Stress can be a serious problem for captive lizards, causing increased susceptibility to illness, anorexia, failure to thrive or breed and, in the worst cases, death. Symptoms vary among species, but include defensive behaviors such as biting, hissing or tail-whipping, closed or sunken eyes, darker than typical colors, and anorexia.
If you're unable to lower the lizard's stress within a few days, a trip to the veterinarian is in order.
Often, a lizard will become stressed because its captive environment is deficient in some area. Be sure the temperature range, lighting, substrate and cage size are appropriate for the species. In many cases, adjusting these parameters is all that's required to relieve the stress.
While there are exceptions, most lizards lead solitary lives and should be housed singly except for breeding trials. Cohabitants can monopolize resources, spread disease and subtly intimidate each other. The stress caused by cagemates can be lethal for some species, notably chameleons (Chamaeleo sp.), and should never be attempted. Bearded dragons (Pogona vitticeps), leopard geckos (Eublepharis macularius) and a few other species can be successfully kept in harem-like groups of one adult male and several adult females. In these cases, it's important to be alert for signs of stress and immediately separate stressed animals from the group.
To a naïve lizard, you are a predator. While some pet lizards will, over time, learn that you're not a threat, other species will always associate you with danger. Animals like Nile monitors (Varanus niloticus), chameleons and tokay geckos (Gekko gecko) are notorious for being aggressive despite the constant, prolonged attempts of their keeper. While some handling is acceptable with species like bearded dragons, ridge-tailed monitors (Varanus acanthurus) and dab lizards (Uromastyx sp.), moderation is the key. In any case, stressed lizards should be handled as little as possible.
Increase Visual Barriers
A number of lizard species that rely on locomotor escape, especially green basilisks (Basiliscus plumifrons), are known to run full-speed into the sides of the cage when startled. The only way to reliably maintain these lizards is by incorporating numerous visual barriers in the cage. By using plastic or real plants and other cage props, your lizard will feel more secure, and its stress level will be lower. Visual barriers help with other species too; chameleons and even bearded dragons will benefit from plants in their cage.
Provide Tight Hide Spots
While ubiquitous among snake keepers, hide spots are often neglected by lizard owners. Though a few species, like green anoles (Anolis carolinensis), will sleep on exposed branches in the wild, most lizards require a secure hide spot. Hide spots needn’t be elaborate, and can be made from paper towel tubes, cardboard boxes and plastic butter tubs. The important thing with hide spots is to remember that your lizard wants to curl up in it tightly; the container should barely contain the lizard. Arboreal species like crested geckos (Correlophus ciliates) will appreciate vertically oriented hides made of cardboard or egg-crate.