An unexpected advantage of whiteness in horses: the most horsefly-proof horse has a depolarizing white coat

Abstract

White horses frequently suffer from malign skin cancer and visual deficiencies owing to their high sensitivity to the ultraviolet solar radiation. Furthermore, in the wild, white horses suffer a larger predation risk than dark individuals because they can more easily be detected. In spite of their greater vulnerability, white horses have been highly appreciated for centuries owing to their natural rarity. Here, we show that blood-sucking tabanid flies, known to transmit disease agents to mammals, are less attracted to white than dark horses. We also demonstrate that tabanids use reflected polarized light from the coat as a signal to find a host. The attraction of tabanids to mainly black and brown fur coats is explained by positive polarotaxis. As the host's colour determines its attractiveness to tabanids, this parameter has a strong influence on the parasite load of the host. Although we have studied only the tabanidhorse interaction, our results can probably be extrapolated to other host animals of polarotactic tabanids, as the reflection-polarization characteristics of the host's body surface are physically the same, and thus not species-dependent.

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

“…Depending on the geographical distribution of different tabanid species in the vicinity of zebras in Africa (Uscher, 1972), these blood-sucking flies can also cause serious health problems for such equines (Leclercq, 1954;Kingdon, 1979;Moss, 1982;Leclercq and Maldes, 1987;Churcher, 1993;Leclercq, 2000;Tegegne, 2004). The tabanid larvae develop in water or mud, and both female Tabanid-proof zebras and male adults of many species are attracted to horizontally polarized light because they detect water by means of the horizontal polarization of water-reflected light (Horváth et al, 2008;Horváth et al, 2010). The consequences of this positive polarotaxis in tabanids are fivefold.…”

The function of zebra stripes

“…We collated literature on diseases carried by biting flies that attack equids in Africa (Supplementary Table 1) and note that equine influenza, African horse sickness, equine infectious anaemia, and trypanosomiasis and are restricted to equids, all are fatal and all are carried by tabanids. Currently, we are unable to distinguish whether zebras are particularly vulnerable or susceptible to biting flies because they carry dangerous diseases or because of excessive blood loss, but we are inclined towards the former because Eurasian equids are not striped, yet demonstrably subject to biting fly annoyance 37 .…”

“…Elsewhere, feral and semidomesticated horses can be plagued by flies 34 . Many herbivores and feral horses are subject to intense fly annoyance that can reduce time spent feeding, and ameliorate attack by moving to different habitats 35 , resting 36 , retreating to shade 37 or grouping together 38 .…”

Sistema constructivo : panel para fachada ventilada con mini aljibe, para enfriamiento evaporativo pasivo estacional

“…(HORVÁTH et al 2004) (HORVÁTH et al, 2008). (HORVÁTH et al, 2010). Existen varios fenómenos de polarización que dan lugar a ondas que oscilan en un único plano como la polarización por reflexión (ley de Brewster) o el dicroísmo que es una absorción selectiva.…”

“…(HORVÁTH et al 2010). Los patrones de rayas blancas y negras en los animales-huesped como las cebras reducen esta polarotaxis a medida que las franjas se estrechan, y lo que es más relevante, esta capacidad de ahuyentar a determinados insectos se produce incluso cuando el patrón sólo puede ser percibido con luz polarizada.…”