The neural substrate subserving magnetoreception and magnetic orientation in mammals is largely unknown. Previous experiments have demonstrated that the processing of magnetic sensory information takes place in the superior colliculus. Here, the effects of magnetic field conditions on neuronal activity in the rodent navigation circuit were assessed by quantifying c-Fos expression. Ansell's mole-rats (Fukomys anselli), a mammalian model to study the mechanisms of magnetic compass orientation, were subjected to natural, periodically changing, and shielded magnetic fields while exploring an unfamiliar circular arena. In the undisturbed local geomagnetic field, the exploration of the novel environment and/or nesting behaviour induced c-Fos expression throughout the head direction system and the entorhinal-hippocampal spatial representation system. This induction was significantly suppressed by exposure to periodically changing and/or shielded magnetic fields; discrete decreases in c-Fos were seen in the dorsal tegmental nucleus, the anterodorsal and the laterodorsal thalamic nuclei, the postsubiculum, the retrosplenial and entorhinal cortices, and the hippocampus. Moreover, in inactive animals, magnetic field intensity manipulation suppressed c-Fos expression in the CA1 and CA3 fields of the hippocampus and the dorsal subiculum, but induced expression in the polymorph layer of the dentate gyrus. These findings suggest that key constituents of the rodent navigation circuit contain populations of neurons responsive to magnetic stimuli. Thus, magnetic information may be integrated with multimodal sensory and motor information into a common spatial representation of allocentric space within this circuit.

About 300 species of mammals have adapted to the dark underground ecotope. Despite a long history of underground existence, many strictly subterranean species have retained structurally normal eyes possessing the capability of image‐forming vision. Moreover, their retinae often feature high cone proportions, an indication of conserved photopic (daylight) vision. Although it has been suggested that low acuity vision plays an important role in predator avoidance, not a single attempt to measure light conditions in burrows has been made so far. Here, we report the first measurements of light propagation in an illuminated artificial tunnel and in experimentally opened burrows of Ansell's mole‐rat, Fukomys anselli in its natural habitat. Only about 0.2–2.5% of the ambient visible light entered the opened burrow. Light intensity attenuated quickly and reached mesopic light levels (at which both cones and rods contribute to vision) within a few centimetres from the burrow opening; scotopic light levels (at which only rods operate) were estimated to be reached at one to a few metres from the opening. Thus, although cones may hypothetically contribute to vision for up to a few metres, they play an indispensable role only in the immediate vicinity of a breach, where rods are fully saturated. Rod‐mediated light sensation in straight tunnels seems to be possible over distances much longer than 100 m, implying that it is the burrow architecture (tortuosity and branching) what limits light sensation under natural conditions. These findings clearly show that light propagating within a breached burrow may serve as a reliable cue providing information about the site of potential predation risk. Both rods and cones contribute to this signalling. The fact that blue light propagated much less efficiently than longer wavelength light suggests that the short‐wave‐sensitive opsin dominance in the African mole‐rats represents a non‐adaptive feature that seems to be associated with arrested cone development.

No abstract

Several groups of mammals use the Earth's magnetic field for orientation, but their magnetosensory organ remains unknown. The Ansell's mole-rat ( Fukomys anselli , Bathyergidae, Rodentia) is a microphthalmic subterranean rodent with innate magnetic orientation behaviour. Previous studies on this species proposed that its magnetoreceptors are located in the eye. To test this hypothesis, we assessed magnetic orientation in mole-rats after the surgical removal of their eyes compared to untreated controls. Initially, we demonstrate that this enucleation does not lead to changes in routine behaviours, including locomotion, feeding and socializing. We then studied magnetic compass orientation by employing a well-established nest-building assay under four magnetic field alignments. In line with previous studies, control animals exhibited a significant preference to build nests in magnetic southeast. By contrast, enucleated mole-rats built nests in random magnetic orientations, suggesting an impairment of their magnetic sense. The results provide robust support for the hypothesis that mole-rats perceive magnetic fields with their minute eyes, probably relying on magnetite-based receptors in the cornea.