Wednesday, November 5, 2008

It's only natural

When I posted the photo of the obese hyena on Halloween, an astute reader posed the question of if she might get diabetes from being so fat. An excellent question!

The short answer is no, probably not. First of all, hyenas can go from "fat" to "normal" and back pretty quickly...they are capable of gaining an astounding 20kg in a single meal (that's 44 lbs. for those not metrically-minded). To give you some perspective (if you still need any after reading "44 lbs."), that's about 130 hamburgers, and at least three times as much as I can eat at Thanksgiving. Obviously that's not all retained as fat, although after such a gorging they will stay fat for days (Navajo, the hyena in the picture, is just now starting to return to her normal, albeit heavy, weight).

But back to the question—can hyenas get diabetes from being so fat, even if they're only fat sometimes? Well, I'm sure it's possible, but the truth is that most wild animals don't contract diseases like diabetes or cancer. That's because the selection pressures (yes, selection as in natural selection) are too strong. In other words, if there were a hyena that got diabetes from obesity—let's call him Carl—it would probably be because Carl had a genetic mutation predisposing him to the disease. Well, Carl certainly wouldn't fare very well against the other hyenas in The Game of Life, since he would be too busy trying not to keel over in a diabetic coma. Carl probably wouldn't produce as many offspring as other hyenas. In fact, maybe Carl would die before he had a chance to produce any. Bummer for Diabetic Carl, but less of a bummer for the hyena population's next generation, because now those genes for diabetes have gone with Carl, without getting passed on. In other words, they've been selected out. This is also why animals don't typically get diseases like cancer.

Okay, so why do we get cancer? By now you have probably caught on and guessed, because if you're reading a blog about hyenas you're most likely in touch with your Inner Science Nerd, but I'll elaborate nonetheless.

Humans are subject to natural selection just like any other species, but we have a special knack for relaxing (weakening) it. We have developed a million different medicines, medical treatments, and other life-saving and life-prolonging tactics to help us overcome things that would have surely done us in a couple hundred years ago. Think about it—a few hundred years ago, if you got the flu, things didn't look good for you. If you had a difficult childbirth, you and your child were in serious danger. And if your body stopped producing insulin, well, you were pretty much out of luck. It's therefore no surprise that life expectancies were half what they are today.

Today, thanks to technological advances, we're able to look natural selection in the eye and say "Pfff! Bite me!" Okay, it's not exactly like that, but the selection that operates on us now is so weak that it can be almost impossible to detect. I've had the flu, as have most of you, and I'm confident that if I can't pop a baby out naturally, my obstetrician can come up with some kind of a solution. And although diabetes is a very serious health condition, it is now something that people can commonly live with if they take proper precautions (and live in a country that doesn't mind insuring all its citizens...whoops, sorry, slip of the tongue). As for cancer, most wild animals—like humans not that long ago—never live long enough for cancer to get them. Those that do get it die pretty quickly, so the ones that are less likely to get cancer are the lucky ones who get to pass on their genes.

I don't mean to imply that natural selection doesn't act on humans at all any more, because it certainly does. It can just be a lot slower and a lot more subtle than it used to be. This is why human overpopulation is such a problem—we've removed many of the natural population controls. But that's a story for another day.

**Special thanks to Andrew S. Flies for his input on this matter.


Anonymous said...

So you contend that hyenas with "issues" simply don't live long enough to give those to their kids. This begs the question, what is the typical lifespan for a hyena in the wild? And what about ones in captivity? And how big of a turkey do you typically cook on Thanksgiving?


James said...

Steve Jones, one of the world's leading geneticists, even suggested recently that humans are no longer evolving (in western societies at least).

What is 20kg as a percentage of a hyena's average body weight? It must be greater a greater percentage than it would be for a human - so we would have to eat even more Turkey at Thanksgiving to achieve the same relative weight increase! Or am I wrong?

I do feel sorry for Carl.

Leslie said...

hyenas can consume up to about a THIRD of their body weight in one meal. adult hyenas range from about 50kg to as much as 70+kg. so, james, it seems like you're implying that we weigh a lot more than a hyena. ahem—speak for yourself. i only weigh more than SOME of the hyenas. :)

Swiftpaw Fatfox said...

Another thing to keep in mind is that the food that many civilized beings (whether human or animal) eat is usually containing so many chemicals most of which don't even exist in nature. So those play a big part to it too.

Michigan State University | College of Natural Science