consequences of the heat-aggression relationship as the world warms.

If ants become increasingly aggressive, Krapf speculates, “they might get rid of other nearby colonies of the same species,” enlarging colonies’ spatial structures. Greater aggression in minnows could lead to less cohesive shoals, making it easier for predators to pick off individual fish. Battle-prone amphibians might keep greater distances from one another, decreasing their population density and potentially making them more vulnerable to declines, Cecala says. 

By subtly tweaking physiology, climate change can influence animal behavior in unexpected ways. For the salamanders of Appalachian streams, fight clubs may be becoming a little more intense.