Why giant animals needed to eat fat instead of sugar

HostIf you look at the skeletons of the big beasts that used to roam the earth, like the woolly mammoth or those giant ground sloths the size of a house, it's hard not to wonder how they kept those massive bodies moving. You would think they just needed to find a giant field of fruit or sugar to keep their energy up, but it turns out fat was the real secret to their survival. Why was fat so much better than sugar for a creature that weighed several tons?

GuestIt really comes down to how much punch you can pack into a single bite. When you're that big, your biggest enemy is actually just the clock. There are only so many hours in a day, and if you're burning through energy just by standing up or taking a step, you need a fuel that lasts. Sugar is like throwing a handful of dry leaves on a fire. It flares up fast, gives you a quick spark, but then it's gone. If a mammoth tried to live on sugar alone, it would've to spend every single second of its life eating just to stay even. Fat is more like a heavy log. It burns slow, it burns hot, and it carries twice as much energy as sugar or even protein does. For a giant, fat is the only way to get enough fuel without having to eat twenty four hours a day.

HostBut we see huge animals today, like elephants, and they seem to spend all day just munching on grass and bark. There's not much fat in a tree branch. If they can make it work on rough plants, why couldn't the other giants just stick to a green diet?

GuestWell, that's a bit of a trick of nature. Elephants actually do live on fat, they just have a very strange way of getting it. They have these enormous guts that act like big fermentation tanks. They eat the grass, and then the tiny bugs living inside their stomachs break that grass down and turn it into short-chain fatty acids. So, even though the elephant puts grass in its mouth, its body is actually running on fat that those bugs made for it. But for the giant hunters or the animals living in the ice age, they didn't always have the luxury of a giant tank and ten hours to sit around and let grass rot in their bellies. They needed to find the fat ready-made.

HostSo it's about how much space you have in the day to chew. But what about the hunters? If a big cat or a short-faced bear caught something, why wouldn't the lean meat, the protein, be enough? We always hear about how protein builds muscle.

GuestProtein is great for building the body, but it's a terrible fuel source if it's the only thing you have. There's actually a limit to how much protein a liver can handle. If a giant bear ate nothing but lean rabbit meat, it would actually starve to death with a full stomach. Its body would get overwhelmed by the waste products from all that protein. They call it rabbit starvation. To stay healthy and keep those huge brains and muscles going, they had to have the fat. It was the only thing that kept the liver from seizing up. If you were a giant predator back then, you weren't looking for the lean muscle of a deer. You were looking for the thick layers of grease and marrow.

HostThat feels a bit backwards to how we think about food today. We're usually told to trim the fat off. So, for these animals, was fat just a way to survive the cold, or did it change how they lived their lives?

GuestIt changed everything. Think about a whale. A blue whale is the biggest thing to ever live, and it survives in some of the coldest water on earth. It doesn't just use that thick layer of blubber to stay warm like a coat. That fat is its bank account. It can eat a massive amount of tiny shrimp in one part of the ocean, turn that into a huge layer of fat, and then swim for thousands of miles without eating a single bite. It's moving a hundred tons of weight across the globe using the energy stored in its skin. Sugar can't do that. You can't store sugar in your body in any big way. If you don't use it right then, it's gone or it has to be turned into fat anyway. For a giant traveler, fat is the only portable battery that holds enough charge to get them where they're going.

HostI guess it's hard to imagine a mammoth trying to carry around enough fruit to make that trip. It would be like trying to power a semi-truck with AA batteries.

GuestExactly. And even the way they grew depended on it. In the ice age, the seasons were brutal. You might have a few months of plenty and then months of nothing but snow and bone-chilling wind. If those animals hadn't been able to bulk up on fat when times were good, they would've vanished the first week the temperature dropped. They were essentially living off the fat of their own bodies for half the year. Their whole design, from their humps to their thick skin, was all about managing that one specific fuel source.

HostIt's wild to think that the very thing we often try to avoid is what allowed the biggest creatures in history to even exist.

GuestThe biggest whales in the ocean today are still proving that a massive body is really just a giant engine designed to turn fat into distance and life.

HostThose walking houses of the ice age weren't just looking for a meal, they were looking for the long-burning logs that could keep their fires going through the dark and the cold.