The parents of Madeleine McCann, the 4-year-old British girl who went missing in Portugal in May, were officially named suspects on Sept. 7 by Portuguese police. The change came after developments in the case, including sniffer dogs detecting the "smell of death" on Madeleine's Cuddle Cat toy and her mother's clothes. They did not, however, find a body. Can you trust a cadaver dog if there's no cadaver?
Not really—especially if a lot of time has elapsed since the body was removed from the scene. Cadaver dogs can find the remains of people who have been dead for years or even decades. But it's much harder for the dogs if the bulk of the remains are gone. In that case, they can pick up the scent from small amounts of body tissue, like a blood stain or nail clippings, or even from materials that came into contact with the tissue. But in the absence of an actual body, the smell of death will dissipate. There's speculation that Madeleine died on the night her parents reported her disappearance—which would mean that she passed away four months ago. It's not clear if a detectable scent could linger on her mother's clothes for all that time.
Researchers are trying to determine how long the scent lingers when the body is no longer present, but there are no conclusive results yet—it may be two weeks, or it may be longer. One former Scotland Yard dog handler talking about the McCann case hypothesized that the scent wouldn't last more than a month.
The dogs couldn't necessarily prove anything even if Madeleine's body had been in recent contact with her mother's clothes. Since they didn't turn up any actual remains, investigators had to rely on the "smell of death" itself, an odor that stems from the decomposition process. Without a body, they can't be certain that the animals didn't make a mistake. Cadaver dogs do mess up from time to time: The McCanns have sought out attorneys who convinced a judge in Wisconsin that certain dogs were accurate just 22 percent to 38 percent of the time. (The prosecution claimed a success rate of 60 percent to 69 percent.)
Cadaver dogs learn to spot the "smell of death" and find its source during the training process, which involves exposing them to either actual human remains—blood, teeth, bones—or pseudoscent, an artificial substance that re-creates the death odor. (One chemical company markets several pseudoscent formulas for training cadaver dogs—recently dead, post-decomposition, and drowning victim.) The dogs also learn to differentiate human remains from animal remains.
A dog's utility depends on the skill of its handler. Identifying false signals is an important part of working with a cadaver dog, and results should be backed up with forensic testing. When a dog gives a signal, such as barking or sitting down, to indicate that it has smelled a corpse, a handler can only say something along the lines of, "My dog is giving an indication consistent with human blood." He can't say definitively that, yes, a body was present, without further confirmation—in the form of a blood stain, for example.
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Explainer thanks Maria Claxton of the South Carolina Search and Rescue Dog Association, Larry Myers of the Auburn University College of Veterinary Medicine, and Andrew Rebmann of K9 Specialty Search Associates.