How young coquí frogs balance the competing demands of growth and fighting disease

The common coquí frog is a small but iconic species in Puerto Rico. Their melodic “co-quí” call is a lullaby for people on the island.

But it’s far less welcome in places like Florida and Hawaii, where the species is invasive. In those states, these little frogs can seem impossible to get rid of.

But in fact, a deadly fungal pathogen has been contributing to declines in more than 500 amphibian species worldwide, including the common coquí.

In Puerto Rico, where this pathogen has been present since 1976, infected frogs tend to be smaller than healthy ones. Smaller frogs of any species are more likely to die.

For tiny frogs, survival is a high-stakes game with no pause button. Any energy put toward immune defense to prevent infection cannot be used to grow, and vice versa. And for young frogs, this energy allocation game is especially critical because getting it wrong can mean game over.

So what processes are driving differences in size and survival between infected and healthy frogs?

We are part of a team of researchers at the University of Florida who study how disease shapes wildlife populations. In our latest research, we integrated field data and mathematical models to examine how young coquís allocate energy as they grow while coping with infection.

We wanted to understand how these trade-offs play out over time and whether the season – that is, whether it’s warm or cool — when frogs hatch from their eggs affects their chances of surviving to maturity and their lifetime reproduction.

Tiny frogs, big challenges

Coquí frogs lay eggs on land, skipping the tadpole stage in a process called direct development. This means they hatch as miniature frogs, smaller than your pinky nail.

Males guard the eggs and newborn frogs, called neonates. Young froglets hatch at less than 0.4 inches (1 centimeter) long. They take about a year to reach adulthood, growing to a bit more than an inch long (2.9 centimeters).

Their small size makes them difficult to track in the wild. That means the most important periods for growth and survival are also the hardest to study directly.

Coquís reproduce year-round, peaking during the warmer months of May through August. During this time, more frogs are infected with the pathogen.

Frogs that hatch during the cooler months of December through April are exposed to fewer infected frogs, but those they do encounter have more of the deadly fungus in their skin. Cooler temperatures alter frog physiology, suppress immune function and disrupt beneficial microbial communities. This leads to higher fungal loads.

Independent of hatching time, all of the coquís are exposed to the deadly pathogen early in life, when they are more susceptible to dying.

A strategic survival game

Because these tiny frogs are so tricky to monitor, we used mathematical models to simulate how young frogs might grow, become infected and survive under different environmental conditions.

The optimization models we used to identify the strategic behaviors for frog survival have been applied across many disciplines, from finance to reproductive ecology.

Our model allowed us to change factors like food supply, the probability of getting infected and mortality risk, mirroring the patterns that we observe in the field and in experiments. For instance, we know that there are more available prey and more infected frogs during warmer seasons, and that the risk of mortality increases in infected froglets.

In our model, we think of this process as a strategic single-player game: At every stage of life, frogs appear to “decide” how to use energy. They can use it to grow, they can use it to fight infection, or they can split it between the two processes.

Of course, the frogs are not consciously making these decisions. Rather, the patterns of frog behavior reflect strategies shaped by evolution. Over generations, individuals who allocate energy in ways that improve survival rates are more likely to reach sexual maturity and pass on their traits.

Using this approach, we identified a few simple rules for surviving as a young coquí frog.

Rule 1: Grow first, until it gets too risky

When infection levels are low, frogs tend to prioritize growth. Growing quickly helps them to avoid predators and reach sexual maturity. As infection levels rise, that strategy shifts. Frogs begin to invest more energy in immune defense, even though it slows their growth. In other words, they tolerate infection up to a point, but once it becomes life-threatening, fighting disease takes priority.

Rule 2: Infection has hidden costs

Our results showed that infected frogs grow more slowly and take longer to reach maturity. These delays reduce survival and lifetime fertility.

This result from our model helped explain the pattern we observed in the wild: Infected frogs are often smaller than healthy ones. Infection does not just determine whether frogs live or die; it also shapes how they grow and develop.

Rule 3: Timing is everything

The time of year when frogs hatch strongly affects survival. In tropical environments, food availability and infection risk vary throughout the year. Frogs born at the start of the warm season, around May, when food is more abundant, grew faster, reached maturity sooner and survived at higher rates. Frogs born under less favorable conditions faced a slower start and less chance of survival.

Why these trade-offs matter

Our findings show that the effects of pathogens go beyond immediate death. Even nonlethal infections can influence growth, development and future reproduction — hidden costs that shape population recovery.

Besides helping us to learn more about coquí frogs in the wild, understanding these trade-offs can also guide conservation. For example, captive breeding programs often release frogs into the wild, and timing releases with favorable environmental conditions could improve their survival chances. On the other hand, our models also identify times when these frogs are more vulnerable to invasive species control measures.

More broadly, this approach helps researchers predict how animals will respond to environmental change, disease outbreaks and shifting climates.

The next time you hear a frog calling at night, remember that its survival depended on a series of invisible “decisions” early in life. These moves are not shaped by conscious choice, but by evolution, timing and the challenges of a changing world.