There's a new Cuban crisis—the island country's rare crocodile is being loved to death by its American cousin, a new study suggests.

Mating Cuban crocodiles and American crocodiles are creating hybrid offspring that threaten the survival of the Cuban species, which has dwindled to about 4,000 wild animals in two isolated Cuban swamps. The ten-foot-long (three-meter-long) reptile is listed as critically endangered by the International Union for Conservation of Nature (IUCN).

"That means any loss of animals—be it loss in fact or loss through hybridization—is a grave concern," said John G. Robinson, executive vice president for conservation and science at the Wildlife Conservation Society.

American crocodiles, which are found throughout the Caribbean, are not considered threatened by IUCN. The animals have increasingly moved into the Cuban crocodile's remaining freshwater habitat as it becomes more brackish—or salty—due to agricultural activities, said Robinson, who was not involved in the research.

The Cuban crocodile is the most terrestrial of the crocodiles—walking instead of waddling on their bellies like other croc species, he added. "They're very cool beasts."

Surprising Croc-Hybrid Discovery

For the study, scientists led by Yoamel Milián-García, of the University of Havana, took DNA samples from 89 wild-caught crocodiles and 2 captive crocodiles.

Surprisingly, the results showed that American crocodiles living in Cuba are more closely related to Cuban crocodiles than to other American crocodile populations in Central America.

This suggests that the American and Cuban species are mating much more than thought. When different species mate to create hybrids, genes mix, and eventually one lineage can cause the extinction of the other.

The scientists haven't done behavioral studies to find out if the hybrids are stronger or more aggressive, which can sometimes happen when species interbreed.

Yet the study "will be a wake-up call" for conservationists in Cuba, who have already put a lot of effort into protecting the Zapata swamp—home to about 3,000 of the Cuban crocodiles, WCS's Robinson said.

One obvious strategy, he said, would be to restore the flow of fresh water into the swamps, which would make the habitat less palatable for the American crocodile.