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Light Perception - Discussion
4 DISCUSSION
The first part of this thesis has presented new results on the visual behaviour of Zambian
Fukomys mole-rats, which show that the eye has seemingly taken over specialized functions in
these underground ‘blind’ rodents. Ethological studies of mole-rat vision, combined here with
a technical study of the optical conditions in a model tunnel, open wide discussion possibilities
in the following.
Demonstrating Light Perception
Our first study gives ethological support for the assumption made on the basis of the recent
morphological findings (Oelschläger et al. 2000; Cernuda-Cernuda et al. 2003; Němec et al.
2004; Peichl et al. 2004) that Fukomys mole-rats are capable to perceive light, that the retina
receives photic cues, and that this light-encoded information can be used to make a
meaningful decision. However, the real adaptive meaning of this ability is far from being clear.
For sure, Fukomys mole-rats do not flee away from light in panic (own observation). Under
our housing conditions, they sleep uncovered on the surface, though they would have the
possibility to transport the substrate to one corner of their cage and hide under it - a
behaviour blind mole-rats (Spalax) or moles (Talpa) would always exhibit (H. Burda, personal
communication). In the field, Zambian mole-rats appear quite rarely above ground (Scharff &
Grütjen 1997), probably e.g. when dispersing, foraging, or during flooding, and they do so also
in the daytime, demonstrating that their surface activity is not strictly nocturnal. They also do
not show any efforts to hide or look for shaded or dark objects (M. Kawalika, personal
communication). Surely these mole-rats do not need light information to know which
direction they should dig in order to hide. Both their vestibular organ and somatosensory
perception provide fast and reliable information on the directional matter as well as on
whether the animal is above ground or fully or only partly in a tunnel. Based on these
considerations, we speculate that the adaptive biological meaning of the observed capacity to
perceive light may lie rather in attentiveness to light than in perceiving and searching darkness.
This approach explains better the paradoxon between the visual system unsuited for above-
ground orientation (or: designed for underground orientation) and the photoreceptor mosaic
adapted to the perception of daylight intensities rather than to a dark environment (Němec et
al. 2007).
In many cases, incidence of light may well indicate a tunnel being opened by predators
and may thus warn the animal not to approach the opening too closely but to instead plug it
(own field observations). This plugging of illuminated tunnels was elicited also under
laboratory conditions in the pocket gopher (Thomomys spp.) (Werner et al. 2005). Note that
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