Winter breeding of the red narrow-mouthed frog in Bangladesh: An unusual ecological strategy?

When frogs ignore the calendar 

For most South Asian frogs, the monsoon is not merely a season—it is destiny. Rainfall between May and September triggers explosive breeding events, synchronising calling, amplexus, egg laying, and larval development with abundant water and food. 

Members of the genus Microhyla, including Microhyla ornata and Microhyla berdmorei, follow this classic pattern closely. 

The Red Narrow-mouthed Frog, Microhyla rubra, has long been assumed to be no different. Yet nature has a habit of quietly undermining our assumptions. In 2015, Ahmad and Alam reported a surprising observation: winter breeding of M. rubra at a perennial stream (locally called a Chhara) below Madhabkunda Eco Park in northeastern Bangladesh. 

At the time, this appeared to be an isolated anomaly—interesting, but perhaps exceptional. However, repeated personal observations in December 2022 and again on 13 December 2024 (from 8am to 8pm) at a Chhara near a Tipra tribal hamlet suggest something far more intriguing. 

Multiple breeding pairs, fresh egg masses, tadpoles at various developmental stages, and ongoing calling activity all point toward regular winter breeding, not a one-off event. 

I will explain what these observations mean, how they fit within amphibian biology, and why Microhyla rubra may be rewriting the seasonal rules of frog reproduction in Bangladesh. 

Meet Microhyla rubra: A small frog with big surprises 

Microhyla rubra is a small, secretive microhylid frog widely distributed across South and Southeast Asia. In Bangladesh, it is commonly associated with forest edges, agricultural land, and moist lowland habitats. 

Like its congeners, it is primarily nocturnal, fossorial or semifossorial, and often overlooked due to its small size and brief surface activity. Traditionally, M. rubra is described as a monsoon breeder, producing eggs in temporary pools, ditches, and slow-moving water bodies. 

Photo: Courtesy

The assumption has been that it depends on warm temperatures and high rainfall to complete its reproductive cycle. My observations challenge this assumption in three important ways: 

1. Timing – Breeding activity in December, well outside the monsoon. 

2. Location – Repeated use of the same perennial Chhara. 

3. Continuity – Presence of tadpoles at multiple developmental stages, implying breeding over several months. 

These three points together strongly suggest that winter breeding is not accidental. 

The field site: A chhara that never dries 

The breeding site you described is ecologically distinctive. The chhara is a slow-moving perennial stream with a wide bend, creating shallow, relatively calm water pockets. This microhabitat appears to be crucial. 

Several features stand out:

 • Permanent water availability 

Unlike rain-fed pools, this Chhara flows year-round, eliminating the risk of desiccation during winter. 

• Reduced current velocity 

Eggs and tadpoles of Microhyla are poorly adapted to strong currents. Slow flow allows egg masses to float and tadpoles to feed efficiently. 

• Structural complexity 

Debris such as wood fragments, furniture remains, leaf litter, and natural stream detritus create shelter from predators and strong currents. 

• Anthropogenic influence 

Waste from nearby tribal households, while ecologically problematic, may increase nutrient levels, promoting algal growth—an important food source for tadpoles. 

Together, these factors create a stable winter refuge, compensating for the absence of monsoon rains. 

Observed breeding behaviour: A familiar ritual in an unfamiliar season 

My December observations reveal that winter breeding behaviour closely mirrors monsoon breeding. 

Amplexus and egg-laying 

Mating pairs were observed in amplexus lasting over an hour.  Females deposited all eggs at once in a loose gelatinous mass. After oviposition, the pairs separated, leaving the eggs unattended—typical behaviour for Microhyla species. 

Each female laid approximately 300–500 eggs, consistent with published accounts for the genus. The eggs were free-floating, likely due to their low density and the gelatinous matrix, allowing them to remain near the water surface where oxygen availability is higher.

Photo: Courtesy

Rapid embryonic development 

Eggs hatched in approximately two days, a remarkably fast rate. This aligns with amphibian developmental biology: embryonic development accelerates at higher temperatures and slows in cooler conditions. Winter temperatures may lengthen development slightly, but the eggs remain viable. 

Tadpole ecology 

The tadpoles were described as tiny, torpedo-shaped, fish-like hatchlings with external gills—classic aquatic larvae. They were actively swimming and feeding on algae and suspended organic particles. Transformation into four-limbed toadlets is expected to take more than a month, consistent with congeners that require 30–90 days for complete metamorphosis. 

Evidence for long-term winter breeding 

Perhaps the strongest argument for regular winter breeding comes from your observation of tadpoles at multiple developmental stages within the same small area (20–25 m²). This suggests: 

• Eggs were laid at various times of the year 

• Breeding occurred repeatedly over weeks or months 

• The population is not relying on a single synchronized event 

When combined with my similar observation in December 2022 and the Ahmad & Alam (2015) report, a compelling pattern emerges. It is entirely plausible that M. rubra has been breeding here during winter for more than a decade, possibly since before 2012. 

Cold-blooded but not cold-helpless 

At first glance, winter breeding may seem contradictory for a poikilothermic (cold-blooded) animal. However, amphibians are not passive victims of temperature. They are behavioural thermoregulators. 

Several factors may allow M. rubra to breed in winter: 

• Microclimate buffering 

Water temperature in perennial streams fluctuates less than air temperature, especially in forested or shaded areas. 

• Daytime breeding activity 

Observation from 8 am to 8 pm suggests that frogs may exploit warmer daytime temperatures. 

• Physiological plasticity 

Some amphibians can adjust metabolic rates and developmental timing in response to cooler conditions. 

• Reduced competition 

Winter breeding may allow larvae to develop with fewer competitors than during the crowded monsoon season. In short, winter is not necessarily hostile—it is merely different. 

Predation and survival: A high-risk strategy 

Winter breeding does not eliminate danger. The tadpoles were observed while being preyed upon by dragonfly larvae, aquatic insects, fish, and birds. This is entirely expected. However, producing hundreds of eggs compensates for high mortality. 

This classic amphibian strategy—high fecundity with low individual survival—remains effective regardless of season. Interestingly, predator communities themselves change seasonally. 

Some monsoon-active predators may be less abundant in winter, potentially balancing overall predation pressure. 

Why winter? Possible evolutionary explanations 

Several non-exclusive hypotheses may explain why M. rubra breeds in winter at this site: 1. Habitat stability hypothesis 

Perennial streams provide consistent breeding conditions year-round. 

2. Competition avoidance hypothesis 

Breeding outside the monsoon reduces competition with other anurans. 

3. Predator dilution hypothesis 

Seasonal shifts in predator abundance may favour winter larvae. 

4. Local adaptation hypothesis 

This population may have adapted behaviourally or physiologically to local conditions. 

5. Climate variability hypothesis 

Gradual warming trends may be extending the effective breeding window. None of these explanations should be accepted alone. Science progresses by stacking evidence, not jumping to conclusions. 

Why this matters 

Winter breeding in Microhyla rubra is not just a curiosity. It has real implications: 

• Species ecology – Our understanding of reproductive timing needs revision. 

• Conservation – Perennial streams may be critical breeding habitats year-round. 

• Climate research – Behavioural flexibility may buffer some amphibians against climate change. 

• Biodiversity monitoring – Winter surveys may reveal overlooked reproductive activity. Small frogs often tell big ecological stories—if we pay attention. 

Listening to frogs instead of calendars 

My observations, combined with previous reports, strongly suggest that winter breeding in Microhyla rubra at certain perennial chharas in Bangladesh is not accidental, not rare, and not recent. 

Rather than forcing species into rigid seasonal boxes, ecology asks us to observe carefully and revise our assumptions. Microhyla rubra appears to be doing what evolution has always favoured: exploiting opportunity wherever it exists. 

In the quiet bend of a winter stream, beneath floating debris and soft morning light, these small frogs are reminding us that nature does not read textbooks—and that is precisely what makes it worth studying. 

Dr Reza Khan is a wildlife, zoo, and safari park specialist