Polarised light signals as a secret communication channel

Already a  while ago (it seems my posts come many days after the event I’m writing about took place!) we went to Fort Sherman where we not only had the unique opportunity to be even higher than the tallest trees with the canopy crane, but we were able to appreciate a great diversity of butterflies, and their beautiful coloured wings. We discussed different issues related to butterflies’ wing-colour patterns, such as Müllerian and Batesian mimicry strategies in relationship to predation, as well as how a habitat’s “background” influences butterfly wing-coloration. Particularly, the butterflies from the Heliconius genus have been a great model system for exploring diverse ecological and evolutionary questions, such as how speciation takes place (and of course they are one of Owen’s and Carlos’s favourite model system). Finally, back in the schoolhouse we had a brief lecture-discussion with Brett Seymoure about animal communication.

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Looking closely at a Blue Morpho (that Owen caught!)

All this (butterflies and animal communication) reminded me of a topic I studied during my undergrad: the use of polarised light in animal communication, where butterflies were one of the organisms found to do so. Strong polarisation can be produced when light is reflected from shiny surfaces such as leaves, wet surfaces, animal skins and scales. And animals who possess polarisation vision or sensitivity (which is not the same but I am not going to go into details for the sake of this post) are able to perceive patterns of polarised light as a visual image (i.e. analogous to color vision). Polarisation vision has been found in a diverse group of organisms, from insects to cephalopods (particularly cuttlefish). One of the things I find most interesting about communicating using polarised light is that as most organisms are not able to reflect and/or detect it, it can be used as a “secret communication channel” between the species who do employ it. This can have great advantages for reproduction and predator avoidance, as I will explain with the following example.

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Figure taken from Douglas et al. 2007. Figure made using false color imaging, where blue indicates no polarisation and green, yellow, red and white indicate an increase in polarisation reflectance. A) shows the non-polarised wings of Danaus erisimus, B) polarised wings of Memphis chaeronia. 

One of the first studies looking at the use of polarised light in terrestrial environments (Sweeney et al. 2003) found that a butterfly species from the Heliconius genus used polarised-light signals for mate recognition. The researchers performed mate-choice experiments with two species of Heliconius butterflies: Heliconius cydno chioneus (iridescent) and Heliconius melpomene malleti (non-iridescent). Female mounted wings were exhibited to conspecifics in the presence or absence of filters that removed the polarisation reflectance, and the study found that H. cydno males approached females significantly less when the depolarisation filters were present, whereas there was no difference for H. melpomene. Furthermore, as these two related species of butterflies inhabit very different habitats (H. cydno is a forest dwelling species while H. melpomene lives in open areas) it has been suggested that polarised signals might be very advantageous within tropical understory habitats, because they are independent of ambient light intensity and can be easily detected against the mostly un-polarized background. Therefore, butterflies that are able to detect and reflect polarised light will have the advantage of appearing very conspicuos to their conspecifics while remaining relatively hidden to the bird predators lacking polarisation vision.

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Figure taken from Douglas et al. 2007. Polarised-light signals seem to be more prevalent (and advantageous) in the dense forest habitat than in the open habitat. This figure shows the % of Costa Rican nymphalid species with polarised and non-polarised wing patterns.

References:

Sweeney, A., Jiggins, C., & Johnsen, S. (2003). Insect communication: polarized light as a butterfly mating signal. Nature423(6935), 31-32.

Douglas, J. M., Cronin, T. W., Chiou, T. H., & Dominy, N. J. (2007). Light habitats and the role of polarized iridescence in the sensory ecology of neotropical nymphalid butterflies (Lepidoptera: Nymphalidae). Journal of Experimental Biology210(5), 788-799.

 

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