While some animals act very skittish around humans, others are seemingly more comfortable in the presence of Earth's super-predators. So why are some wild animals more tolerant of human interaction than others? It turns out that an animal's fear response is largely driven by their surrounding environment and body size, according to researchers from the University of California, Los Angeles (UCLA).

In a recent study, researchers analyzed species' tolerance to human disturbance by comparing how close a human could get to an animal before it fled. This is known as the "flight initiation distance." Using data collected on 212 animal species over the course of 75 studies -- mostly on birds, mammals and lizards -- researchers found birds living in heavily populated urban areas were much more tolerant of humans than their rural counterparts. That said, the second factor -- body size -- revealed results that surprised the scientists. Researchers concluded larger birds, such as pelicans and black-backed gulls, are less likely to fly for the hills as humans approach, compared to smaller hummingbirds, according to a news release.   

"This new finding flips previous recommendations about large-bodied species being more vulnerable to the presence of humans, and shows that large-bodied species are more tolerant," Daniel Blumstein, leader of the recent study and a professor and chair of ecology and evolutionary biology at UCLA, explained in the university's release.

Specifically, researchers found that larger birds are more likely to be disturbed in remote areas. However, they eventually learn that people are not really a threat when human-animal interactions in the area remain benign.

"It is likely costly for animals to respond fearfully to people that are not harming them. The key question to ask now is which species can tolerate humans enough so as to habituate to them," Blumstein added in the release.

Their study, recently published in the journal Nature Communications, essentially sheds light on how human intruders may be altering wild animals' fear responses.   

While urban-rural differences and body size had the greatest impact on an animal's tolerance to humans, other factors including the animal's diet, openness of habitat, and offspring also played a role in their fear response. Their findings could help conservationists target species in need of protection. 

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