Andaman forests hatch a unique frog tale

New Delhi: After spending three monsoons in Andaman forest, an Indo-US team of biologists have unravelled the secret lives of the island’s own Charles Darwin’s frogs, which lay eggs in a “vertical, upside-down posture” on tree cavity walls with their bodies completely out the water to protect the eggs from other marauding males.

“The upside-down spawning is the most remarkable behaviour in this frog. No other frog is known to lay terrestrial eggs inside tree holes in an upside-down position," team leader S D Biju from the University of Delhi, who is currently a Fellow at the Harvard Radcliffe Institute told DH.

The unique reproductive trait is yet another example of the diversity of the amphibians – a keystone species that determines the health of biodiversity hotspots. Unfortunately, the frogs now struggle to survive as the landscape around them changes.

Found in 1998, the Charles Darwin’s frog – scientifically named Minervarya charlesdarwini – is a ground dwelling species that lives on the forest floor. During monsoon, when rain water is accumulated inside tree cavities, the frogs start breeding.

The DU researchers along with their colleagues from Zoological Survey of India spent 55 days in Andaman in three consecutive monsoon seasons between 2019 and 2021 to observe the tiny frogs and noticed their unique mating behaviour.

It began with male frogs producing complex advertisement calls comprising three different call types for attracting females. They also produce an aggressive call. When aggressive vocalizations fail to ward off competing males, physical combat begins.

The fights include kicking and boxing using hands and legs, and biting of body parts or even the entire head. Males compete aggressively to mate with females.

If a male successfully mounts a female, nearby unpaired males may physically fight with the mating pair. They may even try to insert their head between the bodies of the pair from the back side to separate them.

A male Charles Darwin’s frog calling from an unnatural breeding site inside a rain-filled metal food tin littered on the forest floor.

The defending male often kicks the intruding males with his hind legs. Simultaneously to avoid attacks, the female climbs the wall of the tree hole with the male on her back.

The female climbed vertically in an upside down position and lay eggs so that the eggs are protected from other males down below, says the Indo-US team also comprising researchers from Harvard University and the University of Minnesota.

The study - published in the Harvard Museum of Comparative Zoology’s journal Breviora - suggests that the upside-down spawning behaviour in this frog may have evolved as a means of preventing aggressive unpaired males from displacing the embracing pair from behind and disrupting egg-laying.

“This find is an example of the remarkable diversity of amphibians and reproductive behaviours that are still unknown to science, especially from unexplored regions in biodiversity hotspots of tropical Asia.” James Hanken, Curator of Herpetology at the Museum of Comparative Zoology and a professor of Biology at Harvard, who was part of the study, says in a statement.

The species, which is in the vulnerable category in the IUCN red list, struggles for existence because of habitat loss even in the islands.

The research team frequently observed frogs breeding in unnatural sites in disturbed forests, ranging from artificially watered plastic sapling bags in adjacent plant nurseries to rain-filled, discarded plastic, glass or metal containers left as trash at the forest edge.

“The lack of adequate breeding sites due to habitat loss and competition for limited resources may be driving the Charles Darwin’s frog to breed in such unnatural sites,” said Biju, noting that the species might not be able to survive in the face of increasing human dominance and rapidly changing landscapes on the small islands where they live.

"We now need to devise ways to protect the natural breeding sites that are critical for survival of the species.” adds Sonali Garg, a biodiversity postdoctoral fellow at the Harvard’s Museum of Comparative Zoology who co-led the study.