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A new study about tadpoles that turn into cannibals may sound like fodder for nature trivia, but it carries a much deeper message about evolution—and even about ourselves.

The research, in the Proceedings of the National Academy of Science, shows that extreme behaviors like cannibalism in some species may begin as flexible responses to the environment, and then later become hardwired over generations. That has big implications for understanding how not only animals, but humans, respond to extreme change, and how deeply our actions may be shaped by both our genes and our surroundings.

"This study helps shed light on how new behaviors emerge and evolve," said Dante Nesta, the study's lead author and a graduate student in Biology currently working in the Ledón-Rettig Lab within the College of Arts and Sciences at Indiana University Bloomington.

"When compared to physical traits, behaviors may be less readily apparent and are exceedingly flexible and context-dependent. In turn, an overwhelming amount of attention has been paid to the evolution of physical traits, leaving behavior mostly in the dark."

At the heart of the study is a strange but fascinating animal: the spadefoot toad (their name is a misnomer, the spadefoot toad is actually a frog). In one genus of these North American amphibians, Spea, some tadpoles grow large jaws, aggressive temperaments, and start eating their own siblings—not out of desperation, but as a normal part of their development.

Nesta and co-author Cris Ledón-Rettig, Assistant Professor of Biology in the College at IU, compared spadefoot toads from the genus Spea with another species of spadefoot, Scaphiopus holbrookii, that typically do not engage in cannibalism.

The researchers found that the non-cannibal tadpoles could show similar behaviors under crowded conditions; but more importantly, they had hidden activity in their brains that could be triggered by environmental stress.

In other words, the study authors found that the "code" for cannibalism was already there just waiting for the right environmental situation to turn it on.

"If behaviors can evolve this way, it means nature has more flexibility than we used to think," said Nesta. "That's important as an increasing number of animal species, and maybe even humans, face new challenges from things like climate change, natural disasters, and extreme social pressures."

This kind of hidden potential in genes is what scientists call cryptic genetic variation, meaning it doesn't usually show up unless the animal is placed under stress or a new challenge. But once a population does experience a challenge, individuals with the right genetic background end up expressing traits that natural selection can fine tune.

This supports an idea in evolutionary science called genetic accommodation, wherein behaviors or traits might first appear as temporary adjustments to new situations—like growing thicker fur in a cold snap—but over time, those traits get locked in if they help an animal survive and reproduce.

For spadefoot toads, the trigger turned out to be a protein-rich diet of tiny shrimp, which is often found in fast-drying ponds where survival depends on growing fast and outcompeting others. Tadpoles that ate shrimp were more likely to develop into aggressive, carnivorous forms. Over generations, this response appears to have become a fixed part of their development.

To test this, researchers looked at gene activity in the tadpoles' brains under different conditions. They found that even the "non-cannibal" spadefoot toad species had a wide range of responses, depending on their genetic background, when exposed to shrimp or crowding, demonstrating that these behaviors could have emerged from pre-existing flexibility.

They also found that the cannibal species had become more sensitive to a single environmental cue—the shrimp diet—while losing responsiveness to others, like crowding. This suggests the behavior had been refined over time to respond more reliably to a specific trigger.

"This is an example of how evolution doesn't always invent something new from scratch," said Professor Ledón-Rettig. "It often works with variation in a population that is already there."

These findings matter beyond tadpoles because they demonstrate how behavior and biology are not fixed. Rather, they're shaped by a dynamic mix of genes and environment, and that mixture can change across generations.

In a world facing climate change, social stressors, and rapid transformations, knowing how behavior can evolve, and how flexible organisms can be, may help scientists, policy makers, doctors, and mental health professionals prepare for what's next.

To be clear, the study does not suggest that humans are hardwired for cannibalism or that we'll start eating each other in response to rapid, extreme change; rather, the study highlights how hidden genetic potential can shape behavior in surprising ways when environments change.

And while the idea of baby frogs eating each other may seem grim, it reminds us that nature's most extreme behaviors can have surprisingly relatable roots: the struggle to survive, to adapt, and to make use of whatever hidden strengths we carry within us.