Lizards, like all reptiles, are ectothermic, meaning they rely on external sources to regulate their body temperature. This makes cold weather a significant challenge, as their internal temperature fluctuates with surroundings. Unlike mammals, which generate their own body heat, lizards must actively seek out warmth or coolness to maintain optimal body temperature for physiological processes.
Daily Temperature Management
Lizards employ various behavioral strategies to manage their body temperature daily. They bask in the sun to absorb heat and raise their internal temperature. Basking is crucial for activities like digestion, which requires specific warmth. Smaller, younger lizards may heat up and cool down faster, leading them to bask more frequently but for shorter durations.
When temperatures become too high, lizards seek shade or retreat into burrows or crevices to cool down. They can also absorb heat from warm surfaces like rocks through conduction. This continuous shuttle between warm and cool microclimates allows them to maintain their preferred temperature zone, which typically ranges from 75°F (24°C) to 95°F (35°C) during the day, though exact ranges vary by species.
Winter Survival: Brumation
During extended cold periods, many lizard species enter brumation, a state of dormancy akin to hibernation. This strategy is triggered by environmental cues like decreasing temperatures, shorter daylight hours, and sometimes food scarcity. During brumation, a lizard’s metabolic processes slow significantly, conserving energy when external conditions are unfavorable.
Lizards prepare for brumation by seeking sheltered locations, such as burrows, rock crevices, or under leaf litter, which provide insulation. While dormant, they typically do not eat, drink, or defecate for weeks or months, though some may occasionally stir to drink water. Brumation can last from a few weeks to over four months, depending on the species and environmental conditions.
Extraordinary Cold Tolerance
While most lizards are vulnerable to cold, some species have evolved remarkable physiological adaptations to tolerate near or below-freezing temperatures. One adaptation is supercooling, where the lizard’s body temperature drops below the freezing point of water without ice crystals forming within its tissues. This is achieved by preventing ice nucleation, the process by which ice crystals begin to form.
Another mechanism involves producing cryoprotectants, natural “antifreeze” compounds like glucose or glycerol. These substances increase solute concentration within cells, lowering the freezing point of bodily fluids and protecting cell membranes from damage if ice forms in extracellular spaces. For example, the European common lizard (Lacerta vivipara) can tolerate freezing of up to 50% of its total body water and survive exposures to -4°C for at least 24 hours, increasing blood glucose levels during freezing. The Pygmy Short-Horned Lizard burrows into sand and soil to endure freezing during winter months.
When Cold Becomes Lethal
Despite their adaptations, there are limits to a lizard’s cold tolerance. When temperatures drop too low or cold periods are too prolonged, their survival mechanisms can be overwhelmed. A significant danger is ice crystal formation within bodily fluids, which can cause severe tissue and organ damage.
Lizards may become lethargic and immobile when their body temperature falls below their critical thermal minimum, often in the 30s and 40s Fahrenheit (around 0-4°C), a state sometimes referred to as cold-stunning. For instance, iguanas in Florida can fall from trees when temperatures dip below 45°F (7°C) because they lose the ability to grip branches. Prolonged exposure to cold can lead to frostbite, particularly in extremities, and can compromise their immune system, making them susceptible to infections. If cold persists and core body temperature drops to lethal levels, the damage becomes irreversible.