While Dr. Christensen called Costa Rica “the poster child” for its efforts, he said corridors for large mammals were also being created in places like Uganda and China. The World Bank is financing corridor projects in Brazil and Peru; more important, the bank’s transportation planners are working with conservationists to ensure that building highways and laying train tracks so humans can move freely does not destroy that movement for animals, Dr. Christensen said.

Part of the reason that conservationists had in the past focused exclusively on preserves was that there was a lack of good data on the travel and breeding patterns of large animals like jaguars; these big predators favor dense jungles and are nocturnal and extraordinarily shy.

So when new techniques allowed scientists to take a first look at the jaguar genome a decade ago, they were shocked to discover that jaguars from the northern reaches of Mexico had exactly the same genetic makeup as those from the southern tip of South America.

That meant that over time, some jaguars were moving up and down the Americas to breed; otherwise, the isolation of jaguar populations in different regions would have caused their genetic makeups to diverge. At least some males from Colombia were traveling to Panama to mate, and others were moving from Mexico to Belize.

“It was surprising, but it seemed to say they had one continuous habitat,” said Dr. Rabinowitz, the zoologist.

Scientists were convinced that jaguars would never cross a water barrier as wide as the Panama Canal, smack in the middle of their extended habitat. But when they set up cameras to spot jaguars near the canal, they discovered that, every so often, a brave animal took the plunge, ensuring the continuity of genes in the north and south.