Sept. 22 (UPI) -- New research suggests animals begin to lose their fear of predators once they start encountering humans on a regular basis.
For the study, scientists surveyed the findings of 173 peer-reviewed papers on predator avoidance behaviors and traits deployed by 102 species of domesticated, captive and urbanized mammals, birds, reptiles, fish and mollusks.
The analysis, published Tuesday in the journal PLOS Biology, showed predator avoidance traits and behaviors, including vigilance, freezing and fleeing, decreased as a result of exposure to humans.
Researchers found individual variation in anti-predator characteristics increased upon a species' initial exposure to humans, but then gradually decreased after generations of human exposure.
"While it is well known that the fact of being protected by humans decreases anti-predator capacities in animals, we did not know how fast this occurs and to what extent this is comparable between contexts," lead researcher Benjamin Geffroy, biologist at the University of Montpellier in France, said in a news release.
The findings suggest behavioral flexibility allows for the initial increase in the variability of anti-predator traits, but researchers suspect genetic changes solidify declines in predator avoidance as subsequent generations adjust to the presence of humans.
In the studies analyzed by Geffroy and his colleagues, domesticated animals lose their anti-predator traits much more quickly than urbanized animals, which can cause problems when domesticated or urbanized species are released back into the wild.
"We also integrated physiological traits in the study but they were much less numerous that behavioral traits," Geffroy said. "We believe they should be systematically investigated to draw a global pattern of what is happening at the individual level.
"We need more data to understand whether this occurs also with the mere presence of tourists," Geffroy said.