World War Z: Do Zombies Have A Half-Life?

And no, that wasn't (originally) intended to be a pun about zombies being undead, but whatever...

So, the World War Z book was fantastic. It's widely hailed as being one of the zombie properties that really, really thinks through th logistics of such an event, the potential science, the possible social implications, all the ways that humans might survive or react. For those of you who haven't read it, it's not a novel - rather, it takes the form of an anthology of stories, ostensibly collected post-zombie-control, about life during the zombie apocalypse. (If you've only seen the movie, or only read the book, it lost a lot of what made the book great in the adaptation process. Actually, in the adaptation process, it lost almost everything in the book period, except the title and the presence of zombies).

In any case, when people get into debates about the plausibility of zombies, most of the discussion centers around virology and epidemiology. What kind of virus could produce such an effect? What would the incubation period have to be in order for it to successfully spread?

Then, there's the question that always plagued me, the physicist: What about conservation of energy (and mass)? How long could a zombie keep on running, walking, shambling, or crawling towards its would-be victims and food sources? How long could a zombie trapped in a WHO lab with Brad Pitt and Peter Capaldi wander in circles until it literally ran out of energy?

First of all, we need to figure out the energy requirements of a zombie. Since I'm not a biologist and consider things like nutrition to be hand-waving magic (explain how else people can survive years of graduate school on mac and cheese, occasional free pizza, and alcohol!) I'm just going to plug numbers into online calculators until we get to something that we can use for the fun part.

The human body requires a widely varying amount calories a day, depending on the activity level of the person. Zombies have to have metabolisms, still - something has to convert those brains into energy to run around and bite more people, right? Granted, we can assume that a human's basal metabolic rate, or BMR, (that is, the calories spent doing things like powering smooth muscle, or keeping your body at a cozy 98.6 degrees fahrenheit, that even coma patients would burn) can be subtracted from that requirement because once you're a zombie, those body functions are kind of unnecessary.

For the purposes of this thought experiment, we'll assume that the zombie is a perfectly average American man, who (according to the CDC, which apparently know a thing or two about zombies) is approximately 5' 9" or 176 cm and 195 lbs or 88 kg. Because BMR calculations require age, I arbitrarily decided that this average zombie would be 30 years old. And frictionless, and a perfect sphere... also, we're gonna assume that a zombie metabolism converts muscle, fat, and organ tissue to energy just like a human metabolism, because we need a baseline from somewhere and we've got nothing else remotely useful.

Since I wanted to find an accurate BMR calculator, I eventually settled on this one on Wolfram Alpha, which gives a BMR of 1897 calories a day (or about 79 calories an hour), as well as caloric requirements for various other physical activity levels. Zombies that spend most of their time shambling around and not sleeping or resting at all in their endless quest for brains aren't "sedentary" or "lightly active", so subtracting BMR from those numbers, you get a consumption of between 1040 calories a day to 1700 calories a day.

According to the same super-useful Mayo Clinic page, it takes 3500 calories to burn a pound of fat. Since our perfectly average zombie also has a perfectly average body fat percentage of 23%, that means that he has about 45 lbs of body fat, which means that he's got 157,500 calories in his fat alone. That could sustain him from around 3 to 5 months, depending exactly how quickly and frequently this zombie moves.

Of course, once the zombie has metabolized all his fat, his body is going to move on to his muscle, and then start "digesting" its own organs until a human would die of organ failure. But, since Hannibal Lecter is considered an aberration instead of a nutrition expert, there's not much information I can find on how many calories are in a pound of human muscle (or heart or liver or whatever) right now - stay tuned for part 2 of this exercise once I figure that out!