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Do larger creatures get cancer more frequently?

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Sources:
https://motherboard.vice.com/en_us/article/pganzk/whales-have-1000-times-as-many-cells-as-humans-why-dont-they-get-cancer-more
https://www.nature.com/news/massive-animals-may-hold-secrets-of-cancer-suppression-1.12258
https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-017-0401-7
https://www.bbc.co.uk/education/guides/zg2xxnb/revision
https://academic.oup.com/icb/article-lookup/doi/10.1093/icb/icm062
https://www.genome.gov/10001345/importance-of-mouse-genome/
http://rstb.royalsocietypublishing.org/content/370/1673/20140220

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https://commons.wikimedia.org/wiki/File:Tumor_Mesothelioma2_legend.jpg
https://commons.wikimedia.org/wiki/File:Sperm_whale_drawing_with_skeleton.jpg
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SciShow is supported by 23andMe. [♩INTRO] Whales are huge.

By mass, blue whales are literally the biggest animals that have ever existed. And because of that, they have so many more cells than we do.

Which leads to an interesting paradox: We know that every time a cell copies itself, it could make a mistake and turn cancerous. So the more cells you have and the longer you live, the higher your chances should be of developing cancer. Whales are not only bigger than us it’s also totally normal for them to live for over a century.

So you’d think that pretty much all whales would have cancer. But they don’t. And if we could understand why that is, we might be able to solve some of cancer’s biggest mysteries.

Cancer is mostly just abnormal, out-of-control cellular reproduction. If every cell is an opportunity for something to go wrong and trigger the growth of a cancerous tumor, you’d expect whales and other big animals to have way more cancer than humans. But studies have confirmed that cancer rates across species aren’t correlated with body mass or lifespan.

This puzzle is called Peto’s Paradox, named for British epidemiologist Richard Peto, who first described the idea in the 1970s. And we still don’t really know how to solve it. Some scientists don’t think Peto’s Paradox is a paradox at all they think whales actually do get cancer more often.

It’s just that those tumors don’t kill them, because they get destroyed by secondary tumors called hypertumors basically tumor parasites that feed off the original tumor’s blood supply, suffocating it. Yeah... hypertumors: Sounds bad, but is actually good! In an enormous animal like a whale, tumors take much longer to reach a dangerous size, and in theory, that long growth period might lead to these hypertumors popping up.

So maybe they don’t really get less cancer; it’s just that the cancer dies off before it becomes problematic. But the idea hasn’t been tested or proven, and other biologists think that instead, whales probably have some way to keep cancer from forming in the first place. Their size itself might be part of it.

Large mammals tend to have lower metabolic rates, meaning they burn less energy relative to their size. That’s because they have less skin area relative to their body mass, which cuts down on how much energy they lose as heat there’s less room for it to dissipate. And with a slower metabolism, they’re also producing fewer of the damaging, highly reactive molecules that can trigger the growth of cancers.

But on its own, that’s probably not enough to explain their lower cancer rate. And we already know other large mammals fight cancer more directly. Take elephants, for example.

As mammals, elephants share the same set of basic genes with whales that we do. But elephants have tweaked their genomes to have more genes that suppress tumors, called antioncogenes. They have twenty copies of TP53, a gene that triggers damaged cells to self-destruct.

Whales, like us, only have one copy. But they may still be doing something special to keep cancer at bay. Some biologists think they might have better versions of antioncogenes than we do, or other, species-specific cancer-busting genes we don’t know about yet.

In a way, it makes sense that big animals like whales and elephants would evolve special cancer-fighting powers. In giant, long-lived animals that have evolved to invest a lot of resources in just a few offspring, cancer could be a big problem if it interferes with their ability to spend years raising their young. But there’s a trade-off.

More effective anti-cancer genes also seem to lead to decreased fertility, possibly because the energy used in repairing damage and terminating cancerous cells has to be diverted from somewhere. And for animals that aren’t gigantic, and therefore wouldn’t have an increased risk of cancer from all those extra cells dividing, it’s just not worth it. The benefits of producing more babies outweigh the risks of developing cancer before getting the chance to raise them.

In those cases, evolutionary pressure may actually select against more powerful tumor-suppressing genes. Researchers are digging deeper into these ideas because they’re hoping whatever helps whales might be used to help us, too. They’ve studied TP53, for example, and genetically altered mice to over express it.

In some cases, that caused bad side effects like premature aging, but they haven’t given up on finding new ways to use this tumor self-destruct button in humans. So, we don’t yet know exactly what it is about the blue whale genome that prevents cancer. But figuring that out might get us another tool to help fight it in humans.

In the meantime, we do have tools you can use to learn about your own genome. Earlier this year I took a 23andMe DNA test by spitting into a tube, registering my kit and sending it back to their lab. The name 23AndMe comes from the fact that human DNA is organized into 23 pairs of chromosomes.

The results are fascinating and fun to look through, especially since I have offspring. Now, it’s my wife’s turn.

Hank: This is my wife, Katherine. Hi. Katherine: Hello.

Hank: Here is our comparison page for 23andMe. So, you did yours, I did mine, and then we linked them together. And now it tells us stuff about, not just each of us individually, but us together. Katherine: How we compare to each other.

Hank: For example, relationship: We could not detect identical DNA segments between you and Katherine. Katherine: Yeah! Not cousins!

Hank: All right! We can both smell asparagus pee. So... it knows that about us. Katherine: Hoooah.

Hank: A lot of freckles. Katherine: It’s true, that one is real true.

Hank: Little freckling.

Hank: So I’ve got a parent and a grandparent, 100% British. But then all the way back here to the Iberian bit: I most likely have a 4th great grandparent or 5th great grandparent or 6th gr- or 7th... who was 100% Iberian. They were likely born between 1710 and 1800. Katherine: That’s an interesting thing to think about.

So then you go read some historical fiction set in...

Hank: Iberia! Katherine: ...the Iberian peninsula.

Hank: Thanks 23andMe! Thanks for supporting SciShow and thanks for helping Katherine and I know that we are definitely not related to each other. Katherine: Awesome! Looking through 23andMe results with family is a lot of fun.

We had a really good time doing it. The holidays are a great time for family, and a 23andMe kit would make a great gift and possibly settle long-standing disagreements about why there’s a little red in Cousin Gary’s beard. 23andMe has a special holiday offer now through December 26th, so go to 23andme.com/scishow to check it out and to support SciShow. [♩OUTRO]