Underwoodisaurus milii is a species of Gekkonidae also classified as Nephrurus milii. It is commonly known as the thick-tailed or barking gecko. These names come from its distinctive plump tail and sharp, barking defensive call. Thick-tailed geckos are reddish-brown with bands of white and yellow spots and paler underbelly. They usually grow to 120–140 mm in length.[1] Their original tail is black with several pale bands, however regenerated tails have little pattern. When threatened, they will arch their backs and "bark". Thick-tailed geckos are found in rocky outcrops across southern Australia, and are slightly more cold-tolerant than many other Australian gecko species. They are nocturnal, and shelter underneath rocks or in burrows during the day. They feed on insects and small vertebrates.Unusually for reptiles, this species forms aggregations in their retreat sites during the day. The reasons for this are unknown. However, it has been shown that this behavior results in a higher aggregate thermal inertia (they stay warmer) than would be found in solitary geckos of this and related kinds in similar circumstances In the same source, it was suggested that aggregating for physiological benefits may precede the development of other kinds of social behavior. This species, and some other species of gecko have the unusual habit of licking their eyes after eating, presumeably to keep the eyeshield moist and clean.