What's the News: You might think that identical twins have an advantage when it comes to crime---with the same DNA, who could tell them apart? But new research with a squad of scent-trained Czech police dogs reveals that even identical twins have their own individual smells, even if they live in the same house and eat the same food. How the Heck:

Scent line-ups for identifying suspects are regularly used in the Czech Republic, Russia, Denmark, the Netherlands, and several other European countries. Trained dogs are provided with a scent from the scene of a crime and then sniff out the matching scent from sweat samples taken from suspects.

The researchers took sweat samples from two sets of identical twins and two sets of fraternal twins (whose status they verified with DNA testing), as well as plenty of samples from unrelated children. All the samples were taken in the same room and with the same scientist present, so background odors wold cancel out.

Then, 10 trained German Shepherds, police dogs used solely for identifying suspects by scent, were put through a total of 120 scent line-ups by police officers who had no knowledge of which pads were which.

In every single trial, the dogs correctly identified the individuals they were seeking, even when an individual's identical twin was also in the line-up.

What's the Context:

Previous studies investigating whether dogs can tell twins apart were somewhat haphazard, some using untrained dogs or police dogs that were used in a variety of capacities, not just for scent line-ups, or very few dogs. The technique hasn't caught on in many countries because of a lack of scientific back-up, but the researchers think that this study provides a more rigorous proof of the procedure.

What are the dogs smelling? The major histocompatibility complex (MHC), an area of the genome that codes for immune system proteins, is linked to people and animal's ability recognize each other by smell---in a well-known 1995 study, young women overwhelmingly preferred the aroma of sweaty T-shirts of men who had different MHC profiles than they. It's possible that what the dogs are sniffing is some variation in the expression of those proteins, though this study doesn't attempt to address a biochemical mechanism.

The study suggests that even very small differences in environment or experience, such as those experienced by young identical twins in the same household, can lead to biological differences---another mark against DNA determinism.

The Future Holds: The youngest twins were only five years old, indicating that even people who have the same DNA and the same surroundings begin to diverge at a very early age. The scientists suggest that future studies could look into when this starts to happen, with an eye to discovering the mechanism. Reference: Pinc L, Bartoš L, Reslová A, Kotrba R (2011) Dogs Discriminate Identical Twins. PLoS ONE 6(6): e20704. doi:10.1371/journal.pone.0020704Image credit: PLoS One