Darwin’s frogs are breeding in highly unusual places

A recent study has uncovered a unique breeding behavior in the Andamanese Charles Darwin’s frog (Minervarya charlesdarwini), endemic to the Andaman Islands. 

The research was led by a team of biologists from the University of Delhi, the Zoological Survey of India, Harvard University, and the University of Minnesota.

Published in the journal Breviora of the Harvard Museum of Comparative Zoology, the research describes how this frog species exhibits a combination of unusual reproductive traits.

Unique reproductive strategy of Darwin’s frog 

The Andamanese Charles Darwin’s frog, part of the Dicroglossidae family, displays a distinctive reproductive strategy. 

Unlike many frogs, these frogs lay terrestrial eggs inside tree cavities or root buttresses above the water surface. The eggs hatch into tadpoles, which then drop into the water below to continue their development. 

Upside-down spawning: A highly specialized trait 

During egg-laying, the mating pair adopts an unusual upside-down posture, clinging to the vertical walls of the tree cavity, completely out of the water. This behavior is unprecedented among frogs.

“Upside-down spawning is the most remarkable behavior in this frog,” said study lead author Professor S. D. Biju. “No other frog is known to lay terrestrial eggs inside tree holes in an upside-down position.”

Professor Biju noted that this discovery is fundamental for understanding how the species interacts with its environments and which habitats are essential for its survival. 

“Such specialized traits also yield insights into the evolution of reproductive modes and behaviors among anuran amphibians.”

Complex vocalizations used to find mates 

Additionally, the male frogs exhibit complex vocal behaviors, producing three different types of advertisement calls to attract females, along with an aggressive call used during male-male competition. 

When vocal displays fail to deter rival males, physical combat ensues, involving kicking, boxing, and biting. During these altercations, the defending male and the female often move higher up the tree cavity walls to avoid interference from rival males.

Professor James Hanken is curator of herpetology at the Museum of Comparative Zoology and a professor of biology in the Department of Organismic and Evolutionary Biology at Harvard.

“This finding is an example of the remarkable diversity of amphibians and reproductive behaviors that are still unknown to science, especially from unexplored regions in biodiversity hotspots of tropical Asia,” said Professor Hanken.

Unusual breeding sites of Darwin’s frog 

Despite their specialized breeding behavior, these frogs have adapted to breed in a variety of unusual and unnatural sites, especially in disturbed forest areas. 

The researchers observed them breeding in places like artificially watered plastic sapling bags and discarded containers, such as plastic and metal vessels, left at the forest’s edge. 

The adaptation to these suboptimal breeding sites is likely a response to habitat loss and competition for limited resources, posing a threat to the species’ survival.

Conservation efforts to protect Darwin’s frog 

The study emphasizes the urgent need for conservation efforts to protect the natural breeding habitats of Minervarya charlesdarwini. The species is currently listed as “Vulnerable” on the IUCN Red List of Threatened Species, and the degradation of its natural habitat could severely impact its population.

“The frogs’ use of trash for breeding is both surprising and worrying. 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,” said co-lead author Sonali Garg.

The research was conducted over three years on the remote islands of the Andaman archipelago, requiring over 55 nights of fieldwork during the monsoon season. 

The findings are detailed in a paper titled “Tree holes to trash: unique upside-down terrestrial spawning, agonistic interactions, complex mating calls, and unnatural breeding alteration in Minervarya charlesdarwini (Anura, Dicroglossidae),” published in the July 2024 issue of Breviora.

Broader implications of the study 

The research underscores the unique ecological adaptations of Darwin’s frog and the pressing conservation needs of this endemic species. 

The study also highlights the critical role of natural microhabitats and the potential threats posed by human-induced environmental changes.

“Our findings call for conservation attention to this habitat specialist, which, although locally abundant, is an endemic and threatened species of the Andaman Islands,” concluded the study authors.

Image Credit: S.D. Biju

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