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.