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Subterranean mammals seeing or not seeing

About mammals > Adaptations for subterranean life

Sensory perception plays a pivotal role not only in spatial and temporal orientation, foraging, and recognition of food, but also in communication with conspecifics. Like their surface dwelling counterparts, subterranean animals also must find and recognize a mate, recognize kin or intruders, and be warned of danger.

This all is very difficult in a monotonous, dark world where transmission of most signals and cues is very limited. Some senses such as sight are apparently useless, whereas others have to compensate for their loss. One of the most prominent features of subterranean mammals is, no doubt, reduction of eyes and apparent blindness. The question of whether and what subterranean mammals see has been studied by Aristotle, Buffon, Geoffroy, Cuvier, and Darwin, among others. Still, today, no general unambiguous reply can be given.

Thus, for instance, in comparing two of the most specialized subterranean rodents, the east Mediterranean blind mole-rat (Spalax) and the African mole-rat (Cryptomys), both are strictly confined to the underground and are of similar appearance, externally. They both have very small eyes and they both are behaviorally blind, yet whereas the eyes of the blind mole-rat are structurally degenerated and lie under the skin, the eyes of the African molerat are prominent, only miniaturized but in no respect degenerate; on the contrary, they are fully normally developed.

As has been shown by several research groups, they also use different kinds of cone-pigments. Whereas the degenerated subcutaneous eye of the blind mole-rat has apparently adapted to a function in circadian photoreception, the function of a normally developed eye of the African molerat remains enigmatic.

Interestingly, whereas spalacines and bathyergids in the Old World lost their sight and have become completely subterranean, their New World counterparts, geomyids and octodontids, converged to similar habitats and habits retaining their eyes and sight. Unfortunately, the eyes and the sight of most subterranean mammals have not yet been studied.

As do blind people, blind subterranean mammals compensate for loss of sight by well-developed somatosensory perception, which was shown in the blind mole-rat, in the star-nosed mole, and in the naked mole. This somatosensory perception is over-represented also in the brain cortex where it occupies areas dominated in seeing mammals by visual projections.

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