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In a dense crowd where every second matters, bright shade lets females pick the right partner without confusion and prevents males from mistakenly grabbing one another in scramble.
Once chosen, the pair enters amplexus, a posture where the male clasps the female tightly as she lays eggs.
Image Credits: Susanne Stückler and Doris Preininger
Just before the monsoon, when the evenings turn heavy and the air feels thick enough to hold, the forests begins tuning itself for a shift. The soil softens underfoot, the leaf litter grows damp, and small ponds return to life.
And somewhere near these pools, a brown toad slowly brightens into a glowing yellow. The change is so sudden and so striking that it can stop a passer by mid step. Yesterday he looked like any other toad. Today he looks like he has been dipped in turmeric under a pale dawn.
This transformation has fascinated researchers because it appears precisely when the monsoon begins rearranging the landscape. Yet the real reason for the colour shift lies not in symbolism or spectacle, but in survival.
The work comes from a collaborative team led by Prof K V Gururaja, an amphibian specialist from the Srishti Manipal Institute of Art, Design and Technology, alongside researchers Susanne Stückler and Doris Preininger from the University of Vienna and the Vienna Zoo, with additional contributions from Mathew J Fuxjager of Brown University. The team walked across the forests of Uttara Kannada district in Karnataka, India chasing the monsoon makeover of the Common Indian Toads.
Prof K V Gururaja explained the whole phenomenon in detail to News18: The male toad is preparing for the most competitive, high stakes season of his life. His body shifts into yellow not to please an audience but to be noticed by a female in an environment that becomes crowded, noisy, and chaotic the moment the rains set in.
Understanding why this happens, and why it happens right before or during the monsoon, reveals how finely amphibians are tuned to environmental rhythms. Their bodies respond to tiny atmospheric shifts in humidity, soil moisture, and temperature changes that arrive before heavy rainfall. The yellow glow is therefore both a mating signal and a reflection of monsoon linked ecological timing.
The Hidden Biology Behind The Colour Change
For most of the year, male toads stay brown. The shade helps them blend into soil and leaf litter, protects them from predators, and keeps them inconspicuous. But as the breeding season approaches, their bodies undergo a rapid hormonal surge.
Pigment cells in the skin reorganise, reducing the dominance of dark melanin and allowing yellow and orange pigments to spread. Depending on the species, the transformation can take hours or days, but the final effect is always unmistakable. A drab animal becomes a vivid one, as if the forest itself has switched to monsoon mode.
Male toads develop their bright yellow hue through a rapid re-organisation of skin pigment cells known as chromatophores. Stress hormones like epinephrine and norepinephrine trigger dark melanin-rich cells to contract and reflective pigment layers to tilt, exposing yellow and orange pigments beneath the surface.
This bright coloration is a direct result of breeding hormones rising in response to environmental cues. As humidity climbs and temperatures dip slightly, the toads receive signals that temporary pools will soon form.
The result is a swift change from brown to neon yellow in minutes, enabling the male to stand out during the brief breeding window.
The breeding season requires these shallow waters because eggs need a safe, moist environment and tadpoles must develop quickly before the pools dry. The yellow colour therefore emerges at exactly the moment when males need to begin competing for mates.
Why the Monsoon Triggers This Transformation
The monsoon is not simply a season for these amphibians. It is the core of their reproductive cycle. Their survival depends on synchronising their breeding with the return of water. Toads do not wait for heavy rain. They respond to the earliest signs of its arrival.
Moist soil, the first light showers, changes in the smell of the air, cooler nights, and the appearance of puddles all act as signals that breeding grounds will soon be ready.
As these cues intensify, the hormonal change within the male accelerates. His body prepares for breeding even before the first downpour. The yellow coloration appears at this intersection of biology and climate, making it seem as though the toad is announcing the monsoon.
In truth, both are responding to the same environmental conditions. Nature sets the stage. The male toad steps into character.
A Season of Calls, Competition, and Urgency
Once the males turn yellow, the monsoon breeding frenzy begins. Shallow pools fill overnight, and males gather in large numbers, each calling loudly to attract females. The soundscape becomes a layered chorus, with dozens of calls overlapping across the water surface.
It is an environment where visibility matters just as much as sound. The yellow body is an advantage here. It allows the male to stand out in murky water and helps a female quickly identify and choose a partner.
The competition is intense. Males jostle, attempt to climb over one another, and often engage in physical struggles to reach receptive females. A female usually selects a male based on proximity, strength of call, visibility, and overall condition.
Once chosen, the pair enters amplexus, a posture where the male clasps the female tightly as she lays eggs. He fertilises them externally while the monsoon provides the water needed for the eggs to survive.
The yellow coloration fades once the breeding season ends. When the hormones drop and the rains settle into a steadier rhythm, the males gradually return to their usual brown, blending again into the forest floor.
The yellow shift in males serves a very simple purpose. It helps females quickly recognise who is male in the chaotic rush of the breeding pools. The colour is not a badge of strength, fertility, or superiority. It is just a visual shortcut.
In a dense crowd where every second matters, the bright shade lets females pick the right partner without confusion and prevents males from mistakenly grabbing one another in the scramble. The colour, in the end, is nothing more than a practical signal in a fast-moving monsoon race.
When Love Meets 3D Printing
Researchers created lifelike 3D printed models of male toads to understand how females respond to the sudden yellow transformation. These models were made in both brown and bright yellow versions, allowing scientists to test whether the colour shift alone influences female choice without the distraction of movement or calls.
By placing the models in breeding pools and observing how real females approached, the team could isolate coloration as a factor in mate selection. The experiments confirmed that the vivid yellow males were consistently more visible and more likely to attract female attention during the monsoon breeding rush.
What the Colour Change Tells Us About the Monsoon Itself
Although the yellow glow is fundamentally a mating strategy, it also reveals much about the monsoon’s progression. Because the transformation depends on environmental triggers rather than fixed dates, it often reflects how early, late, strong, or weak the season is.
In years when pre-monsoon showers arrive on time, males begin turning yellow earlier. In years when rainfall is delayed or uneven, the colour change is also delayed or appears irregular. When the monsoon is patchy, breeding activity becomes scattered and inconsistent. If the rains fail, the transformation may never reach full intensity.
This makes the behaviour an ecological indicator. The toads do not predict the monsoon, but their bodies react to the same shifts that bring rain. Observing them closely reveals subtleties that meteorological instruments may miss at the micro level, especially in forested regions and foothill zones where moisture patterns vary sharply.
Toads and Their Delicate Relationship with Climate
Amphibians are among the most climate sensitive creatures on the planet. Their skin absorbs water. Their movements are tied to moisture levels. Their reproduction depends entirely on rainfall. Any disruption in the monsoon ripple directly through their life cycles.
Changes in rainfall patterns now create challenges such as delayed breeding pools, shorter windows for eggs to hatch, pools drying too early, and mismatched timing between hormonal readiness and suitable habitat. In extreme cases, entire breeding seasons can fail.
Because the yellow colour change is so tightly linked to the male’s readiness to mate, irregularities in the timing of this transformation often hint at deeper environmental instability.
When the yellow comes too early or too late, or when males do not turn fully vibrant despite the season, it reflects stress in the ecosystem. Tracking this behaviour over years can help researchers understand how climate change is altering local monsoon rhythms.
A Signal We Used to Notice Without Instruments
For generations, rural communities read the arrival of rain through the chorus of frogs and toads. Their calls filled the nights just before storms. Their sudden appearance on pathways, fields, and bunds marked the beginning of the wet season.
The colour change in some species fits into this intuitive system of observation. It is a natural event that coincides with rain linked shifts and reveals the moment the forest accepts that the season has turned.
Many communities have long treated frog calls as the first sign of approaching rain. But the colour change actually happens before the croaking begins.
The males turn yellow first, then the chorus starts, and only after that does mating begin. This makes the colour shift an even earlier and more delicate signal of the monsoon’s arrival, a quiet cue that appears before the landscape gets loud.
Today, we live in a world that depends heavily on satellite data, numerical models, and long range forecasts. Yet the small signals still matter.
They offer immediacy and local truth. The yellowing of a male toad is one such signal. It is not a prediction. It is a response. But it is a response so finely woven into the fabric of the monsoon that it becomes one of the most reliable natural events tied to the season.
In a time when climate variability makes the monsoon unpredictable, these old signals carry renewed value. And in the quiet glow of a small toad turning yellow, the forest still announces the rain long before the thunderclouds appear.
November 14, 2025, 13:04 IST
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