"Say, someone’s been murdered and dumped in a rubbish bag," says biologist Leonie Bergwerff. "The bag has been taped shut and the police discover cat hairs attached to the tape. The victim didn’t own a cat but a potential suspect does." The police can decide to send those hairs to the "Non-Human Biological Traces area of expertise", a department of the Netherlands Forensic Institute (NFI), where Bergwerff graduated and now works.

There, a whole team of biologists and scientists in related fields examines Nature’s "silent witnesses", ranging from grains of pollen on someone’s car roof and algae on someone’s clothes to cat hairs – anything that could lead to a crime being solved. Bergwerff and her colleagues are responsible for making sure the answers are found.

"If you own a cat, you can’t stop its hairs sticking to you", says the biologist. "How often do the police ask us to check out cat hairs? At the moment, about thirty times a year, but that was before we had published anything on them." That is about to change: the science journal Forensic Science International: Genetics is about to publish Bergwerff’s paper on the hairs. "Mind you, not all thirty cases involved murders", she stresses. "Some involved robberies or animal cruelty."

When you pull out a cat hair, there is a follicle at the bottom which is composed of cells. The nuclei of those cells contain DNA, the genetic material that the cat inherited from her mum and dad. Current DNA technology is sufficiently advanced to make it easy enough to find out that the follicle belonged to Mr Fluffy, the suspect’s beloved pet. Regrettably, such lovely informative hairs are rarely found at the crime scene.

"The hairs that are retrieved have almost always fallen out, they’ve not been pulled out", continues Bergwerff. "And the follicle is lost." As a result, the NFI has to make do with the next best thing.

The fallen hair contains another type of DNA: the cell nucleus is surrounded by mitochondria, tiny cell organs that are responsible for the cell’s energy management. Those mitochondria also contain a tiny bit of DNA, mitochondrial DNA (mtDNA), which can be extracted from the hairs and then analyzed.

Humans and animals do not inherit that type of DNA from both parents, only from their mothers, so you, your mother, your brothers and sisters, your granny, your cousins and other relatives stretching back a good many generations on your mother’s side all have the same mtDNA. That means that mtDNA can’t be used to link a hair to one specific person or one specific cat.

Bergwerff adds: "Even if the retrieved mtDNA doesn’t match that suspect’s cat’s DNA, for example, you can learn something. And if there is a match, you can then ask: how rare is it?" Some variants of mtDNA are more common than others.

It’s Bergwerff’s task to find out how those variants are distributed. "The United States and England have already built databases, each one studying one specific piece in the mtDNA chain. But we’re examining several pieces. We have compared how what we have found matches the information in their databases. But there’s too much difference: we can’t use the foreign databases here. The Dutch feline population differs genetically to that of the feline populations in America and the United Kingdom."

The NFI’s cat-based strategy has identified six main categories, subdivided into 37 different mtDNA-variants. To find out about their occurrence, the institute has collected a database of Dutch kitties. Bergwerff explains: "The NFI staff swabbed their cats while I visited cat breeders to collect samples of the different breeds."

The proportions of the occurrence of those mtDNA variants in the Netherlands are clearly different to those of other countries. The most common variant occurs in as much as 54 per cent of Dutch cats. Other variants, called "haplotypes" by biologists, are rarer: one in a hundred cats has one of eighteen other haplotypes.

Consequently, the researchers can’t say whether the hair came from Mr Fluffy – or possibly Mr Fluffy’s identical twin – but they can establish that the mitochondrial DNA in the hair belongs to the haplotype NL-C5: the same as Mr Fluffy’s. And that the haplotype only occurs in 0.9 per cent of Dutch cats. Bergwerff adds: "That information could be used as supplementary evidence in court."