Sensory ecology of electromagnetic radiation perception in subterranean mole-rats
Subterranean living animals must handle orientation in their habitat with limited cues, among which is light scarcity. Zambian mole-rats belong to the rodent genus Fukomys and spend the majority of their lifetime underground in extensive burrow systems. These mole-rats have generally been considered as functionally blind, but recent morphological findings have suggested that their visual capabilities must have been underestimated. The odds of cue scarcity underground have also apparently led to the use of a sensory system for orientation quite uncommon in mammals: magnetoreception. This thesis deals with new findings on both the visual and the magnetic sense in small Zambian mole-rat species of the Genus Fukomys – two senses coupled to the minuscule, inconspicuous eye. While the retina provides the basis for light perception, the cornea is probably the site where magnetic perception takes place. Firstly, I show in part A that the formerly thought ‘blind’ Fukomys mole-rats can distinguish between light and dark even until at least a light intensity of 0.6 -mol photons m−2 s −1 × × (approximately 33 lux). This thesis has also undergone the first approach to measure wavelength propagation in a tunnel, showing that long wavelengths (600-700 nm) travel, as expected, furthest in a horizontal tunnel, and that photons in this spectral range can still be detected, though at a very low (scotopic) level, at 70 cm apart from a tunnel opening illuminated with a light intensity resembling that on a clear day. Secondly, in part B, I also show that the hitherto poorly understood transduction mechanism of the magnetic sense in mole-rats is based on magnetite rather than on biochemical processes. The site of the respective magnetite-harbouring receptors can be confined to the ocular region, more specifically the cornea, where ferrous inclusions with magnetic properties might be coupled with the receptors for mediating magnetic information. The magnetic sense is not lateralised. This thesis also contributes to a further understanding of the neuronal processing of magnetic information in mammals, finding that hippocampal structures, e.g. structures coordinating spatial memories, are also involved in magnetic orientation. Magnetic cues might supply the animal with both directional compass and map information.
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