The blind mole rat (Spalax ehrenbergi) is a short-legged, mole-like rodent that lives entirely underground. Presumably as a result of this subterranean lifestyle, its eyes are atrophied and covered over by skin. The retina contains very few ganglion cells, and brain structures involved in image processing are either absent or dysfunctional. However, it does have functional suprachiasmatic nuclei, the site of the primary biological clock in mammals. On page 655, Russell Foster and colleagues (University College London) show that these 'blind' mammals actually sense light and regulate their body clocks accordingly.

The authors isolated a functional cone-like pigment from the degenerate eye of the blind mole rat, which they show entrains circadian rhythms to light. In the laboratory, upon entrainment to a 12:12 light/dark cycle, the majority of animals show greater periods of spontaneous locomotor activity (running on a wheel) during the subjective day. When the animals are then given a 15-minute light flash, they entrain their activity to this single stimulus. Most animals then become active before the pulse, almost as though they were anticipating 'dawn'. When the eyes are removed, this photoentrainment does not occur.

In the wild, blind mole rats are solitary and highly territorial. They dig extensive burrows with separate nesting, storage and defecation sites. During the breeding season, females build 'breeding' mounds, surrounded by smaller mounds connected via tunnels that the males occupy. Their habits raise a question: how would they be exposed to light outside the laboratory? The best guess is that this happens during mound building when they clear debris from their tunnels.