Missing in Costa Rica: female crocodiles

SAN JOSE, Costa Rica — Where have all the female crocodiles gone?

Scientists in Costa Rica are concerned about the dwindling number of female crocodiles on the Pacific coast.

They warn that a dramatic switch-up in the sex ratio could lead to the crocodile's demise in that region within the next 25 to 30 years.

The sex of the American crocodile (Crocodylus acutus) is determined by temperature during egg incubation, with males hatching in warmer temperatures. Scientists say that climate change and deforestation are raising temperatures enough to create a sex imbalance.

While the ratio usually skews toward females by 3:1, a study of the Tarcoles River found the ratio had shifted to 1:1. Then a study in the Tempisque River showed males are dominating by as much as 5:1.

Laura Porras, the biologist who led the Tempisque studies, said the difficulty in finding female mates has spurred aggressive behavior among the males.

Costa Rica sees an average of eight crocodile attacks per year, which she says is high for such a small country. An average of three fatal attacks occur each year, said Juan Rafael Bolanos, a statistician and member of the Association of Central America Crocodile Specialists.

When a crocodile attacks a human or livestock, people tend to retaliate by killing off the nearby crocodiles, Porras said.

Beyond the threat of slaughter, the male-female ratio could tip the balance too far for the population to sustain itself. Bolanos gives Tampisque’s crocodiles until 2040 at the latest to survive under these conditions.

The United Nations forecasts the planet’s temperatures will rise one to six degrees over the next century. Many scientists predict hotter and drier days could decimate plant and animal life in Costa Rica, one of the world’s most biologically diverse regions. Amphibians and other types of species have already disappeared.

Researchers in Costa Rica say local environmental damage is creating localized climate change. They say over-farming and land development have encroached on the rivers, clearing away vital plant cover that provides shade for crocodile nests, heating the eggs enough to create a sex imbalance in a species whose females are expected to outnumber males.

“There are very large companies — producers of watermelon, cantaloupe, rice, sugar cane — and they don’t respect the riverbanks. The crops grow right up to the river,” said Bolanos, who participated in the Tempisque study.

Cattle farms, he added, have also crept dangerously close to the creeks, allowing cows to trample nests and roam the waters where crocodiles are known to attack.

Female crocodiles nest 5 to 10 meters from the river. They test out as many as seven sites before digging a hole and laying some 30 eggs. Of those, just 3 percent may hatch.

Both scientists are also concerns about the effects of deforestation.

Clearing plant cover removes the earth’s natural coolant. Cement keeps temperatures higher while trees and plants absorb the heat, trapping carbon dioxide and releasing oxygen. While deforestation could have caused the immediate problem of removing shade on the riverbanks, scientists say it could also speed up climate change by ridding the planet of nature’s sink for harmful greenhouse gas.

While many residents and farmers might like to see the huge, sharp-toothed reptiles just disappear, their demise would be harmful for the habitat in which they live, Porras argues. As top-level predators, crocodiles are vital to maintaining their ecosystem, helping control the population of other species and nurturing aquatic plant life with their excrement.

The findings on the sex imbalance came about unexpectedly during an experiment with crocodile translocation, in which Porras and her team relocated crocodiles to different rivers to observe how they lived in a different habitat. Forcible translocation of animals by humans is a management tool used to save a species from extinction or sometimes to protect a human population from dangerous predators.

Distances of up to 20 kilometers didn’t matter, Porras said. The crocodiles found their way back to their families. (Good for crocodile survival; bad for residents, farmers and prey who want crocs out of there.)

During the experiment, the scientists studied the species’ genetic diversity. It was during that study that the sex ratio appeared out of whack.

The crocodile experts say with more funding they intend to continue monitoring the species in different habitats around the country and they hope that with raised awareness Costa Ricans will work to prevent them from becoming another dinosaur in the natural science museum.