Many are domestic—like cats—but coyotes, monkeys, and other wild species are also finding their way onto public transportation.

These animal "commuters" are motivated more by food and security than anything else, said Suzanne MacDonald, a psychologist and biologist at York University in Toronto, Canada, who studies urban wildlife.

"If they're sitting there thinking, 'I gotta catch that 3 o'clock bus in order to make it to the movies,' then we're in deep doo-doo," quipped MacDonald, who receives funding from the National Geographic Expeditions Council.

Instead, animals that end up on buses or trains usually realize "if they get on this thing and get off this thing, in between they [can be] treated pretty well and rewarded for it."

Indeed, urban-wildlife ecologist