The sensory ecology of polarized light

For orientation, birds use a number of different compasses based on the geomagnetic field, stars, the sun and the related pattern of skylight polarisation. In collaboration with scientists at Eötvös University in Budapest we have performed measurements of polarized skylight under different sky conditions and studied the orientation of birds and insects. It has been shown that birds use polarized light for detecting open waters in high arctic pack-ice areas, and also that skylights affected by for example forest fire smoke have a disorienting effect on animal orientation. Furthermore, according to Icelandic Sagas, Vikings used a sunstone to tell the direction to the sun at sea while sailing in mist and under overcast conditions, a physical prerequisite which we have shown is reasonable given low degrees of linearly polarized skylight is transmitted through clouds.
 
Studying insects, we have found that tabanid flies (horse flies) use reflected linearly polarized light to find hosts for a blood meal, preferring horses with black coat reflecting linear polarized light, over horses with depolarizing white coat. We plan to continue this project with extended work on the sensory ecology and evolution of parasite-host interactions and measurements of polarized skylight during orientation studies. We will also study the use of polarized light cues for orientation and food detection in birds.

Dark coat preferred over white

Dark coat preferred over white

The reason why tabanids are attracted to brown or black horses is that the dark coat of the horse reflects polarized light from the sky in a way much similar to a water surface, whereas a white coat does not. The tabanids are attracted to water to drink and to lay their eggs, and use information from reflected polarized light also to locate hosts, and it is the female tabanids that are attracted to the host to suck blood, using this meal to produce eggs. Tabanid attacks can be severe and cause spreading of diseases to humans and grazing animals as well as reduce milk production in cattle. Photo: courtesy Gabór Horváth.