How many carrots do Santa's reindeer eat?

All of us here at The Naked Scientists leave a carrot out for rudolf (and the other reindeer). But how many carrots would those hardworking reindeer require to circumnavigate the globe? Otis Kingsman catches up with Gabriela Wagner from theNorwegian Institute of Bioeconomy Research, she's no stranger to speculating over Santa's handsome and furry helpers...

Gabriela - Hi, this is Gabi Wagner. I work at the Norwegian Institute of Bioeconomy Research and I research reindeer husbandry.

Otis - Hello, my name is Otis from The Naked Scientists. I need your help to figure out how many carrots Rudolph needs to eat in order to circumnavigate the whole globe in one night.

Gabriela - Well, magical flying reindeer may be a bit difficult to get hold of.

Otis - So what you're saying is we'd have a better answer if we could test it?

Gabriela - Theoretically, yes.

Otis - Okay then. Time to steal a reindeer.

Gabriela - I'm sorry, what was that?

Otis - Let's grab one and meet Gabi in Norway.

Gabriela - I hope he won't be too long. What's that? In the sky, it looks like it's heading straight towards me.

Reindeer - [crashes]

Gabriela - Are you okay?

Otis - I'm ok. I found us a reindeer to rest with. Apparently he's owned by a Vix? En? That's what it says on the name tag.

Gabriela - That's not what I was expecting, but when is science ever something you can accurately predict? This is interesting. Usually only females have antlers in winter, but this is a male. He's castrated of course, otherwise he would already have lost his antlers about a month ago.

Otis - I think he's built to pull sledges.

Gabriela - That would explain why he is so tame. He didn't beat you up that badly.

Otis - He's bigger than I imagined. How heavy is he?

Gabriela - He's 140 kilos. He is in very good condition. He must have had some Christmas food to weigh that much at this time of year.

Otis - Ok, but back to the main question. How many carrots would it take for him to get all the way around the world?

Gabriela - Obviously he couldn't circumnavigate the earth at the equator. He can't sweat after all, and he can't lose any heat with that fur. He has a thick coat of under-wool and thicker longer hairs, called 'guard hair'. Not only can he trap air between the hair of his coat, but each one of the guard hairs is hollow. In addition, that gives him superb insulation against the cold.

Otis - I was wondering why he didn't seem phased by the cold.

Gabriela - -40°C is no problem. But anything above 5°C would be a challenge and he'd have to massively increase his metabolic rate to get rid of the heat.

Otis - Is that the only feature they have to survive cold temperatures?

Gabriela - Reindeer have special noses, which they used to save energy. They have scrolled structures in their nose. It's a bit like a heat pump and they can use the extra surface to warm up the cold outside air in winter. When it's too hot or when they work hard, they have to pant like dogs. We couldn't get him to walk anywhere along the equator; he'd die of heat stroke.

Otis - So does that mean they're mostly situated in colder countries?

Gabriela - You can find them in Scandinavia, Russia, Siberia, Northern America in Greenland. You can even find them in Iceland and Scotland.

Otis - That's a lot of places. Did they just walk there?

Gabriela - Walking on frozen ground or snow is of course no problem. Their feet act like snow shoes. That's why they're so good at pulling sledges. They can also use their hooves like chisels to dig for food under the snow. Most importantly for us, their feet can also act as paddles. Reindeer are very powerful swimmers. The hollow hair of their fur acts also like a built-in buoyancy aid. That means rivers and fjords are not a problem along the route. The oceans however, are, so we just have to assume he could take a terrestrial route for our calculations.

Otis - On the flight over here, the reindeer was galloping in the air, so it stands to reason that it will require the same amount of energy.

Gabriela - They have been documented to walk up to 5,500km or 3,410 miles in a year. Now travelling on Christmas foods, that's special reindeer pellets for you, with about 11,000 kilojoules of digestible energy per kg of food. Our big boy here working hard, would need about 5kg of food a day. He is after all a ruminant who needs a lot of rest to digest his food. At an average walking distance of about 25km or 15 miles a day, he would need about 640 days to circumnavigate the Arctic circle to cover the caloric needs with only about 1,700 kilojoules in a kg of carrots, we need about 20.4 tons of carrots.

Otis - Let's scale that up. If we base our calculations off the fact that the Earth is 40,075km in circumference, and that Santa would need to travel across the sea from country to country. I've estimated that the distance Rudolph would need to travel to be 160,000km. Of course this doesn't factor in the distance between each street and each city, but based off this, how many carrots would a reindeer need to eat?

Gabriela - If this reindeer was to circumnavigate the whole globe he'd need 204 tons of carrots or 40,080,000 carrots. I hate to break the news to you, but reindeer don't eat carrots. He can't bite the carrots as he hasn't got the teeth for that.

Otis - But tradition states you leave a carrot for the reindeer. I can't believe I stole this reindeer for nothing.

Gabriela - You stole the reindeer. How? And from where?

Otis - I just found one in the North Pole. It was standing next to this, rather large man with a white beard dressed in red.

Santa - So it was you who stole my poor reindeer.

Otis - Wait. No, please. It was all in the name of science. I swear.

Santa - I'm going to put you in a ho-ho-holding cell.

Otis - Nooooooo.