Ocular regression conceals adaptive progression of the visual system in a blind subterranean mammal

THE mole rat, Spalax ehrenberghi, is an extreme example of natural visual degeneration in mammals: visual pathways are regressed and incomplete1, and the absence of visual cortical potentials or an overt behavioural response to light have led to the conclusion that Spalax is completely blind2–4. But structural and molecular investigations of the atrophied, subcutaneous eye suggest a functional role for the retina in light perception5,6, and entrainment of circadian locomotor and thermoregulatory rhythms by ambient light demonstrates a capacity for photoperiodic detection2,7–9. We report here that severe regression of thalamic and tectal structures involved in form and motion perception is coupled to a selective hypertrophy of structures subserving photoperiodic functions. As an alternative to the prevalent view that ocular regression results from negative or nonselective evolutionary processes10–12, the differential reduction and expansion of visual structures in Spalax can be explained as an adaptive response to the underground environment.