Rats rely on their ability to squeeze through narrow gaps for survival. Without this skill, rats wouldn’t be able to access food stores or escape from predators as effectively as they do. Of all the tricks rats have up their sleeves, this is one of the most impressive and essential.
While rats can grow to a much larger size than mice, they are still relatively small in the grand scheme of things. However, they can pass through narrow horizontal gaps that look as if they should be far too tight for them to squeeze under. How do rats pull off these seemingly impossible feats? The answer can be found in their skeletons. If you ever see an x-ray of a mouse or rat skeleton, you will notice some unusual features. Rat skeletons essentially look like long cylinders; they look quite different from how most people assume they should.
But it isn’t just the shape of the rat’s skeletons that’s important; it’s what they can do with their bones. Like mice, rats have an impressive ability to compress their skeletons, enabling them to collapse their legs against their bodies or splay themselves out completely flat. Evolution has provided rats with the adaptations they need to make their way through incredibly narrow gaps. Contrary to what many people think, rat ribcages won’t collapse like a coil; they still need to protect their internal organs, and they have to be able to breathe while navigating narrow spaces.
Their whiskers and long hairs on the snout enable rats to determine whether they can fit through a gap or not. This is why you often see them poking their heads through an entrance before deciding whether to go through.
The exact size of a hole that would be too narrow for a rat to get through varies according to several factors. But under optimal conditions, an adult rat can get through a 12mm horizontal gap or 25mm round hole.