Lightning during a recent thunderstorm in Norway killed a herd of 323 reindeer — 70 of them calves — making it one of the deadliest strikes ever. Hunters in a remote area discovered the bodies last Friday, according to the Norwegian News Agency.

We usually hear about lightning striking people, but it does kill animals, too. Two scientists at an Australian research institute have found that everything from seal pups to wild turkeys to elephants and giraffes can be killed by lightning.

Was this a freak accident, or is it common? And how does this happen? I spoke to John Jensenius, a lightning safety expert from the National Oceanic and Atmospheric Administration, to learn about the science behind these deadly herd strikes. (This interview has been edited and condensed for clarity.)

Angela Chen: First, how likely is it that it really was lightning that killed those reindeer? Is there a way to know without having seen the strike directly?

John Jensenius: It isn’t that unusual to see farm animals, or wild animals such as reindeer, being killed by lightning. Of course, 323 is a rather large number, though we’ve seen reports of 654 sheep being killed in one spot.

Animals do tend to group together in storms and huddle under trees. If lightning strikes the tree or somewhere nearby, the entire group can be killed. We don’t know how common this is because it’s hard to track, though usually it’s herds of 10 or 20 animals that get killed.

In the case where the animals are huddling under a tree, oftentimes you’ll see some visible signs on the tree, though you may not see any visible signs on the animals themselves. In this case, it’s hard to know where lightning struck based on the pictures, but there may be an animal among the dead animals that has visible signs, like a bit of charring on the skin.

How did lightning kill all of those reindeer at once? Did they need to be touching for this to happen?

When animals or people are in groups, most are being killed by the ground current. First, there’s a direct strike — this is what most people think of when they think of lightning — that hits the tree or maybe the ground nearby. The energy then spreads along the ground surface, and if you’re anywhere near that lightning strike, you absorb it and get shocked.

Lightning goes up one leg and down another. Animals are more vulnerable because their legs are spread out more, so the ground currents travel more easily in their bodies. It doesn’t matter if they’re touching, or exactly how close they are, it matters that they were all in the area hit by lightning. Ground currents are the thing that’s responsible for the most lightning deaths and injuries in both people and animals.

How far can the ground current travel? When are you safe?

That’s one question we’re often asked, and it’s a difficult question because it depends on a lot of factors, including the strength of the actual lightning strike.

In this case, the animals seem to be in an area that was 50 to 80 meters in diameter and on a hillside, which gives you some idea that lightning can travel a good distance and still be deadly. Lightning doesn’t always travel deep into the ground.

What exactly is it about lightning that kills these animals?

It’s the electricity going into your body. It passes through the nervous system and your nerves, and the deadly part is that it stops the heart. In the case of people, many can be revived with CPR if tended to immediately but with reindeer, it just would have stopped their hearts.

What are some other types of lightning besides the ground current and the direct strike?

There’s the side flash. That’s when an animal or person is standing close to the tree, the tree is hit by lightning, and then the lightning jumps from tree to person or animal. The side flash usually kills one or a small number of animals, not large ones like with ground currents.

There’s also something called a "wall conduction," which is when something plugged into the wall is a direct connection to a wire outside. So if the wire outside is struck, the lightning will follow the wire and you can be shocked.

Are lightning fatalities, in people at least, going down?

Yes, they’ve been dropping over recent years. If you go back over the 1930s and 1940s, we had about 300 to 400 people killed every year in the United States. Nowadays, our 10-year average is about 31 people per year. This year so far we’ve had 32.

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