January 26, 2023

Pet cats on the case: these pets trap human DNA in their fur, allowing for suspect identification

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Cats might be able to help police pinpoint crime suspects, according to new research. It won’t be because these pets are especially forthcoming with what they’ve seen, but rather, because their fur can retain the DNA of people that have been in close proximity to the animals.

Pet Cats On The Case: These Pets Trap Human DNA In Their Fur, Allowing For Suspect Identification
Image credits Mabel Amber.

New research explains that the fur of cats can retain enough material shed by the people who have shared a space with the animal, even fleetingly, to allow for DNA to be recovered and help identify the individuals. Due to this, they could play a valuable role in helping identify criminals in certain cases.

The paper is the first one to look at how pets can act as vectors of DNA transfer, meaning that the results will likely invite more detailed research and conclusions in the future. But even at our current stage of research, these animals provide a reliable mechanism that forensic science can employ to help real-life police investigations.

Special investigators

“Collection of human DNA needs to become very important in crime scene investigations, but there is a lack of data on companion animals such as cats and dogs in their relationship to human DNA transfer,” says forensic scientist Heidi Monkman of Flinders University in Australia, first author of the paper. “These companion animals can be highly relevant in assessing the presence and activities of the inhabitants of the household, or any recent visitors to the scene.”

Our understanding of genetics has come a very long way since its earliest days. So, too, has our ability to recover, process, and interpret genetic material from living (or once-living) cells. Modern equipment can pick up even tiny traces of DNA for medical, archeological, or forensic science.

The trick, then, is how to reliably obtain such genetic material in the first place. Luckily for forensic scientists, human beings leave quite a rich trace of DNA wherever they go. Even brief physical contact with an object can leave traces of genetic material on its surface, in the shape of “touch DNA”. While such material is usually not enough to directly identify a suspect by itself, it can be valuable in conjunction with other evidence by, for example, helping to rule suspects out.

But, despite its name, touch DNA doesn’t necessarily require an individual to actually touch an object for a transfer of genetic material to take place. Such evidence can also be transported by a number of other means, such as shed skin cells or strands of hair. The thick fur of pets is an ideal trap for such material, in the eyes of the authors.

Together with Flinders University’s Mariya Goray, an experienced crime scene investigator, and forensic scientist Roland van Oorschot of the Victoria Police Forensic Services Department in Australia, Monkman set out to check whether we could realistically extract those DNA traces from the fur of pet cats.

The team worked with 20 cats from 15 different households. They swabbed the fur on the right side of each cat twice (at the home of the participants), and collected DNA from most of the human participants — one participant, a minor child, was not sampled. Additionally, each household filled out questionnaires on the cats’ daily behavior and habits, including how often the cat was touched and by whom.

Roughly 80% of the cat swab samples showed detectable levels of human DNA. There was no significant difference in the amount of DNA present among the samples. The length of a cat’s hair didn’t have any bearing on this quantity of DNA, nor did the amount of time since the cat was last in contact with a human.

The team was able to generate DNA profiles that were complete enough to be linked to a particular human from 70% of the cats. Most of these were individuals in the cat’s own household, but 6 cats harbored DNA from unknown individuals (that were not part of the households). Two of these cats did spend a lot of time each day in the bed of the child whose DNA was not sampled, which could explain where part of this unknown DNA came from. But that still leaves 4 cats that had the genetic material of unknown persons on their fur; none of their households has had any visitors for at least two days prior to the swabs.

Among the cats, the team highlights one particularly interesting case: a two-person household that had two cats. One of them, a hairless sphynx, carried DNA from an unknown human. But the other one, a short-haired ragdoll, did not. Both of the cats had roughly equal interactions with all individuals in their household, and the source of the third individuals’ DNA is unknown. Some of the possible sources that the team is taking into consideration is direct transfer from an unknown individual through patting, or indirect transfer from the cat brushing against a contaminated surface. In other words, it is possible that the DNA present on the cat’s skin has been there since the last time it came into contact with a visitor.

“The mode of transfer of this DNA to the cat, and its persistence on them, is unknown,” the researchers write. “Further research is required on the transfer of human DNA to and from cats, and the persistence of human DNA on cats and what may influence the varying levels of DNA found on cats such as behavioral habits, and shedder status of the owners.”

On one hand, such research could help untangle some confusing cases in the future. But it also makes you wonder: just what are our cats doing behind our backs that they’re getting covered in DNA from unknown people?

The paper “Is there human DNA on cats” has been published in the journal Forensic Science International: Genetics Supplement Series.