Sweating is a mechanism of evaporative cooling requiring specialized eccrine glands to secrete fluid onto the skin. Lizards and the vast majority of reptiles do not possess these specialized sweat glands. Since lizards are ectotherms, they cannot cool down via sweating when their body temperature rises. Instead, they rely on behavioral and physiological adjustments to manage body heat and prevent overheating.
Why Lizard Skin Lacks Sweat Glands
The scaly skin of a lizard is primarily designed for water retention, making the mechanism of sweating counterproductive to its survival. Reptilian skin is highly keratinized, meaning it has a tough, protective outer layer composed of the protein keratin. This layer forms an effective, nearly impermeable barrier against water loss, which is a necessity for animals living in arid environments.
This watertight barrier is the opposite of the porous, glandular skin needed for evaporative cooling. The specialized eccrine and apocrine glands that mammals use for cooling are absent in the lizard integument. The scales themselves contribute to water retention, creating a dry outer surface resistant to water movement.
Behavioral Strategies for Temperature Control
Lizards manage their body temperature primarily by controlling their location and posture, a process known as behavioral thermoregulation. Because lizards are ectotherms, they rely on external heat sources to reach their optimal body temperature.
A common strategy is basking, known as heliothermy, where the lizard absorbs heat directly from solar radiation. They will orient their bodies perpendicular to the sun’s rays and flatten themselves to maximize the surface area exposed to the sun.
Conversely, to cool down or avoid overheating, they move into shaded areas, burrows, or under rocks, a strategy called thigmothermy. This involves gaining or losing heat through conduction by direct contact with a substrate. Lizards also use postural adjustments, such as lifting their bodies off a hot surface, to reduce heat gain. This constant shuttling between warm and cool patches allows them to maintain a stable body temperature throughout the day.
Internal Physiological Cooling Mechanisms
When behavioral strategies are no longer sufficient to lower body temperature, lizards activate internal physiological responses. The most effective of these is evaporative cooling through the respiratory system, known as thermal panting or gular fluttering.
During panting, the lizard opens its mouth and rapidly vibrates the gular area (throat), which increases airflow over the moist mucous membranes. The evaporation of water from these surfaces dissipates excess heat from the body core. This mechanism can be effective, with some lizard species able to depress their body temperature by 2–3°C below the ambient air temperature.
Lizards also utilize their circulatory system to regulate heat transfer between the core and the surface. When a lizard is too warm, blood vessels near the skin dilate (vasodilation), increasing blood flow to the surface to dump heat into the environment. Conversely, they constrict these vessels (vasoconstriction) to reduce heat loss when cold. Panting, while costly in terms of water loss, is a necessary defense against reaching the critical thermal maximum, the lethal upper temperature limit.