Ultraviolet (UV) light is a segment of the electromagnetic spectrum invisible to the human eye. When certain animals are exposed to UV light, they appear to emit light. This display, often described as a “glow,” reveals a hidden world of natural illumination.
Understanding the Glow
The “glow” observed in animals under UV light is due to fluorescence. This process occurs when a substance absorbs high-energy light, such as UV radiation, and then immediately re-emits it as lower-energy, visible light. Absorbed UV photons excite electrons within fluorescent molecules; as these electrons return to their ground state, they release energy as visible light, which our eyes can detect. This re-emission is instantaneous, meaning the glow stops as soon as the UV light source is removed.
Fluorescence differs from other forms of biological light production. Bioluminescence involves a chemical reaction within an organism that generates light, as seen in fireflies. Phosphorescence, another light-emitting process, differs because its emitted light persists after the exciting light source is removed, due to a slower energy release.
Animal Examples That Fluoresce
Many animals across various taxonomic groups exhibit fluorescence under UV light. Among arachnids, scorpions are a classic example; their exoskeletons glow brightly, often blue or green, due to fluorescent compounds in their cuticle. This fluorescence continues even in fossilized specimens.
Amphibians also show fluorescence, including several species of frogs and salamanders. The South American polka-dot tree frog (Boana punctata) fluoresces blue and green due to compounds in its skin and lymph. Certain chameleons, such as the panther chameleon, possess bones that fluoresce under UV light, with the glow visible through their translucent skin. Some sea turtles, including the hawksbill, fluoresce in vibrant red and green, possibly due to fluorescent proteins in their shells and soft tissues.
Mammals can also exhibit fluorescence, a discovery relatively recent for many species. The platypus, a unique Australian mammal, has fur that fluoresces blue-green under UV light. Similar fluorescent properties are found in the fur of flying squirrels, which glow pink under UV, and opossums, whose fur also shows a pinkish hue.
In the marine environment, many invertebrates like corals and jellyfish are known for their fluorescence, often attributed to specific fluorescent proteins. Some fish species, including certain gobies and blennies, also display fluorescent patterns on their skin. Even some insects, like the wings of certain moths and butterflies, contain fluorescent pigments that become visible under UV light.
The Purpose of Fluorescence in Animals
While fluorescence is well-documented in many animal species, its precise biological purpose remains an area of ongoing scientific investigation. One hypothesis suggests that fluorescence plays a role in communication, potentially serving as a signal for attracting mates or deterring rivals. The specific patterns or colors of fluorescence could convey information about an individual’s health or genetic fitness.
Another theory proposes that fluorescence aids in camouflage or counter-illumination, helping animals blend into their environment, particularly in habitats where UV light penetrates, such as coral reefs. Conversely, it might also serve as a form of predator avoidance, acting as a warning signal to potential threats or making the animal appear less appealing. Some researchers speculate that fluorescence could help certain animals lure prey by creating a visible signal in otherwise dark or murky conditions.
Fluorescence might also offer a form of UV protection, where harmful UV light is absorbed and re-emitted as less damaging visible light, thereby safeguarding sensitive tissues. For some species, the ability to fluoresce or perceive fluorescent light could enhance their sensory capabilities, allowing them to navigate or detect objects in low-light environments. The functional significance of fluorescence likely varies considerably among different species and their respective ecological niches.