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Comment by mysterypie | Hacker News Reader

> They must be able to see [their beaks glowing under UV light] — that's the only reason it would exist.

I don't understand why he's so certain about this. Does it never happen under evolution that you get a property that is useless, but is otherwise harmless so it does not evolve away? In this particular case, could the chemical that gives the beaks a certain color just happen to be UV fluorescent, yet the birds can't see it? If we examine millions of features in animals, it seems we should come across some funny or interesting features that are incidental.

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Agreed. Scorpions also glow under UV light and scientists don't seem to have come up with definite theory explaining why this may be. Maybe these are just random occurrences of proteins that glow under UV, but have other properties beneficial to the animal in question.

My theory about these scorpions is that it is protective coloration (aposematism) that warns potential predators, in this case likely birds and reptiles that are capable of seeing uv colors due to tetrachromacy. See [1] and [2].

[1] https://en.m.wikipedia.org/wiki/Aposematism

[2] https://en.m.wikipedia.org/wiki/Tetrachromacy

Tetrachromacy does not necessarily imply the ability to see UV colors. There's a non trivial number of women that are tetrachromats[1] and they can just see more shades in the same color spectrum as most people.

[1] http://www.bbc.com/future/story/20140905-the-women-with-supe...

Keep in mind that puffins and scorpions are fluorescing. So the excitation is UV light, but the emission is in the visible spectrum. So tetrachromacity doesn't necessarily come into play. The ability to resolve the emission colors through the contrast of the day is the relevant capability for observers.

Once you learn to recognize florescence, you notice it on some objects with optical brighteners in daylight (e.g., blaze orange products, certain laundry detergents on clothing).

For the life of me I can't remember the term, but this does exist. Basically genes can be linked together and as one gets selected, it brings along the other, non-related gene (and it's traits).

If I can think of the term and some examples I'll edit. Hopefully a fellow HNer with a better memory can help me out.

Are you looking for vestigial? I would be inclined to agree with you, except that birds have tetrachromatic vision (as opposed to our trichromatic vision), and it is possible that the beak coloration is visible after all.

i think it’s genetic drift

I think you're right, but after learning about the birds' fourth photopigment, I think this is probably regular evolution at work and there is some selective advantage to it since they can (probably) see the colors in some capacity.

I wonder if they've done any experiments yet as to whether the birds actually can detect that. It definitely seems possible to do such an experiment.

I do think it's odd that in the article they just say the birds "must" be able to detect it, without any evidence that they actually can.

For example you could have three boxes, only one of which has food in it. Paint that box with UV fluorescent material like that which is on their beaks and repeatedly show the bird where the food is and let it eat it. Then stop showing it and let it go to the box on its own. Then start switching around the location of the boxes and show the bird which box has food in it. Then keep switching the location of the boxes but don’t show the bird where the food is. Do this in all of the different lighting conditions that the bird would typically come across in nature. See if under any of these conditions the bird can consistently identify which of the boxes has food in it. If it can and if it’s not due to smell or sound then it has to be because of what it can see. Ensure that other than the fluorescence there is no other visual distinction that makes the box identifiable. Do this by making new boxes every now and then and replacing the old boxes with those.

In order to eliminate the possibility of sound or smell being a factor we could make three new boxes, one of which is fluorescent and the other two not as before but now instead of putting the food in the box we keep it in a separate bag and we reward the bird for going up to the fluorescent box by feeding it from our bag.

Bluetits (parus caeruleus) are sexually dimorphic under UV light only, and apparently someone did some experiments on this too:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1688915/

Do puffins eat insects? Many insects see UV, so maybe it attracts lunch?

I think it's mainly fish and other sea creatures.

Seeing UV is not relevant here. The beaks _fluorescence_ under UV. They absorb UV and emit visible light. They don't emit UV.

Doh, good point. Not quite giving up on the dietary utility though. Fluorescence only needs enough energy to re-emit, and the emission could be activated by other actions. Is it immediate or delayed?

The activation of fluorescence by UV may not be the only means of activation. Lingulodinium polyedrum, for example, bioluminesces normally under pressure changes, but also under acidity changes (brilliant but lethal: https://en.wikipedia.org/wiki/Lingulodinium_polyedrum).

Turns out puffins eat fish. Is it possible the puffin's beak glows underwater as it swims, attracting fish? Either due to salinity or pressure change?

Here's a nice paper on the penetration of UV in saltwater: https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo....

I'm confused, if the fluoresce as shown doesn't that mean they look brighter in light visible to us? is it such a stretch to believe the birds can't see that?

Yes, fluorescent chemicals are distinctly included in laundry detergent for the same reason: to make the colors visibly stand out in daylight. As such, you can use laundry detergent to write "secret" notes visible under UV.

Also handy for Halloween props

Given how few natural things glow under UV light I would venture to say that those chemicals are not particularly common or more useful than others in general.

Agreed. There is no reason why the fluorescence isn't a harmless and coincidental byproduct of growing a beak in this animal.

The broad question you're asking is called adaptationism.

https://en.wikipedia.org/wiki/Adaptationism

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