In the Andaman Islands off the east coast of India, a species of frogs mate and lay their eggs upside down, according to a new study.

Both male and female Charles Darwin frogs are placed upside down on the walls of tree cavities, with their bodies completely out of the water, the scientists from India and the US say in the article. The hatchlings drop into the water below and develop through a free-swimming stage.

“This is remarkable. Laying eggs upside down is the most unique behavior in this frog. No other frog is known to lay eggs on inner walls of tree holes in an upside-down position with the bodies completely out of the water,” says SD Biju of Delhi University and currently a fellow at the Harvard Radcliffe Institute.

“This discovery is fundamental to understanding how this species interacts with its environment and which habitats are essential for its survival.”

Almost all of the world’s 7,708 species of frogs mate and breed in water and other terrestrial environments. They also use external fertilization: the female lays eggs during mating, while the male releases sperm to fertilize them.

A team of Indian and American biologists from the University of Delhi, the Zoological Survey of India, Harvard University and the University of Minnesota camped for 55 nights over 3 years during the monsoon rains to study the reproductive behavior of the tiny frogs Charles Darwin in the remote Andaman Islands. Their study appears in the latest issue of the scientific journal Breviora.

The uniqueness of the species doesn’t end with how they mate—even the mating calls are different.

Most frog calls are simple mating calls of one type, although some species have complex calls with multiple types.

Researchers found that in their mating frenzy, determined male Charles Darwin frogs produce three types of “complex” calls to attract females. When “aggressive” calls fail to “fend off” competing males, they start fighting – using arms and legs and biting off parts of the body or even the entire head.

If the male manages to mount a female, other males nearby may fight the pair. They may even try to put their heads between the couple’s bodies to separate them, according to the researchers.

“In our observation these fights rarely lead to deaths. This was surprising considering the extent of aggression seen in this species, such as biting body parts and even the entire head for long periods of time,” says Mr. .Biju, who led the study.

“Also, all of these fights take place in a small tree hollow filled with small amounts of rainwater, unlike most other species where fights are known to occur in larger open water ‘bodies.’ It’s amazing how frogs have evolved such unique strategies,” he adds.

The study suggests that reverse mating behavior evolved to prevent other aggressive males from disrupting spawning by displacing the pair.

“This finding is an example of the remarkable diversity of amphibians and reproductive behaviors that are still unknown to science, especially from unexplored areas in the biodiversity hotspots of tropical Asia,” says James Hanken of Harvard University, who participated in the study.

The Charles Darwin frog, named after the famous naturalist, is endemic to a few islands, the Andaman Islands and found nowhere else. It is rare and restricted to specific forest habitats and is listed as “vulnerable” by the International Union for Conservation of Nature (IUCN) due to population fragmentation and habitat loss.

Scientists have observed the frogs breeding in disturbed forests in artificial locations – from watered plastic bags of seedlings in neighboring nurseries to rain-filled discarded containers left as litter at the edge of the forest.

“Frogs’ use of garbage for breeding is surprising and disturbing. Now we need to learn its causes and long-term consequences and devise ways to protect the natural breeding sites that are critical to the survival of the species,” says Sonali Garg, a fellow at Harvard University who co-led the study.

A lack of adequate breeding grounds due to habitat loss and competition for limited resources may be driving the frog to breed in such unnatural locations, the researchers say.

With information from BBC News