Endangered Charles Darwin’s frog exhibits unique upside-down spawning behaviour

An Indo-American team of biologists has discovered an extraordinary reproductive behaviour in the Charles Darwin’s frog (Minervarya charlesdarwini), an endangered species endemic to the Andaman Islands.

In a pioneering study featured in the Harvard Museum of Comparative Zoology’s journal Breviora, an Indo-American team of biologists has discovered an extraordinary reproductive behaviour in the Charles Darwin’s frog (Minervarya charlesdarwini), an endangered species endemic to the Andaman Islands.

This discovery not only highlights the rare mating ritual, but also raises concerns about the loss of natural habitats, leading the species to thrive in human trash in forests disturbed by anthropogenic activities.

The research, undertaken by biologists from the University of Delhi, Zoological Survey of India, Harvard University, and the University of Minnesota, reveals that the Charles Darwin’s frog exhibits an unparalleled reproductive behaviour: upside-down spawning. Unlike other frogs, this species deposits its eggs in a vertical, upside-down position on the inner walls of water-filled tree cavities or root buttresses. Once laid, the eggs eventually fall into the water below, where they develop into tadpoles.

“This is a remarkable frog and its reproductive behaviour is unique for a number of reasons. No other frog lays terrestrial eggs in an upside-down position within tree holes,” explains Professor S.D. Biju from the University of Delhi, who led the study, and is currently a fellow at the Harvard Radcliffe Institute and an Associate of the Harvard’s Museum of Comparative Zoology.

“Such specialised traits also yield insights into the evolution of reproductive modes and behaviours among anuran amphibians. This discovery is crucial for understanding the species’ interaction with its environment and its essential habitats.”

The study also delves into the complex vocal behaviour of male Charles Darwin’s frogs, who use intricate calls to attract females. These calls include three distinct types: advertisement calls, aggressive calls, and combat calls. In cases where vocalisations fail to deter rival males, physical confrontations ensue. Males engage in fierce battles involving kicking, boxing, and biting, competing intensely for mating opportunities.

When a male successfully mounts a female, unpaired males often attempt to disrupt the mating pair, leading to aggressive skirmishes. The mating pair’s upside-down position may serve as a defensive adaptation, preventing rival males from displacing them and disrupting the egg-laying process.