Call it the case of the incredible shrinking cod. Thirty years ago, the cod that swam in the Baltic Sea were brag-worthy, with fishing boats hauling in fish the size of human toddlers. Today, such behemoths are vanishingly rare. A typical Eastern Baltic cod could easily fit in someone’s cupped hands.
Experts have suspected that commercial fishing might be to blame. For years, the cod were intensely harvested, caught in enormous trawl nets. The smallest cod could wriggle their way out of danger, while the biggest, heaviest specimens were continually removed from the sea.
One simple explanation for the phenomenon, then, was that the fish were not actually shrinking: Rather, they were simply eliminated as soon as they grew big enough to be caught.
But a new study suggests that intense fishing was driving the evolution of the fish. Small, slow-growing cod gained a significant survival advantage, shifting the population toward fish that were genetically predisposed to remaining small. Today’s cod are small not because the big individuals are fished out but because the fish no longer grow big.
The data, which were published on Wednesday in the journal Science Advances, add to a growing body of evidence that human activities like hunting and fishing are driving the evolution of wild animals — sometimes at lightning speed.
“Human harvesting elicits the strongest selection pressures in nature,” said Thorsten Reusch, a marine ecologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany and an author of the new paper. “It can be really fast that you see evolutionary change.”
The imprint that humans are leaving on other species is not always quite so visible. In a second study published this week, researchers at the Field Museum of Natural History, in Chicago, reported that over the past century, increasing human development may have driven changes in the skulls of local rodents. But some of these changes were subtle, and they were not the same across species.
“We are comparing two species in the same area that were supposedly exposed to the same pressure,” said Anderson Feijó, the assistant curator of mammals at the Field Museum and an author of the rodent study, which was published in the journal Integrative and Comparative Biology on Thursday. “But the way they dealt is totally different, because their biology is different.”
Go Fish
In the new cod study, researchers studied a set of unusual biological specimens: a collection of otoliths, the tiny, bonelike structures located in the inner ear of most fish. Otoliths grow in size over the course of a fish’s life, adding rings much as a tree trunk does. By examining these rings, scientists can estimate the age and growth rate of individual fish.
The researchers used a newly developed chemical technique to analyze otoliths collected from Eastern Baltic cod harvested between 1996 and 2019, when the collapse of the population prompted a fishing ban. They found that fish harvested in more recent years were significantly smaller, with slower growth rates, than those from the beginning of the period. From 1996 and 2019, the average length of the cod declined by 48 percent.
Then, the researchers sequenced and analyzed the DNA of each individual fish. For the older specimens, this was a tricky task, requiring the researchers to recover degraded DNA from otoliths that had been stored in paper bags, at room temperature, for decades. “We had to work with a little dirt, a little slime, some blood traces that were sticking to the otoliths,” Dr. Reusch said.
Ultimately, the team identified a variety of genomic regions and variants associated with growth rate. A statistical analysis revealed that over time, these variants were changing in correlated, nonrandom ways — suggesting that there was some external selective force acting on the genome and the population.
That is a “signal of selection,” said Kwi Young Han, a postdoctoral researcher at GEOMAR and an author of the paper.
The results didn’t prove that fishing is what drove this selection; warming temperatures would also be expected to favor smaller cod. But the size changes that the scientists documented far exceed what would be expected from temperature alone.
The genetic changes could have long-term consequences for the population and help explain why it hasn’t bounced back since the 2019 fishing ban. “It’s 2025 right now, and we don’t see any big fish still,” Dr. Han said.
Rodent Roundup
In the second study, researchers examined hundreds of rodent specimens contained in the collection of the Field Museum of Natural History. The specimens had originally been collected from around the Chicago metropolitan area between 1898 and 2023. The scientists focused on the skulls of two different species: eastern chipmunks and eastern meadow voles. Each skull was analyzed for specific characteristics, including its particular collection site and how highly developed the area was.
The researchers found that over time, as Chicago grew more urban, the chipmunks’ skulls became larger — but their rows of teeth grew shorter. These seemingly opposing trends may have been driven by a change in diet, the scientists said. Urbanization, with its abundance of human food and trash, could have made it easier for chipmunks to pack on weight year round, leading to larger body sizes. At the same time, the robust teeth that helped chipmunks extract calories from nuts and seeds may have become less essential.
Voles, in contrast, did not show significant changes in skull size over time, perhaps because their more restrictive diets — mostly grasses and other plants — and reclusive natures made them less likely to dine on human food, Stephanie Smith, a research scientist at the Field Museum and an author of the study, said. “Voles are kind of much more secretive,” she said. “They’re not as out-and-about and stealing people’s French fries.”
But their skulls showed signs of other changes. Vole skulls collected from more urban areas were flatter than those found at less developed sites. The bony structure that houses parts of the middle and inner ear — known as the auditory bulla — also tended to be smaller in vole skulls from urbanized areas.
There is some evidence from other species that larger auditory bullae may be associated with enhanced hearing. Perhaps urban voles evolved smaller auditory bullae to help dampen the urban din, Dr. Smith said.
Voles live “down in the ground, near all of the train noises, all of the vibrations from people walking around, cars, buses, everything,” Dr. Smith said. “So our thought here is that, potentially, this change in the auditory bulla could be related to filtering out excess sound.”
It’s just a hypothesis, Dr. Smith stressed, and one that requires much more study. But the findings help illustrate the enormous diversity of ways in which humans are inadvertently reshaping other species, whether out in the open ocean or in our backyards.
“There is evolution happening everywhere, all the time,” Dr. Smith said. “You just have to know where to look for it.”
Emily Anthes is a science reporter, writing primarily about animal health and science. She also covered the coronavirus pandemic.
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