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Light Perception - Discussion
tuning of the shortwave sensitive cone type to the UV-light spectrum should be successively
examined, because it has been discussed in Peichl et al. (2004) that the Zambian mole-rat S-
opsin may be UV sensitive. This functional shift might be one explanation of the unusual
presence of short-wave sensitive opsins in mole-rats. In mammals, ultraviolet vision has been
found in e.g. bats (Winter et al. 2003) and in a number of rodents (Jacobs et al. 1991). Though
the UV sensitivity of the S-opsin is undoubtedly an ancestral mammalian condition (Hunt et
al. 2001), the adaptive meaning in those rodent groups that have retained it, e.g. whether its
formation has been driven by the use of territorial urine marks (Chávez et al. 2003), remains
an object of discussion.
The wavelengths propagated best, i.e. attenuated least in a tunnel, are green and red,
with red being detectable even at 70 cm distance from the light source. Though the amount of
detected red light was meagre, it was twice the photon catch in the blue or green range, and
well in the area of scotopic vision (see above). Thermal radiation, i.e. infrared radiation, was
always detected. This finding did also not, as to the low frequency of long wavelengths,
surprise; and it fits also well the findings by Sun et al. (2003), who describe that far-red and
near infra-red light might leak out of plant tissue. Here, a second explanation for the blue-light
sensitive cone opsin, though a highly speculative one, should be introduced. It is principally
imaginable that the blue cones are a kind of infrared-light detector for orientation along
longwave radiation within the burrow system. Physically, it would be possible that IR quanta,
which are, particularly at low flux rates, difficult to detect for a warm-blooded animal due to
thermal contamination, activate photoreceptors in a multi-stage quantal process that would
provide the necessary energy for excitation of the cis/trans transfer in the ‘blue’ visual
pigments (K. Götz, personal communication). Another option is the so-called “blue eyes”
effect, recently firstly presented at a conference (Kaernbach & Scheibelhofer 2006). The name
of the effect is deduced from the blue halos that occur on both sides of a red light spot
focused on in a dark room; the authors suggest this effect as being retinal. If both a transfer of
this human study to mole-rats and wild speculations are allowed, the rodent could see ‘blue’,
and that is even a symmetrical blue form, by viewing only a single red light spot. By moving
the head and perceiving red light from different spatial positions, the shape of the blue halos
could then yield information on intensity, direction, and distance of the light source.
In any case, whether mole-rats ‘see’ the (infra)red light or detect it by another sensory
system, it would be advantageous for an animal to recognize an open or collapsed tunnel as
soon as possible in order to plug or maintain it. As there are no air currents in an open tunnel
(see above), detecting the opening from a distance of already 60 to 70 cm would allow the
animal to quickly react and plug the tunnel system. Regarding the red light present in a tunnel,
a system sensitive enough to integrate even low photon shares of 0.02% of the total light
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