When in Florida an unusually cold night arrives and the thermometer drops to around 4-5 °Cyou may come across an unexpected scene: rigid, motionless iguanas raining down from the trees. It is not a “freeze” in the proper sense of the term, nor a planned hibernation. It is the biological limit of a tropical animal that temporarily stops working when the body cools down too much: in practice, they are no longer able to move and simply lose their grip on the branches, falling. Cold waves don’t just produce this spectacular effect. Research shows that, after an extreme episode, iguanas survived they can tolerate lower temperatures than before. Within a few days the minimum threshold at which they can still move changes. Since their metabolism depends on temperature, in hot periods – when they are more active and feed more – the impact on the environment increases: they consume large quantities of vegetation and intensify excavations and movements, amplifying damage to ecosystems and infrastructure.
Florida authorities have issued an extraordinary order allowing citizens to collect green iguanas, which were stunned due to the low temperatures. The initiative led to the delivery of 5,195 specimens in just two days, which were later suppressed. The operation was deemed necessary because green iguanas are one non-native invasive speciesconsidered harmful to the local economy and ecosystem.
Iguanas don’t “freeze”, they slow down
We think of iguanas and lizards in general as “cold-blooded” animals, but this formula, although convenient, is not very precise. It’s not their blood that’s cold: it’s the way their body works. They do not maintain a constant internal temperature, like mammals. If the air warms up, they also warm up and become active; if the air cools, the metabolism drops and movements become slow. If the cold comes suddenly and drops too much, coordination fails: muscles and nervous system no longer respond effectively. In fact, the body of lizards and iguanas is below a minimum temperature threshold he can no longer control his movements. Some scholars calculated this minimum threshold on the night of January 22, 2020, when a Miamia minimum of 4.4 °C was reached, a rare figure for that area, as reported by the study An extreme cold event leads to community-wide convergence in lower temperature tolerance in a lizard community published on Biology Letters.
In the days following the event, the scientists evaluated the minimum temperature at which a lizard can still right itself when turned on its back (CTmin). The measurement is obtained gradually cooling the animal and observing the exact moment when loses the righting reflex. It’s a concrete way to figure out how much cold a reptile can handle before the body stops functioning effectively. Before the cold wave, the species studied showed average CT valuesmin ranging roughly between 8 and over 11 °C. Immediately after the event, the values detected were lower, around 6-7 °C. AND it was not a passing effect: even after ten weeks, the threshold remained reduced. After the cold, therefore, the surviving lizards were able to tolerate lower temperatures than before.
Iguanas falling from trees: death or temporary paralysis?
Iguanas spend the night motionless among branches and vegetation. During a sudden drop in nighttime temperature, their body may drop below the neuromuscular functioning threshold. They can no longer coordinate and they can lose their grip. This state is called cold-stunning: a cold shock that causes temporary immobility. It’s not freezing in the literal sense of the word (they don’t become blocks of ice), but a sort of paralysis from hypothermia.
In a scientific work it is reported that, after the 2020 event, both dead animals and “cold-stunned” individuals who later recovered were observed. Mortality can occur if cold exposure lasts too long or exceeds the animal’s physiological limits.
Natural selection or rapid adaptation?
The researchers of the Miami lizard study considered two possible explanationsi for lowering the CTmin:
- Natural selection: only the most cold-resistant individuals survive, so the average population becomes more tolerant.
- Physiological plasticity: Surviving individuals change temporarily their physiology to adapt to the cold, that is, they have the ability to change some characteristics without changing their DNA: like a flexible response to the environment.
The study However, it does not definitively conclude which of the two mechanisms prevailsbut it demonstrates that a single extreme climate event can cause measurable, synchronized change in multiple species at once
Invasive green iguanas: a problem beyond the cold
Green iguanas, Iguana iguana, did not exist in Florida until recently. They arrived from tropical regions of Central and South America and have begun to appear steadily starting from the 1960s. Over time they have expanded into many counties and today they are considered invasive: they do not belong to the local fauna and their presence represents a big problem.
A review published on Frontiers in Amphibian and Reptile Science in 2025 describes the expansion of green iguanas in Florida and their impact, especially the problems they cause on crops, agricultural production, electricity and even ornamental plants and how these extreme climatic events can affect their population. Green iguanas, in fact, are one invasive species in Florida: they dig burrows that weaken embankments and foundations, damage crops and can cause blackouts by climbing on electrical systems. They also consume numerous plant species, with effects on local ecosystems.
The cold, therefore, is not only a curious event, but can influence the dynamics of an invasive species that already has significant economic and environmental effects. Furthermore, the authors of the study point out that extreme climate events, such as heat or cold waves, are expected to increase in frequency and intensity with climate change. It seems like a paradox, but a warmer global climate does not eliminate the risk of episodes of localized intense cold. For the survival of tropical species ectotherms like iguanas, whose body temperature depends greatly on the external environment, these oscillations can be decisive.
