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. (C) 2013 Elsevier Inc. All rights reserved

Spottier Targets Are Less Attractive to Tabanid Flies: On the Tabanid-Repellency of Spotty Fur Patterns

During blood-sucking, female members of the family Tabanidae transmit pathogens of serious diseases and annoy their host animals so strongly that they cannot graze, thus the health of the hosts is drastically reduced. Consequently, a tabanid-resistant coat with appropriate brightness, colour and pattern is advantageous for the host. Spotty coats are widespread among mammals, especially in cattle (Bos primigenius). In field experiments we studied the influence of the size and number of spots on the attractiveness of test surfaces to tabanids that are attracted to linearly polarized light. We measured the reflection-polarization characteristics of living cattle, spotty cattle coats and the used test surfaces. We show here that the smaller and the more numerous the spots, the less attractive the target (host) is to tabanids. We demonstrate that the attractiveness of spotty patterns to tabanids is also reduced if the target exhibits spottiness only in the angle of polarization pattern, while being homogeneous grey with a constant high degree of polarization. Tabanid flies respond strongly to linearly polarized light, and we show that bright and dark parts of cattle coats reflect light with different degrees and angles of polarization that in combination with dark spots on a bright coat surface disrupt the attractiveness to tabanids. This could be one of the possible evolutionary benefits that explains why spotty coat patterns are so widespread in mammals, especially in ungulates, many species of which are tabanid hosts

Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes

The characteristic striped appearance of zebras has provoked much speculation about its function and why the pattern has evolved, but experimental evidence is scarce. Here, we demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents. Such biting flies are prevalent across Africa and have considerable fitness impact on potential mammalian hosts. Besides brightness, one of the likely mechanisms underlying this protection is the polarization of reflected light from the host animal. We show that the attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in horizontal or vertical homogeneous grey surfaces. Tabanids have been shown to respond strongly to linearly polarized light, and we demonstrate here that the light and dark stripes of a zebra's coat reflect very different polarizations of light in a way that disrupts the attractiveness to tabanids. We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids. Further, we demonstrate that the stripe widths of zebra coats fall in a range where the striped pattern is most disruptive to tabanids. The striped coat patterns of several other large mammals may also function in reducing exposure to tabanids by similar mechanisms of differential brightness and polarization of reflected light. This work provides an experimentally supported explanation for the underlying mechanism leading to the selective advantage of a black-and-white striped coat pattern

Cavemen Were Better at Depicting Quadruped Walking than Modern Artists: Erroneous Walking Illustrations in the Fine Arts from Prehistory to Today

<div><p>The experts of animal locomotion well know the characteristics of quadruped walking since the pioneering work of Eadweard Muybridge in the 1880s. Most of the quadrupeds advance their legs in the same lateral sequence when walking, and only the timing of their supporting feet differ more or less. How did this scientific knowledge influence the correctness of quadruped walking depictions in the fine arts? Did the proportion of erroneous quadruped walking illustrations relative to their total number (i.e. error rate) decrease after Muybridge? How correctly have cavemen (upper palaeolithic <em>Homo sapiens</em>) illustrated the walking of their quadruped prey in prehistoric times? The aim of this work is to answer these questions. We have analyzed 1000 prehistoric and modern artistic quadruped walking depictions and determined whether they are correct or not in respect of the limb attitudes presented, assuming that the other aspects of depictions used to determine the animals gait are illustrated correctly. The error rate of modern pre-Muybridgean quadruped walking illustrations was 83.5%, much more than the error rate of 73.3% of mere chance. It decreased to 57.9% after 1887, that is in the post-Muybridgean period. Most surprisingly, the prehistoric quadruped walking depictions had the lowest error rate of 46.2%. All these differences were statistically significant. Thus, cavemen were more keenly aware of the slower motion of their prey animals and illustrated quadruped walking more precisely than later artists.</p> </div

Spottier Targets Are Less Attractive to Tabanid Flies: On the Tabanid-Repellency of Spotty Fur Patterns

During blood-sucking, female members of the family Tabanidae transmit pathogens of serious diseases and annoy their host animals so strongly that they cannot graze, thus the health of the hosts is drastically reduced. Consequently, a tabanid-resistant coat with appropriate brightness, colour and pattern is advantageous for the host. Spotty coats are widespread among mammals, especially in cattle (Bos primigenius). In field experiments we studied the influence of the size and number of spots on the attractiveness of test surfaces to tabanids that are attracted to linearly polarized light. We measured the reflection-polarization characteristics of living cattle, spotty cattle coats and the used test surfaces. We show here that the smaller and the more numerous the spots, the less attractive the target (host) is to tabanids. We demonstrate that the attractiveness of spotty patterns to tabanids is also reduced if the target exhibits spottiness only in the angle of polarization pattern, while being homogeneous grey with a constant high degree of polarization. Tabanid flies respond strongly to linearly polarized light, and we show that bright and dark parts of cattle coats reflect light with different degrees and angles of polarization that in combination with dark spots on a bright coat surface disrupt the attractiveness to tabanids. This could be one of the possible evolutionary benefits that explains why spotty coat patterns are so widespread in mammals, especially in ungulates, many species of which are tabanid host

Statistical comparisons (binomial χ² test) between the numbers (<i>N</i>_incorrect) of incorrect quadruped walking illustrations (Table 1) to test differences between various ages (modern, post-Muybridgean, pre-Muybridgean, prehistoric), 2- and 3-dimensional depictions, and the random choice.

<p>For the random case the unity walking matrix has number 1 in its every cell (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049786#pone.0049786.s044" target="_blank">Table S9</a>). Then the numbers of correct and incorrect quadruped walking illustrations are: <i>N</i>_correct = 16, <i>N</i>_incorrect = 44, total <i>N</i> = <i>N</i>_correct+<i>N</i>_incorrect = 60, resulting in an error rate of <i>N</i>_incorrect/<i>N</i> = 73.3% corresponding with the pure accident.</p

A prehistoric elephant depiction.

<p><i>Left</i>: Prehistoric illustration of an elephant from the Libian Tadrart Acacus (<a href="http://www.galuzzi.it" target="_blank">http://www.galuzzi.it</a>, the permission from the photographer, Luca Galuzzi is found in the Supporting Online Material). <i>Right</i>: Contour of the elephant. Here there are three possibilities (A, B, C) for the alignment of the assumed ground line: In cases (A), (B) and (C) the walking depiction falls in cells <i>Be</i>, <i>Cf</i> and <i>Bf</i> of the walking matrix, respectively, all being incorrect.</p

Testing a polarimetric cloud imager aboard research vessel Polarstern: comparison of color-based and polarimetric cloud detection algorithms

Cloud cover estimation is an important part of routine meteorological observations. Cloudiness measurements are used in climate model evaluation, nowcasting solar radiation, parameterizing the fluctuations of sea surface insolation, and building energy transfer models of the atmosphere. Currently, the most widespread ground-based method to measure cloudiness is based on analyzing the unpolarized intensity and color distribution of the sky obtained by digital cameras. As a new approach, we propose that cloud detection can be aided by the additional use of skylight polarization measured by 180° field-of-view imaging polarimetry. In the fall of 2010, we tested such a novel polarimetric cloud detector aboard the research vessel Polarstern during expedition ANT-XXVII/1. One of our goals was to test the durability of the measurement hardware under the extreme conditions of a trans-Atlantic cruise. Here, we describe the instrument and compare the results of several different cloud detection algorithms, some conventional and some newly developed. We also discuss the weaknesses of our design and its possible improvements. The comparison with cloud detection algorithms developed for traditional nonpolarimetric full-sky imagers allowed us to evaluate the added value of polarimetric quantities. We found that (1) neural-network-based algorithms perform the best among the investigated schemes and (2) global information (the mean and variance of intensity), nonoptical information (e.g., sun-view geometry), and polarimetric information (e.g., the degree of polarization) improve the accuracy of cloud detection, albeit slightly

How could the Viking Sun compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment

Vikings routinely crossed the North Atlantic without amagnetic compass and left their mark on lands as far away as Greenland, Newfoundland and Baffin Island. Based on an eleventh-century dial fragment artefact, found at Uunartoq in Greenland, it is widely accepted that they sailed along chosen latitudes using primitive Sun compasses. Such instruments were tested on sea and proved to be efficient hand-held navigation tools, but the dimensions and incisions of the Uunartoq find are far from optimal in this role. On the basis of the sagasmentioning sunstones, incompatible hypotheses were formed for Viking solar navigation procedures and primitive skylight polarimetry with dichroic or birefringent crystals. We describe here a previously unconceived method of navigation based on the Uunartoq artefact functioning as a 'twilight board', which is a combination of a horizon board and a Sun compass optimized for use when the Sun is close to the horizon. We deduced an appropriate solar navigation procedure using a twilight board, a shadow-stick and birefringent crystals, which bring together earlier suggested methods in harmony and provide a true skylight compass function. This could have allowed Vikings to navigate around the clock, to use the artefact dial as a Sun compass during long parts of the day and to use skylight polarization patterns in the twilight period. In field tests, we found that true north could be appointed with such a medieval skylight compass with an error of about +/- 4 degrees when the artificially occluded Sun had elevation angles between +10 degrees and -8 degrees relative to the horizon. Our interpretation allows us to assign exact dates to the gnomonic lines on the artefact and outlines the schedule of the merchant ships that sustained the Viking colony in Greenland a millennium ago