SAN JUAN, Puerto Rico -- Visitors to the lush greenery of this island may notice a peculiar two-note sound similar to "co - key" and assume it comes from a bird.
But this sound is actually made by a tiny frog known as el coquí. Being a nocturnal frog, the coquí hides during the day and emerges at sundown to delight residents with its melody.
The common coquí or the Eleutherodactylus genus is an important aspect of Puerto Rican culture and many consider it the unofficial mascot of the island.
"It is not only a national symbol but also Puerto Ricans are very proud of the coquí. It defines what we are, our culture and our heart," says Danny Muñiz, deputy director of the Puerto Rico Tourism Company.
When some Puerto Ricans want to express their nationality, they say: "Soy de aquí como el coquí" (I'm as Puerto Rican as a coquí).
"Some tourists think it's another type of animal. There are stories about people trying to shake a tree to see a little bird flying and they are always surprised when they find out it is a frog," added Muñiz.
You can find in an area of two acres about 22,000 coquís singing their little hearts out at night. According to Prof. Rios-Lopez, the number of coquís on the island is in the billion range. There are 17 species of coquí on the island but only two produce the famous two-note call, named co-quí. Full-grown coquís measure between 1 inch and 3 inches.
The sound made by males serves two functions. The CO part of that note is to deter other males and establish territory while the QUI serves to attract females. Coquís reproduce year-round but breeding activity is in the wet season.
"Being an island, Puerto Rico doesn't have big predators and these little animals are a very important component of the ecological process of the island," says Associate Professor Neftalí Rios-Lopez from the University of Puerto Rico at Humacao, who started studying the species in 1992.
The best place to find the coquís are wooded areas including El Yunque National Forest, the only tropical rain forest in the United States National Forest System, and other mountainous areas of the island.
ABC News' Brian Fudge and Arthur Niemynski contributed to this episode.