Four puffins on a rock

This week I learned: animals in ultraviolet

Just a quick one this week, as it’s been a very busy week (some of the busyness might be leading to interesting things, but nothing confirmed enough yet to post about).

Some people might choose to celebrate the end of a long week by getting down at the disco. Not really my scene of choice (though I do love a dance!), but here are some party animals who look great under a bit of UV lighting.

Birds that brighten up

Ornithologist Jamie Dunning says he was having a “troubling” day at the lab. Looking for a distraction, he took the carcass of a puffin out of the freezer (as he told the Independent, “I’m the kind of guy people send dead birds to”) and put it under ultraviolet light.

The result was striking: the pair of curved yellow stripes on the bird’s colourful beak lit up like glow sticks.

Dunning’s impulse to shine disco lights on a dead bird wasn’t as eccentric as you might think. He already knew that crested auklets, another species in the same family of seabirds, have beaks that light up under UV. In fact, many birds have some element to their look that reflects UV light.

UV light can be used to indicate the age of an owl, because their feathers light up different colours depending on their age. The male and female Picui Dove, which look pretty much identical to us, show sexual dimorphism under UV. And budgerigars are so fluorescent that they don’t even need UV to shine (though they light up under UV too).

In fact, a research review in 2009 found a stunning total of nine hundred and sixty-eight different species recorded as having UV-reflective plumage – and those are just the ones scientists have taken the time to check.

Seeing a broader spectrum

But given birds don’t typically hang out at black light discos, what’s the use of lighting up under UV?

Recently I talked about sensory perception and the different umwelten inhabited by different species (and different people). Birds are a great example of this. Where we (usually) have three cones in our eyes for seeing colour – red, green, and blue – birds have four. Unlike us, they can see into the ultraviolet end of the spectrum naturally.

That doesn’t mean they’re living in a permanent disco – that glow-stick brightness is just how we perceive UV-reflective materials when we shine specially-designed black lights on them, because under normal conditions we can’t see UV reflection at all. But to a bird’s eye, UV-reflective plumage would be visible under the ultraviolet wavelengths found in normal sunlight – just the same way that we see a leaf as green because it reflects sunlight’s green wavelengths. And that means what birds see are a range of colours we can’t even imagine.

And just like the other colours in a bird’s umwelt, ultraviolet can provide important visual signals. Blue tit females choose a mate based on the brightness of his UV-reflecting crown – the brighter his UV, the healthier the bird. Chicks in the nest have bright UV mouths to guide their parents: insert food here! But that feature can cut both ways, with the parent birds in at least some species using UV brightness to decide which chicks to feed first. UV can even tell a bird when fruit is ripe enough to eat (wouldn’t that be useful for us sometimes?).

A world of colour

Birds aren’t the only animals to see into the ultraviolet. Butterflies similarly use UV to advertise their health as a mate; bees look for markings on flowers – invisible to us – to tell them where to land; even reindeer use UV vision to find food. And scorpions light up gloriously under UV light – although why they do that is a total mystery, since they’re nocturnal and there’s hardly any UV around at night.

Really, for such a vision-based species, humans are missing out. Imagine how different the world would look if we could see what a bird sees!

Meanwhile, Jamie Dunning’s next task is to prove that puffins’ light-up beaks are found in live birds, not just dead ones. And to that he has, very importantly, made sunglasses for them.

Image source: Pixabay
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