This time of year, “Rudolph the Red-Nosed Reindeer” is a nearly inescapable earworm. Rudolph, the old song goes, is bullied for having a nose so bright it glows (like a lightbulb!). But one Christmas Eve, his much-mocked nose makes Rudolph a hero. Using it as a beacon, Rudolph guides Santa’s flying sleigh through the foggy night to deliver gifts around the world.
Rudolph’s superbright snout might seem as fantastical as his ability to fly. But a light-up body part needs no holiday magic. Many animals glow through bioluminescence, and a built-in red headlight would make a great adaptive trait for a sleigh-pulling reindeer. Though thanks to physics, Rudolph’s nose might not even look red to someone on the ground.
Most bioluminescent animals use the same chemical reaction to glow. It takes only two main ingredients: a compound called a luciferin and an enzyme called a luciferase. “When oxygen is present in the cell, [the pair] react together and give off light,” says Danielle DeLeo, a marine biologist at Florida International University in Miami.
Bioluminescent reactions light up the lanterns of anglerfish a bluish-green, illuminate fireflies’ flashy backsides yellow and allow some insects and deep-sea fish to radiate red.
The odds of this trait evolving in reindeer is “very, very low,” DeLeo says. Most glowing animals are found in the ocean — and among the land animals that do give off their own bioluminescent light, none are mammals. Still, it’s not impossible for bioluminescence to emerge in a new species. “It’s evolved at least 100 times across the tree of life,” DeLeo notes.
If a sleigh-pulling reindeer were going to glow, red would be the best color for it, says Nathaniel Dominy, an evolutionary biologist at Dartmouth College.
With the longest wavelengths of any color we can see, red light is least likely to get scattered by airborne water droplets. As a result, “red light is going to allow [Rudolph] to navigate under foggy conditions more effectively than any other light,” Dominy says.
To someone on the ground who spied Rudolph soaring through the sky, his nose may not look red at all. That’s because when a light source is moving toward you, its light waves get squished and look bluer. It’s only when the source is moving away that light waves get stretched out and appear redder.
“We don’t see that typically around us, because things have to travel really fast for the redshift or the blueshift” to occur, says Laura Driessen, a radio astronomer at the University of Sydney. Santa’s sleigh could be an exception. That’s because it would have to hit extreme speeds to visit houses around the world in a single night, she says.
Say Santa travels at 10 percent the speed of light. As Rudolph approaches a house, his nose would be blueshifted to look orange. As he flies away, his nose would redshift to nearly the deepest crimson red that human eyes can see — so dark it would look almost black.
Rudolph’s nose wouldn’t be the only thing blueshifted and redshifted, either. For example, blueshifted brown hues, like reindeer fur and a wooden sleigh, would take on a greenish tinge. “We’d see a green sleigh and reindeer coming towards us,” Driessen says. As they flew away, Santa and his reindeer would almost disappear as they moved beyond visible light into infrared.
Such fast movement and a brightly glowing nose would cost Rudolph a lot of energy, Dominy says. “I would want to make sure that he could get as much energy as possible — sugary foods.” So anyone planning to leave out treats for Santa this Christmas Eve should be sure to leave out plenty of cookies for his reindeer, too.
