A lovak fehérségének egy nem várt előnye - A leginkább "bögölyálló" ló depolarizáló fehér szőrű

A fehér kültakarójú lovak gyakran szenvednek az ultraibolya napsugárzással szembeni nagy érzékenységükből fakadó rosszindulatú bőrrákban és látórendszeri betegségekben. Ráadásul a vadon élő fehér lovakat a ragadozók könnyebben elejtik, mert fehérségük miatt kevésbé tudnak rejtőzködni, mint sötétebb színű fajtársaik. Nagyobb sebezhetőségük ellenére a fehér lovakat az emberek évezredek óta nagy becsben tartják, éppen a természetbeni ritkaságuk miatt. TDK dolgozatomban megmutatom, hogy a bögölyök kevésbé vonzódnak a fehér lovakhoz, mint a sötét színűekhez. A bögölyök számos egészségügyi és gazdasági problémát okoznak az embereknek és állatoknak egyaránt, mivel a nőstényeik betegségek kórokozóit terjesztik, miközben a gerincesek vérét szívják. Azt is bizonyítom, hogy a bögölyök a vérszívásra alkalmas gazdaállatot részben az annak kültakarójáról visszavert poláros fény segítségével találják meg. A bögölyök főként fekete és barna kültakarójú lovakhoz való vonzódása a nemrégiben fölfedezett pozitív polarotaxisukkal, vagyis az erősen és vízszintesen poláros fényhez való vonzódásukkal magyarázható. Mivel a gazdaállat színe meghatározza a bögölyökre kifejtett vonzerejét is, ezáltal kihat a gazdaállat kórokozó terhelésére is. Habár kizárólag a bögölyök és lovak közti vizuális kölcsönhatást vizsgáltam, eredményeim érvényesek lehetnek a polarotaktikus bögölyök más gazdaállataira is, mivel a gazdaállatok kültakarójának fénypolarizáló sajátságai fizikailag azonosak, így nem fajfüggőek

Stripes disrupt odour attractiveness to biting horseflies : Battle between ammonia, CO2, and colour pattern for dominance in the sensory systems of host-seeking tabanids

As with mosquitoes, female tabanid flies search for mammalian hosts by visual and olfactory cues, because they require a blood meal before being able to produce and lay eggs. Polarotactic tabanid flies find striped or spotted patterns with intensity and/or polarisation modulation visually less attractive than homogeneous white, brown or black targets. Thus, this reduced optical attractiveness to tabanids can be one of the functions of striped or spotty coat patterns in ungulates. Ungulates emit CO2 via their breath, while ammonia originates from their decaying urine. As host-seeking female tabanids are strongly attracted to CO2 and ammonia, the question arises whether the poor visual attractiveness of stripes and spots to tabanids is or is not overcome by olfactory attractiveness. To answer this question we performed two field experiments in which the attractiveness to tabanid flies of homogeneous white, black and black-and-white striped three-dimensional targets (spheres and cylinders) and horse models provided with CO2 and ammonia was studied. Since tabanids are positively polarotactic, i.e. attracted to strongly and linearly polarised light, we measured the reflection–polarisation patterns of the test surfaces and demonstrated that these patterns were practically the same as those of real horses and zebras. We show here that striped targets are significantly less attractive to host-seeking female tabanids than homogeneous white or black targets, even when they emit tabanid-luring CO2 and ammonia. Although CO2 and ammonia increased the number of attracted tabanids, these chemicals did not overcome the weak visual attractiveness of stripes to host-seeking female tabanids. This result demonstrates the visual protection of striped coat patterns against attacks from blood-sucking dipterans, such as horseflies, known to transmit lethal diseases to ungulates

Accuracy of the hypothetical sky-polarimetric Viking navigation versus sky conditions: revealing solar elevations and cloudinesses favourable for this navigation method.

According to Thorkild Ramskou's theory proposed in 1967, under overcast and foggy skies, Viking seafarers might have used skylight polarization analysed with special crystals called sunstones to determine the position of the invisible Sun. After finding the occluded Sun with sunstones, its elevation angle had to be measured and its shadow had to be projected onto the horizontal surface of a sun compass. According to Ramskou's theory, these sunstones might have been birefringent calcite or dichroic cordierite or tourmaline crystals working as polarizers. It has frequently been claimed that this method might have been suitable for navigation even in cloudy weather. This hypothesis has been accepted and frequently cited for decades without any experimental support. In this work, we determined the accuracy of this hypothetical sky-polarimetric Viking navigation for 1080 different sky situations characterized by solar elevationθand cloudinessρ, the sky polarization patterns of which were measured by full-sky imaging polarimetry. We used the earlier measured uncertainty functions of the navigation steps 1, 2 and 3 for calcite, cordierite and tourmaline sunstone crystals, respectively, and the newly measured uncertainty function of step 4 presented here. As a result, we revealed the meteorological conditions under which Vikings could have used this hypothetical navigation method. We determined the solar elevations at which the navigation uncertainties are minimal at summer solstice and spring equinox for all three sunstone types. On average, calcite sunstone ensures a more accurate sky-polarimetric navigation than tourmaline and cordierite. However, in some special cases (generally at 35° ≤ θ ≤ 40°, 1 okta ≤ ρ ≤ 6 oktas for summer solstice, and at 20° ≤ θ ≤ 25°, 0 okta ≤ ρ ≤ 4 oktas for spring equinox), the use of tourmaline and cordierite results in smaller navigation uncertainties than that of calcite. Generally, under clear or less cloudy skies, the sky-polarimetric navigation is more accurate, but at low solar elevations its accuracy remains relatively large even at high cloudiness. For a givenρ, the absolute value of averaged peak North uncertainties dramatically decreases with increasingθuntil the sign (±) change of these uncertainties. For a givenθ, this absolute value can either decrease or increase with increasingρ. The most advantageous sky situations for this navigation method are at summer solstice when the solar elevation and cloudiness are 35° ≤ θ ≤ 40° and 2 oktas ≤ ρ ≤ 3 oktas.</jats:p

Adjustment errors of sunstones in the first step of sky-polarimetric Viking navigation: Studies with dichroic cordierite/tourmaline and birefringent calcite crystals.

According to an old but still unproven theory, Viking navigators analysed the skylight polarization with dichroic cordierite or tourmaline, or birefringent calcite sunstones in cloudy/foggy weather. Combining these sunstones with their sun-dial, they could determine the position of the occluded sun, from which the geographical northern direction could be guessed. In psychophysical laboratory experiments, we studied the accuracy of the first step of this sky-polarimetric Viking navigation. We measured the adjustment error e of rotatable cordierite, tourmaline and calcite crystals when the task was to determine the direction of polarization of white light as a function of the degree of linear polarization p. From the obtained error functions e(p), the thresholds p* above which the first step can still function (i.e. when the intensity change seen through the rotating analyser can be sensed) were derived. Cordierite is about twice as reliable as tourmaline. Calcite sunstones have smaller adjustment errors if the navigator looks for that orientation of the crystal where the intensity difference between the two spots seen in the crystal is maximal, rather than minimal. For higher p (greater than p(crit)) of incident light, the adjustment errors of calcite are larger than those of the dichroic cordierite (p(crit)=20%) and tourmaline (p(crit)=45%), while for lower p (less than p(crit)) calcite usually has lower adjustment errors than dichroic sunstones. We showed that real calcite crystals are not as ideal sunstones as it was believed earlier, because they usually contain scratches, impurities and crystal defects which increase considerably their adjustment errors. Thus, cordierite and tourmaline can also be at least as good sunstones as calcite. Using the psychophysical e(p) functions and the patterns of the degree of skylight polarization measured by full-sky imaging polarimetry, we computed how accurately the northern direction can be determined with the use of the Viking sun-dial under 10 different sky conditions at 61° latitude, which was one of the main Viking sailing routes. According to our expermiments, under clear skies, using calcite or cordierite or tourmaline sunstones, Viking sailors could navigate with net orientation errors [Formula: see text]. Under overcast conditions, their net navigation error depends on the sunstone type: [Formula: see text] , [Formula: see text] and [Formula: see text]