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Hi everyone. I'm a sensory ecologist and I'm starting to get interested in ultraviolet light (also polarized light). I understand that birds and some insets (e.g. moths and butterflies) can sexually advertise in ultraviolet wavelengths. Still, there are so many diverse taxa of animals that can see UV that it seems like there have to be some more diverse uses for it other than prey identification and sexual advertising. I'm looking to create a bit of library of things that are strong reflectors of UV light and I'd really appreciate any you can contribute as mentions or more detailed references. Thanks so much.
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Urine can be "seen" in the UV helping birds of prey find rodents. Berries differ in their UV reflectance. Some example images of flowers and berries are available at http://www.uv4plants.org/gallery/plants-and-flowers-in-vis-and-uv-images/ as pairs of visible and UV images. Images are from Lasse Ylianttila.
Below are links to some publications I am aware of, but my expertise is more on plants than on animal vision.
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Anuran tadpoles respond to chemical cues of predation reducing foraging and swimming activity. In many cases this behavior is produced by a predation event releasing different kind of cues, in particular alarm cues, coming from tadpole itself, seem to play a key role in elicit antipredatory responses, even if they often need to be associated to kairomone (from predator) to have the whole response.
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In terms of actual chemical characterization, none have been described (to my knowledge). However, Fraker et al 2009 (Horm Behav) showed a nice neuroendocrine response from a skin-released compound in larval amphibians. Fish have been well described (e.g., ostariophysan alarm pheromone). The Fraker paper has a nice summary of what is known as well. I doubt my answer helps, but I would also love to know if you have any updates on this question.
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Exaptation is the co-option of a biological trait from it's original function into a new context. Sensory bias (biases inherent in a sensory system) evolve in order to convey an advantage to the individual carrying them, for example foods with a certain smell. If these sensory biases are co-opted into a new function (for example from food acquisition to predator avoidance) is this then an example of exaptation?
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I have to admit I have serious problems with the concept of "exaptation", or at least that people can publish in Nature saying that "exaptation" is more common than previously thought (see Corrie Schoeman's comment). In my view EVERY adaptation deserves also the name "exaptation" when analysed on all possible levels. Because none of the adaptations was originally "designed" for a function, because there is no designer creating adaptations. This is not a new discovery, but the foundation of darwinian evolutionary theory itself. Every adaptation starts with random variation of existing structures, if structures are changing and fulfil now a new function (because there is a natural selection pressure towards another optimum), then this is an adaptation as well as an "exaptation" (because it always derives from a former structure, which often has a different function, and before that there was another structure and so forth). As you can see the word "exaptation" doesn´t give us any new insight because it basically describes the darwinian mechanism; new structures evolve from existing structures.
A last word on the example with the evolution of the feathers: People trying to distribute the (quite unnecessary) word "exaptation", including Barve and Wagner 2013 (the Nature paper), claim that the feathers are one example. They were first used for thermo-regulation and later in evolution for flight. Yes, this is a very clear example how natural selection works it fits very well in Darwins original theory, no need to say "this is not adaptation". However, people say now, the feathers originally did not evolve for flight, therefore we call it "exaptation". Interestingly, in the feather-example it all gets blurry now: Adaptation/Exaptation for what? The feathers are even in this definition still adaptations, for thermo-regulation, and would be exaptations for flight. Really would they? What is with the 150 million years that passed since Archaeopteryx, aren´t the feathers of modern birds very well adapted to flight? So as you can see the wording you are using depend only on the scale you are looking at. Modern birds clearly have feathers that are adapted to flight, feathers may have originally evolved because of a selection pressure to keep the body temperature constant, however, also feathers didn´t fall from the sky they evolved out of other structures, with different functions as well.
Bottom line, I would really hope that evolutionary scientists don´t waste to mcuh time talking about a purely semantic concept, which does not give us any new insights or explanations.
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Colour photoreceptor cells are found as double, triple or even quadruple cones in the retina of some birds, fish, amphibians and reptiles. I would love to have a clearer idea as to the purpose/advantage that this may have.
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There have been other proposed roles, in addition to the various functional roles that Quirin mentioned above, including facilitation of an optimum mosaic distribution (to presumably improve photon catch) and changes during development, see e.g. http://jcs.biologists.org/content/s3-98/42/189.full.pdf+html
These are references you have likely seen but good overviews of some of the proposed functional roles of the cone types including some experimental studies:
Cheers-cjb