There are always mold spores wherever you go and if they land on something they can eat, they start growing. Normally an organism such as a rat has an immune system that will kill any bacteria, fungi, or viruses that attack it. However, once an organism dies, it can no longer fight off invaders and it becomes the food source of whatever gets to it first.Dead animals are the perfect food source for bacteria and fungi because all of the molecules they need are “pre-made” by the dead animal so they can get them easily. The special thing about molds, which are a type of fungus, is that they specialize in eating dead animals and plants. In this way, they are basically the garbage-men of the living world.

The reason that it takes a few weeks is that it takes a while for the mold to grow big enough that you can see it. You can only see the mold after its “full-grown” since the green color is from the part of the mold that sends spores into the air.

I’m surprised it took that long for the dead rat to get moldy.

We are hosts to all sorts of tiny life forms, bacteria, fungi (including mold), viruses, etc. There are actually more bacterial cells in and on your body right now than your own cells. While we’re alive, our immune systems keep all of these other life forms in check. Sometimes our immune systems can’t keep up and we get sick.

After we die, there is no security system to keep all of these other life forms in check. They can just take over. They can even be joined by decomposers—living things that eat dead stuff. Flies lay eggs that hatch out as maggots and eat the body. Beetles eat the decomposing bodies and lay their eggs too. Vultures aren’t usually interested in something as small as a rat, but eat larger carcasses (dead animals).

If conditions are warm and moist, things can break down really fast. If the environment is very dry, the decomposers can’t do their jobs. Carcasses can dry out and become mummies.

Anyone who was not grossed out by this may want to explore how biology is useful in fighting crime. Specialized biologists study how bodies decompose under various conditions so that they can determine time and place of death. It just may be the career for you.

We are surrounded by microorganisms – bacteria and fungus spores smaller than we can see, on surfaces and in the air we breathe. These microorganisms are typically dormant until they find the food and water they need to grow and reproduce. A rat (and any other plant or animal, including you) has defenses that keep these bacteria and fungus from growing on them. Some are passive, like tough skin and hair that make it difficult for microbes to grow. Most plants and animals also have friendly bacteria that live on their skin and fight off other microbes that might harm them. Some defenses are active, like the white blood cells in your immune system or fevers that make it difficult for bacteria or fungus to live inside the body.

When a rat dies, it no longer actively defends against these microbes, which begin to break apart cells to get at the water and sugars inside. Because the rat’s heart is no longer beating, blood is no longer transporting oxygen throughout the body, so enzymes and wastes build up in the tissues and speed the break down of cells. As more water and nutrients are released from the cells, more microbes can grow – the most colorful and distinctive of which are fungal molds. For more information, check out:

Spores of fungi land on the rat and grow inside of it, eating the dead body from the inside. The mold you see are their spore-producing organs to colonize other dead matter.