Lizards are a diverse group of reptiles found globally, serving an important ecological function as both insectivores and prey. Their survival is a complex balancing act influenced by numerous environmental, biological, and human-related factors. Understanding the various causes of mortality for these ectotherms provides a clearer picture of the threats they face. Causes of death range from natural predators to chemical toxicity and are often interconnected.
Natural Predators and Disease
Lizards face a constant threat from a wide range of natural predators. The most formidable aerial threats are birds of prey, such as hawks, falcons, and owls, which strike from above with great efficiency. On the ground, snakes are a primary danger; both venomous species and non-venomous constrictors consume lizards, sometimes swallowing prey far larger than their own heads.
Predation pressure also comes from smaller, less obvious sources, including larger invertebrate species. Scorpions and carnivorous ants may prey on small or hatchling lizards, and adults are often cannibalized by larger members of their own species. Internal biological threats compromise a lizard’s health and make it an easy target. A heavy parasite load from mites, ticks, or internal organisms like roundworms can cause anemia and severe debilitation, reducing the ability to flee or forage.
Viral and bacterial infections are also significant killers, especially in populations under environmental stress. Septicemia, a bacterial infection in the blood, is a common cause of death resulting from a minor injury or trauma. Viruses like Flavivirus, transmitted by infected insects, may lead to liver disease and neurological issues. Other pathogens like Adenovirus cause severe weight loss and internal lesions. Protozoans, such as Entamoeba invadens, are particularly pathogenic and can spread rapidly, causing severe digestive distress.
Environmental and Climate Extremes
As ectotherms, lizards are extremely sensitive to fluctuations in environmental temperature, making them highly vulnerable to climate extremes. Their bodies rely on external heat sources to regulate metabolism, setting a hard limit on their thermal tolerance. Extreme heat can quickly push a lizard past its Critical Thermal Maximum, leading to fatal overheating and dehydration, which is especially dangerous for desert species.
The most vulnerable life stage is often the embryo, which is immobile and cannot escape a lethal temperature spike. Eggs developing in nests buried in the soil can die if exposed to 110°F (43.3°C) for even a short period. Conversely, sudden or prolonged extreme cold can be equally devastating, causing the body temperature to drop below its Critical Thermal Minimum, resulting in a loss of locomotor function and freezing.
Resource scarcity, often driven by habitat loss, functions as a major environmental mortality factor. Deforestation removes the forest canopy, eliminating the crucial shade and cooler microclimates lizards rely on for thermoregulation. Without these thermal refuges, lizards must reduce active foraging time to avoid overheating, leading to starvation and reduced reproductive success. The loss of vegetation also directly removes the insect populations that form the base of the lizard’s diet.
Accidents Involving Human Activity
Human presence introduces a unique set of physical dangers. One of the most significant causes of accidental death is the domestic cat, an introduced predator. Free-roaming house cats and feral populations kill small reptiles globally, and a single pet cat can locally exterminate a lizard population in a suburban area. Dogs also contribute to this mortality, though to a lesser extent than cats, which are highly efficient and opportunistic hunters.
Accidents involving human infrastructure are another major mortality source, notably road mortality. Lizards are often struck by vehicles when basking on warm asphalt or moving across roadways. Beyond vehicles, common yard equipment poses an unseen threat; lizards concealed in long grass, such as the slow-moving blue-tongue lizard, are frequently killed by lawnmowers or string trimmers.
Lizards also risk being trapped or injured by household and garden fixtures. They can become stuck behind appliances, fall into uncovered window wells, or be crushed by movable objects in a yard. Intentional killing by humans who perceive lizards as pests also contributes to mortality in some regions, though this is distinct from the unintentional deaths caused by infrastructure and introduced pets.
Chemical Exposure and Secondary Poisoning
Chemical agents introduced into the environment by humans pose a toxic threat to lizards. Primary poisoning occurs when a lizard directly consumes a toxic substance intended for another pest. For example, some species, like the blue-tongue lizard, consume snail or slug bait containing molluscicides, leading to internal bleeding and death. Ingestion of concentrated pesticides applied to gardens or crops remains a direct danger, even if the acute toxicity of some rodenticides is low for certain species.
Secondary poisoning is a more widespread threat, occurring when the lizard consumes prey that has already ingested a toxic substance. Insectivorous lizards risk accumulating poison by eating insects exposed to insecticides. This is particularly problematic with anticoagulant rodenticides, which are designed to slowly kill rodents over several days.
A lizard consuming a poisoned rodent or a large number of poisoned invertebrates can receive a lethal dose. Compounds like brodifacoum accumulate in the lizard’s tissues, disrupting blood clotting and causing internal hemorrhaging. This exposure pathway is difficult to monitor and diagnose but significantly contributes to the decline of lizard populations near human residential and agricultural areas.