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marks are not volatile; they may be difficult or impossible to locate using smell. Desert iguanas avoid this problem by combining a unique type of visual signal with their scent marks.

One striking property of desert iguana scent marks is that they strongly absorb ultraviolet light. Although these wavelengths are invisible to human eyes, they appear dark to animals able to see ultraviolet light -- much as ultraviolet-absorbing honey guides on flowers look black when UV-sensitive camera film is used to view them. Recent studies have shown that desert iguanas are able to see long-wave ultraviolet light, and they may use this adaptation to detect scent marks from a distance. After scent marks are localized using visual cues, desert iguanas can approach and investigate them in more detail through tongue-flicking. Although it is not known to occur in mammals, visual sensitivity to ultraviolet light has been shown in certain insects, spiders, fish, frogs, and birds. The ability of desert iguanas to detect ultraviolet light may help them solve some of the problems associated with finding scent marks in a desert environment.

In contrast to those of desert iguanas, the scent marks of green iguanas contain a variety of volatile chemical compounds, and they do not absorb ultraviolet light. Behavioral studies indicate that green iguanas, unlike desert iguanas, can detect these scent marks by smell alone. Because the chemical components of green iguana scent marks remain active and transmit well under the humid conditions of tropical forests, green iguanas do not appear to need a visual cue in order to locate scent marks. Research on both iguana species demonstrates how the environment in which animals live can influence the nature of the communication signals they employ.


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