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For decades, scientists believed animals on islands evolved smaller brains relative to body size to save energy. But most Channel Islands foxes—tiny predators no bigger than a house cat—defied that rule, evolving larger brains than their mainland cousins.

The findings, in PLOS One by researchers at the USC Dornsife College of Letters, Arts and Sciences, suggest that brain size may hinge less on isolation and more on the demands of survival.

Island syndrome refers to a suite of traits including reduced size, brain shrinkage, loss of flight in birds and tamer behavior. Until now, island brain size has mainly been studied in fossils of herbivores, which face different pressures than carnivores. The island fox provided an opportunity to bridge that research gap.

Lead author Kimberly Schoenberger said the results upend long-held assumptions about how animals adapt to island life.

"It was most surprising to discover that island syndrome isn't one-size-fits-all," said Schoenberger, a Ph.D. candidate in biology. "When we looked at carnivores like Channel Island foxes, the pattern of smaller brains didn't hold."

Channel Islands offer a natural experiment

The Channel Islands, an eight-island chain off the California coast, six of which are home to foxes, offered researchers a rare natural experiment to test whether island syndrome holds true across environments. The biologists compared the brains of foxes from those six islands with mainland gray foxes—their closest living relatives and likely ancestors—as well as with one another.

The analysis revealed that the brain-to-body ratio appeared to be shaped less by isolation or island size alone—long thought to drive island syndrome—and more by local habitat conditions.

On five of the islands—Santa Cruz, Santa Rosa, Santa Catalina, San Clemente and San Miguel—foxes had relatively larger brains despite their smaller bodies. Their brains also showed slightly deeper folds and ridges in areas tied to motor control and spatial processing, traits that may help the foxes navigate rugged terrain and compete for food and shelter, especially on two islands where they share space with rivals like the spotted skunk.

San Nicolas Island, the most remote and resource-limited of the six, was the exception. There, foxes had smaller relative brain sizes. With no predators, little biodiversity and limited food, researchers believe the animals faced fewer cognitive demands and may have conserved energy for basic survival rather than retaining traits like enhanced motor coordination or spatial processing.

"The Channel Island foxes show that brain size reduction is not a universal feature of island life," Schoenberger said.

Fox brains adapt to island living

To understand how brain size changed in the island fox, Schoenberger and research collaborators at the Natural History Museum of Los Angeles County (NHM) studied more than 250 skulls from six island fox subspecies and four mainland gray fox subspecies. The specimens came from collections at NHM and the Santa Barbara Museum of Natural History.

First, they estimated body weights from skull measurements, then compared those estimates to recorded weights from live island foxes. The two sets aligned closely, giving the team a solid basis for analyzing brain-to-body ratios. The brain size was measured using microbeads to approximate the internal volume of the skull, a highly accurate proxy for brain size.

Next, the researchers used CT scans to create 3D models of the skulls' interiors. By capturing the shape and volume of the braincase—the hollow cavity that once housed the brain—they could confirm the accuracy of the microbead brain size estimates and examine the structure of brain surface features such as folds and ridges imprinted on the inside of the skull.

The digital scans revealed a surprising feature: Island fox brains had shorter, more compact frontal areas than their mainland relatives. The researchers theorize that this likely relates to their shorter snouts, which reduce the brain space at the front of the skull. To compensate, these foxes developed slightly deeper folds and more pronounced ridges in this region. This preserves key motor control and spatial awareness skills particularly needed to climb trees and forage for food.

Schoenberger explained that while both the island and gray foxes are the only canines known to climb trees, island foxes relied more on foraging in trees compared to mainland foxes, which have more abundant food options on the ground.

The researchers also found that brain size didn't differ between male and female foxes, suggesting that environmental pressures rather than mating competition were the likely drivers of these adaptations.

Island foxes were likely pest control, not pets

DNA and carbon dating suggest that foxes first arrived on the northern Channel Islands about 9,000 years ago, likely by riding natural debris rafts or swimming when sea levels were lower and distances between islands were shorter. Thousands of years later, archaeological evidence shows that Indigenous peoples may have transported foxes to other islands, perhaps to help control pests.

Despite this long association with humans, island foxes likely never became truly domesticated—a fact researchers say may help explain why their brains did not shrink, as often happens in domestic species. In both brain size and behavior, they more closely resemble wildcats than early domestic dogs, which had already begun to lose brain volume as they grew dependent on people.

Broader lessons from island-bound foxes

As climate change and habitat loss create more fragmented, island-like environments, understanding which traits help animals adapt could guide conservation efforts. The island fox suggests that intelligence and cognitive flexibility may play a key role in survival but may only be an option if their habitat has enough natural resources.

Still, their future is uncertain. Previous research had found that low genetic diversity could leave the foxes especially vulnerable to new threats such as disease, raising concerns about how well they'll weather the next wave of environmental change.