Humans are not the only mammals capable of deception. A new study published in the Journal of Mammalogy suggests that some Cantabrian brown bears deliberately exaggerate their size to intimidate rivals and avoid physical confrontations.
Researchers from Spain's National Research Council (CSIC), the Foundation for the Conservation of the Brown Bear (FAPAS), several Spanish and European universities, and the Principality of Asturias found evidence that a small number of brown bears "fake" their size by leaving scent and visual marks much higher than they could normally reach.
How do the bears deceive rivals?
Brown bears typically leave claw marks, scent marks and other territorial signals at the highest point they can reach while standing on their hind legs—usually around two metres above the ground.
However, researchers discovered that some bears climbed trees, tree trunks or other elevated surfaces before marking them, making it appear as though a much larger bear had left the sign.
Out of 164 male bears tracked between 2021 and 2024, only seven—about 4.3%—used this strategy. Among them, 57.1% were adults and 42.9% were subadults.
The team monitored 14 marking sites in Spain's Cantabrian Mountains for a total of 117,552 hours using camera traps. During the study, they found one mark nearly three metres above the ground—far beyond the reach of a bear standing normally. A broken branch beneath the mark suggested the bear had climbed up before leaving it.
According to the researchers, "These results suggest that brown bears can manipulate index signals that were previously thought impossible to fake, which adds complexity to their communication system."
Why would bears fake their size?
The researchers believe the behaviour gives smaller or less dominant males a competitive advantage.
"For a non-dominant male or a young individual, this strategy could offer advantages since, by appearing larger, they could deter bigger rivals, reducing the likelihood of a direct confrontation, avoiding the costs this may entail and even increasing their reproductive opportunities. It is a strategy reminiscent of deceptive behaviour described in other mammals, but one that has never before been documented in bears," said researcher Vincenzo Penteriani from the National Museum of Natural Sciences.
Researchers urge caution
Despite the findings, scientists stressed that the behaviour was rare and should not be interpreted as evidence that brown bears are generally deceptive animals.
"Our work highlights a fascinating behavioural capacity in bears that justifies further observational and experimental studies," Penteriani said.
Brown bear population recovering
Spain's two brown bear populations, found in the Cantabrian Mountains and the Pyrenees, now number more than 400 animals. The species had come close to extinction during the 1970s and 1980s but has recovered significantly due to conservation efforts. Although still considered endangered, the bears are no longer classified as critically endangered.
