The feral cat population of GLMZ's Circuit District numbers approximately four thousand individuals, distributed across an estimated three hundred loosely territorial colonies. They are, in most respects, ordinary domestic cats — Felis catus, the same companion species that has cohabited with humans for ten thousand years. They hunt, they sleep, they fight, they breed. They are cats. But their eyes glow.

The bioluminescent modification originated with a cosmetic geneware product marketed in the early 2180s under the brand name "Starlight Eyes," manufactured by a now-defunct subsidiary of NovaPharma. The product was intended for domestic cats — a simple modification that caused the tapetum lucidum, the reflective layer behind the retina that gives cats their natural eyeshine, to emit a soft bioluminescence instead of merely reflecting ambient light. The product was popular for approximately three years before reports emerged that modified cats were passing the trait to offspring. NovaPharma quietly discontinued the product line. By then, enough modified cats had been abandoned or escaped to establish breeding populations throughout the Shelf.

Three color variants have been documented. The most common produces a green bioluminescence, visible in low light as a steady, soft glow emanating from the cat's eyes — not the brief flash of reflected light that all cats exhibit, but a sustained emission. The second variant, found in approximately 15% of the population, produces an amber glow. The third and rarest variant, found in less than 3% of observed individuals, produces a blue luminescence of striking intensity. Blue-eyed ferals are considered lucky by Circuit residents, and sighting one is treated as an auspicious event. The cats are indifferent to this designation.

The modification has had unintended functional consequences. The bioluminescent tapetum appears to have expanded the cats' visual sensitivity into near-ultraviolet wavelengths. Circuit ferals have been observed tracking prey — primarily augmented cockroaches — in conditions where no visible light is present, suggesting they are perceiving reflected UV or detecting the faint electromagnetic emissions of the cockroaches' metallic exoskeletons. They are seeing the world in spectra their ancestors could not access. They are, through an accident of cosmetic vanity and environmental contamination, better predators than any cat that has ever lived.

Most intriguingly, the Circuit ferals have developed vocalizations outside the normal feline range. Standard domestic cats vocalize between 25 Hz and 1,520 Hz. Circuit ferals have been recorded producing calls at frequencies up to 3,400 Hz, and infrasonic rumbles as low as 12 Hz. These vocalizations appear to serve communicative functions across distances that exceed normal feline social range — colony-to-colony signaling across multiple city blocks. The content of these communications, if they are communications, remains undeciphered. The cats are talking to each other. We do not know what they are saying.