A Chain Swarm has the potential to be the most deadly aspect of a zombie apocalypse, after The Panic. And yet, it is something that has not been covered in detail. This page will attempt to rectify that.

The basic idea behind a Chain is quite simple, and relies on a few assumptions of zombology. The first is that zombies moan when they find prey. The second is that zombies react to other zombies moans, and on some level understand the difference between a regular moan and a "food moan".

So one zombie can potentially bring others to your location. Not intentionally, I'm not suggesting that level of conscious intelligence. "Bob, there's humans over here. Let's eat!" But more as a useful evolutionary mechanism to ensure propogation of the species. Much in the way that ants leave chemical trails for others to follow to the food, zombie moans allow zombies to find food more easily.

Obviously having more zombies descend on your location is a Bad Thing. But just how bad could it be? How far could a moan carry?

Well potentially all the way back to a large city, containing several million zombies. How?

Let's use New York as an example. The metro population is just under 19 million. Given most people expect survival rates to be between 0.5% and 3%, with the zombie population at between 20% and 65% and dead people/zombies 30% and 75%. Best case scenario, there are 3.8 million zombies in the metro area. Worst case, 12.35 million zombies in the metro area.



Going with the middle numbers, let's say 8 million. And in the beginning, they had enough food to occupy them. But eventually, that food will run out, and they will begin to wander aimlessly. Most will stay within the city. A few will not.

How many will wander away? Small percentages at first. So small they are not worth looking at. But eventually the numbers will get significant. Let's start at 1/100th of a percent (0.01%), or 800 zombies.

Assuming they can leave from all directions (and more on that later), that makes 2.22 zombies per degree, or 0.45 degrees per zombie. I have stated before the average speed of a shambling zombie is 16 miles a day. So the zombies after one day are 16 miles from the edge of the city, but they are only 0.13 miles, or 686.4 feet from the zombies either side of them.

How far does a moan travel? Well using very rough calculations on the dB of a moan, scream, shout or yell the high end would be 110dB, average of 90dB and low end of 40dB. And 90dB measured at 3 feet would be 43dB at 686 feet. Obviously this number is largely dependant on background noise, air density, wind, etc. But there would not be much background noise once humanity is gone. What is really disturbing is that the same 90dB would still be a very audible 24dB a mile away.

How long would it take for the zombies to move one mile apart? 9 days. And that is if they carry on in straight lines for the full 16 miles every day. If they move slower because of terrain, or deviate from their path, it could be even longer.

And don't forget that every day, more zombies will leave the city, wandering aimlessly just as the zombies on the first day did. So behind the first zombie will be another one. How far behind?

Remember the "all directions" that I stated earlier? Well no city can be built on a completely flat, traversible piece of land. The terrain dictates which directions a zombie can take. Hills and mountains, rivers and cliffs all have the potential to block their path. Would a zombie follow a road rather than wander through a field? Possibly. But those 360 degrees from the earlier example suddenly because a lot fewer. Maybe as few as 100. But the number of zombies doesn't decrease because of this. They simply get funneled into taking the same routes.



So let's take those 800 zombies, and restrict them to just the 100 degrees. And rather than have them all leave at exactly the same time, let's distribute them a little more evenly. One zombie leaves every 1 minute, 48 seconds, taking 0.125 degrees from the last one. The maths is a bit more complex here, but after 24 hours of the first zombie leaving, how far away is the zombie nearest it? 217 feet. And the next zombie is 217 feet away from that one. And the moan of 90dB? 52.81dB for the zombie hearing it.

How long would it take for these zombies to be a mile away from each other? 30 days or more. By which time they have travelled 480 miles from the city.



The numbers get more complex the more terrain you throw in the way, but essentially every obstacle will strengthen the Chain, reducing the directions a zombie can travel and forcing them to take the same paths. Rivers are the worst. Assuming zombies don't feel inclined to swim or otherwise try and cross a river, they will likely walk along the side of it. If they encounter the river head on, there is a 50/50 chance on which direction they will take. But if they approach it at an angle, they are more likely to continue in a direction similar to the one they are facing. This brings them into line with zombies that are further ahead and behind, reducing the distance between them and strengthening the Chain.

The Chain Swarm

But one or two zombies in a line isn't a Swarm. The issue comes when the moan travels down the Chain, causing the zombies to do two things. The first is to change direction, heading for the source of the moan. The second is to moan themselves, passing the moan down the Chain.

But again, you will be dealing with a couple of zombies an hour until the Chain stops, right? No big deal.

Unless the Chain reaches a large population of zombies, like a city.

Imagine if 8 million zombies, aimlessly wandering a city in search of food, hear a "food moan" coming from one direction. They turn in it's direction, moan, and start walking.

But a single moan won't keep them moving hundreds of miles in a single direction, right? Eventually they are going to go back to passive aimless wandering. Yes, they will. Until another moan gives them motivation to go on.

Where is that second moan going to come from? And the third? Fourth?

Your base.



Here is the scenario. You have established a base that is a couple of hundred miles from any large city. You have a water supply, crops, a guard rotation and a small community. It has been a while since any zombie encounters, and you believe the worst to be over.

Then one day, there is a moan from an incoming zombie. And a little further off, two more. No big deal, you have rifles and a guard tower. The first zombie is dispatched in short order, and fifteen minutes later, two more. The people relax, and the guard feels good about only using 5 bullets for 3 targets despite the lack of practice.

A couple of hours later, it happens again. Two more zombies alert your guards with their moans, then a lone zombie a further way off. This time your guard is 3 for 3. There is a bit of concern, but nothing is really worrying you. The guards are well armed, and you have line of sight all around your base.

But this pattern continues every few hours for the next six days, and shows no sign of stopping. You have more than enough ammunition to contend with it, and you even had the foresight to buy subsonic rifle rounds and suppressors to eliminate the likelihood of gunfire attracting unwanted attention. But still you have the feeling that something untoward is going on. Eight days into this strange event, you decide to venture out with two of the guards to see if you can find an explanation, and hopefully a solution. You travel on horseback, travelling ten miles perpendicular to the direction the zombies are coming in before turning and heading parallel to them.

There is a hill 20 miles out that will give you a good vantage point, hopefully without drawing attention to yourselves. You reach the top and stare along the trail towards your base.

You cannot believe what you are looking at. Wandering along the same route, you see first a handful of zombies, maybe thirty of them scattered along the ten miles to the base. Behind them, what you would have considered until recently a large horde, Fifty or sixty zombies, bunched closer together. And behind them, thousands, then tens of thousands, stretching down the route back to where you know the city is still fifty miles out of sight.



Much like a repeated pulse, each time a zombie encounters your base and moans, that moan is sent back down the Chain. It renews the motivation for those zombies following it, giving them direction and bringing them that much closer to you.

How can you stop it? You need to break the Chain. But you can't just kill your way down the line. As the zombies encounter you they will moan and bring up more zombies behind them. You need to think a little more about disrupting the Chain itself, not just making it happen sooner.

Circling around a Chain and picking off several of the middle links is a good start. The large gap will mean the zombies behind it don't hear the moans of those further ahead, making them go back to passive shambling. But they will still be facing in your general direction, and even if they disperse there is a chance the Chain will reform, closer to your base giving you less warning time.

What you need to do is turn them around, probably not all the way since you don't want to risk running into the Swarm at the back. But certainly 120 degrees away from your base, so that when they go passive they will be facing away from your base, reducing the likelihood they will aimlessly stumble across it. This means first breaking the Chain, letting the front continue on to your base (where hopefully the guards you left will be able to handle it) and you leading the new beginning of the Chain round in a wide circle before outrunning them and sneaking back to your base.

This is not a plan for the risk averse, and yet is the only way that I can see to combat a Chain Swarm that doesn't run the risk of them stumbling into your base at a later point in time.