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The Charles Darwin, a frog species endemic to the Andaman Islands, is displaying a curious mating, as well as egg-laying behavior, which is concerning in the context of climate change according to a set of India and United States researchers.
The scientists reached the conclusion after an extensive study conducted over three consecutive monsoons. They found that the frog species, scientifically known as Minervarya charlesdarwini, engages in upside-down mating and egg-laying behaviour.
The changing patterns are part of the frog species’ attempts to adapt to the rapidly changing environment on the sensitive Andaman archipelago.
The researchers, who hail from both Harvard University and the University of Delhi, noticed that these frogs spawn upside down and use human waste as their breeding grounds.
The detailed study, which involved scientists from the University of Delhi, the Zoological Survey of India, Harvard University, and the University of Minnesota, was published in the Harvard Museum of Comparative Zoology’s journal Breviora on Monday.
S D Biju, currently a fellow at the Harvard Radcliffe Institute, told Down To Earth (DTE) that his team found both male and female Charles Darwins adopting an inverted position on the walls of tree cavities, keeping their bodies completely above the water while mating as well as laying eggs.
The hatchlings descend into the water below and undergo a phase of being free-swimming tadpoles as they mature, Biju, who hails from Kerala and is a faculty member of the University of Delhi, added.
“The upside-down spawning is the most distinctive behaviour of this frog now, and it’s extraordinary. No other frog species is known to lay eggs on the inside walls of tree holes while being upside-down and with their bodies totally outside of water,” he explained.
“This finding is crucial for comprehending the species’ interactions with its surroundings and identifying the indispensable habitats for its survival,” Biju noted.
These findings have significant implications for the conservation of this species, which is already listed as vulnerable on the International Union for Conservation (IUCN) of Nature Red List.
“The study revealed that these frogs increasingly use artificial objects as breeding sites, such as plastic sapling bags and human trash comprising discarded plastic, glass, and metal containers. This shift is likely a response to the loss and fragmentation of forest habitats, forcing the frogs to adapt to the rapidly changing environment on these small islands,” said Biju.
According to him, this is the biggest study on the species so far. The biologists spent 55 days in the Andamans, during three consecutive monsoon seasons between 2019 and 2021, to observe the tiny frogs and notice their unique mating behaviour.
Biju said the mating calls of this frog species are complex, with multiple call types.
The team found that determined Charles Darwin males produced three types of ‘complex’ calls to woo females. When the ‘aggressive’ calls fail to ward off competing males, they begin fighting—kicking and boxing, using forelimbs and hind limbs, and biting off body parts or even the entire head.
If the male successfully mounts a female, the bachelor males nearby may fight with the mating pair. Researchers found that they may even try to insert their heads between the pair’s bodies to separate them.
“In our observation, these fights rarely lead to deaths. This was surprising considering the extent of aggression observed in this species, such as biting off body parts and even the entire head for long durations,” said Biju.
Charles Darwin’s frog, named after the famous naturalist, is endemic to a few Andaman Islands and is not found anywhere else. It’s uncommon, restricted to specific forest habitats, and listed as ‘vulnerable’ by the IUCN due to fragmented populations and habitat decline.
What worries the scientists is that the frogs are breeding in disturbed forests at artificial sites—from watered plastic sapling bags in neighbouring plant nurseries to rain-filled discarded containers left as trash at the edge of the forest.
“The frogs’ use of trash for breeding is surprising and worrying. We now need to know its causes and long-term consequences and devise ways to protect the natural breeding sites critical for the species’ survival,” said Sonali Garg, a fellow at Harvard University who co-led the study.
Researchers said the lack of adequate breeding sites due to habitat loss and competition for limited resources may be driving the frog to breed in such unnatural sites.
Charles Darwin’s frog belongs to the family Dicroglossidae, a large group of Asian frogs with over 220 species.
“This discovery highlights the remarkable diversity of amphibians and reproductive behaviours that are still unknown to science, especially in unexplored regions of biodiversity hotspots,” according to James Hanken, who was part of the study.
He also said observing the species adapt to human trash in forests disturbed by human activities is worrying.
The study calls for increased efforts to conserve the endemic and threatened species and protect their specialised and vulnerable microhabitats to maintain natural breeding sites.
“The lack of adequate breeding sites due to habitat loss and competition for limited resources may drive Charles Darwin’s frog to breed in unnatural sites. Protecting natural breeding sites is crucial for the species’ survival,” says Biju.
The research also highlights how human disturbances can impact the characteristic features of poorly known species.