If you need to find a corpse, there are lots of ways to look: you can comb the woods in a line search or hunt for hidden graves with ground-penetrating radar, for instance. In most cases, though, the most versatile and reliable method has four legs and a wet nose. No man-made machine can reliably identify the odour of decomposition, but properly trained Human Remains Detection (HRD) dogs can.
However, scientists aren't sure exactly which chemicals make up the scent that HRD dogs recognise. A decomposing human body releases 478 different chemical compounds, and researchers are still trying to figure out which ones really matter to HRD dogs. Learning the answer could help improve canine training, which could help find crime victims and missing persons.
Whatever the chemical signature is, it's present through the whole process, from fresh corpses in the first few hours after death to skeletons several years old. It's also present in several types of tissue, including blood, bone, and fat. Properly trained HRD dogs can identify the scent not just in whole bodies, but in blood spatter, bone, and even cremated remains. They can even pick up the scent left behind in the soil after a body has been removed from a grave.
The signature scent of human death is also unique to humans. Trained HRD dogs can tell the difference between human remains and animal remains.
Of course, in order to recognise and find the scent of human remains in so many contexts, HRD dogs need a lot of training, including practice finding human remains in as many forms as possible — from fresh blood to old, dry bone, and from ashes to whole bodies — so they can recognise remains on a real search, no matter what shape the body is in.
So, how do they do it?
What's That Smell?
Two of the best known chemicals are cadaverine and putrescine, chemical compounds produced by the breakdown of amino acids during decomposition, but they tell only a fraction of the story.
A 2004 study by Arpad A. Vass at the University of Tennessee Anthropological Research Facility — better known as the Body Farm — sampled chemicals in grave soil and in the air just above graves. Vass and his colleagues found over 400 compounds, but no measurable amounts of cadaverine or putrescine. These two compounds don't seem to be a key part of the scent of death, after all, at least not at every stage of the process.
Faking the Scent of Death
But for years, scientists thought these chemicals were the source of the distinctive scent of human remains. There are even synthetic versions on the market, which are still used in some HRD canine training, despite a lot of debate among canine handlers.
In order to learn to find something by scent, dogs need to practice with the real scent. That's why, for example, handlers who train drug dogs are allowed to work with actual narcotics for training. So, to learn how to find dead people, HRD dogs need to practice on — you guessed it: actual dead people, or body parts. Getting access to real body parts is, predictably, not easy. That's why "pseudo-scents" like synthetic cadaverine and putrescine are still so popular; they're a lot easier to come by, but science seems to indicate that they're not actually a good substitute for the real thing.
Is It In Our Bones?
Bone may hold at least part of the answer. A 2008 study at the Body Farm compared chemical vapours given off by bones from humans, dogs, deer, and pigs, and found that each type of bone produced noticeably different ratios of certain classes of chemicals. Researchers said that these scents probably contributed to the overall scent of decomposition, but it's not yet clear whether these same compounds could are present in, for example, blood or ash. That research followed a 2006 study of soil from the campsite of the ill-fated Donner Party, which found that phosphates may be a measurable by-product of the breakdown of human bone.
Vass and his colleagues have done a series of studies on decomposing bodies, from 2004 to 2012, and the results make up the Decompositional Odor Analysis Database. It's safe to say that science is still trying to sniff out the answer.
Meanwhile, although the exact mechanism isn't yet understood by humans, it's very clear that correctly trained HRD canines can reliably find human remains, from hidden graves to disaster zones.