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

Abstract

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 blind^2–4. But structural and molecular investigations of the atrophied, subcutaneous eye suggest a functional role for the retina in light perception^5,6, and entrainment of circadian locomotor and thermoregulatory rhythms by ambient light demonstrates a capacity for photoperiodic detection^2,7–9. 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 processes^10–12, the differential reduction and expansion of visual structures in Spalax can be explained as an adaptive response to the underground environment.