The European mole (Talpa europaea)
The European mole (Talpa europaea) is a mammal of the order Eulipotyphia. It feeds on earthworms, centipedes and even some mice and shrews that are caught opportunistically in long tunnel systems. The mole has a cylindrical body and is 11 to 16 cm long, weighing 70 to 130 g. Females are typically smaller than males. The fur is usually dark but can be any colour as there is no disadvantage in having off-coloured fur. Moles are energetic diggers and can create up to 200m of tunnel shifting 540 times their own body weight of earth per day. Typically they work in patterns of 4-hour shift cycles. This means 4 hours working, 4 hours sleeping; all day every day. So, within a 24-hour period, they will work 3 shifts of 4 hours respectively.
Moles are not normally exposed to cold temperatures so the fur is not thick. Also they do not have any need for 360 degree threat warning. As a consequence the fur of moles is very simple with only two types, underfur and guard hairs that fold over and protect the coat during digging. Both have internal banding. The underfur hairs are helical or spiral in habit with an average of 4 rotations and has spines that lock the hairs together to form a spring mattress effect.
An artists impression of a single mole hair is shown below but no attempt has been made to depict the helical nature. For the infrared analysis the main features have been illustrated. The band spacing oscillates and chirps to set up interference disturbances to stimulate photon emission. The cross-section of the hair is elliptical and becomes flatter near the tip.
When the band spacing is measured there is some variation from hair to hair but the pattern follows a consistent template of oscillations superimposed on a chirp over the range from 10 to 6 microns. Here are two examples:
Mole hair follows a pattern of oscillations based on 2nd order Bragg interference with abrupt transitions to diffract the photon to the escape angle. For 2nd order transmission the band spacing relationship with wavelength is
d = 0.75λ/n
where d is the band spacing, λ the wavelength and n the effective refractive index. The abrupt transition shown below is typical of mole hair and this example shows a disturbance for ejecting a photon of approximately 15.4 um. We can only estimate the wavelength because we do not know the refractive index accurately. The hair diameter is a lot less than the tuned wavelength and much of the energy is outside of the keratin so a value of 1.35 is assumed for this example compared to pure keratin – 1.58.
Detailed measurements were taken to establish the statistical validity of the assumptions regarding the transitions. 13 hairs (5 dorsel and 8 ventral) were mapped in detail with 83 recorded transitions averaging 1.215 and 0.808 (standard deviation 0.05 and 0.07 respectively). In addition 21 abrupt transitions were photographically analysed (as above) yielding values of 1.226 and 0.793. The mean transitions of 0.80 and 1.22 abound in hair anatomy and probably result in the diffraction of photons to the escape angle.
Taking into account all the mole data we can calculate the tuned wavelength of the European mole to be in the range of 13.5 to 17 microns assuming a effective refractive index of 1.30 (following a value derived from mouse data).
Is the cuticle pattern also infrared engineering?
There is a general correlation of the cuticle dimensions with the banding in the animals studied so far. There is also a wave-like pattern to the cuticle in three phases which “looks” optical. The pattern rotates in most samples and becomes very difficult to measure. In the mole one of the phases is enlarged into a spine so is relatively easy to measure. The hair of moles does not appear to contain air voids and has very clear banding making it an ideal subject to perform detailed characterisation.
The spines are not locked to the bands but roughly follow the pattern with an average ratio of 3.6. Assuming a 3 phase arrangement the ratio of spines to bands averages 1.2 so with an effective refractive index of 1.2 the internal photon wavelength matches that of the free space wavelength. It is probably not as simple as this though. Any infrared adaptation in the cuticle pattern still needs to be proven but it may be a way of controlling the impedance of the surface or controlling the radiation lobe.