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

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.

Unique reproduction

“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.”

Amplexus (breeding) pair of Charles Darwin’s frog moving towards a tree hole.   | Photo Credit: S.D. Biju

Complex behaviour

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.

Adding to the study’s significance, researchers observed that the frogs are increasingly breeding in artificial and discarded containers such as plastic and metal trash. This shift is attributed to habitat loss and competition for natural breeding sites. “The frogs’ use of trash for breeding is both surprising and worrying,” notes Sonali Garg, a biodiversity postdoctoral fellow at Harvard’s Museum of Comparative Zoology, who co-led the study.

“We now need to know its causes and long-term consequences, and devise ways to protect the natural breeding sites that are critical for the survival of the species,” she added.

Charles Darwin’s frog is classified as ‘Vulnerable’ by the International Union for Conservation of Nature (IUCN).

 A male Charles Darwin’s frog calling from an unnatural breeding site inside a metal food tin littered in the forest.   | Photo Credit: G. Gokulakrishnan

Fieldwork for this study, spanning over three years, involved more than 55 nights of observation during the monsoon season across the Andaman archipelago.

David C. Blackburn, an expert in frog behaviour and developmental biology, emphasises the significance of such detailed field research. “There are more than 7,700 known species of frogs, but just because a species has a scientific name, it does not mean that we know much about its biology,” he points out.

The curator of herpetology at the Florida Museum of Natural History also underscores the pertinent questions raised by the study about how human disturbances, including loss or alterations of habitats, can impact the characteristic features of poorly known species.