The keepers at Blackpool Zoo had thought the two 10-year-old animals in the enclosure were both female - but nature proved them wrong.
It is notoriously difficult to identify the gender of the yacare caiman - a South American member of the crocodile family - a zoo spokeswoman said.
Staff at the zoo have not yet named the eight-inch youngster.
'Eat constantly'
The baby yacare caiman has been placed in a special care tank in the feeding kitchen where staff say it is doing remarkably well.
Ann Williams looks after the feisty youngster, who is constantly hungry and snaps his tiny jaws at anyone passing in the hope of being fed.
"The baby eats small pieces of fish, locusts and pinkies (baby mice) three times a day, but it would eat constantly if given the chance," she said.
"Caiman are not endangered, although they are hunted in the wild for their skins for bags and shoes.
"They normally live between 25 and 40 years."
She added that in the wild they eat tadpoles and small mammals which they catch by sensing vibrations in the water.
The zoo would be happy to receive suggestions for a name for the baby.
