The Scent of Fear
I get a lot of questions from folks regarding dog searches so I figured today would be a good day to address one big difference between search methods. There are two basic types of "people" dogs can search for; living or dead. The training is different for both. That's because the "evidence" or scent the dog is searching for is different between living and dead victims. NecroSearch International and it's dog handlers have been investigating this issue for 25 years and other handlers have been going much longer. Dog tracking is an old "art" and we've learned a lot over the years. The one things we don't know with certainty is the exact molecules the dogs rely on most. That's for one simple reason. Dogs can't talk. That's okay though because we still learn a lot from working with well trained dogs and handlers.Each handler trains their dog to give a specific "alert" when they find the scent they are looking for. One famous handler I have known over the years trains his dogs to stop and sit.
When searching for living persons the dog is tracking their unique "scent". Millions of skin cells are sloughed off every day in our various activities and a well trained dog (like a bloodhound) can tell the difference between each person's "scent". To begin these searches the dog handler will introduce something from the victim (like an article of clothing) for the dog to acquire the scent. This item is called a scent article. A well trained dog may then follow that scent trail (sometimes many miles) to the source or until it becomes too degraded or diffused in the environment. The key aspect of this search is that the dog is searching for a unique "scent" given off only by the one person being searched for.
Searching for a deceased body is an entirely different process. When searching for a deceased person the dog is looking for "decomposing human scent", not the scent given off by a living person. Put simply, this is because the cells are breaking down and decomposing (not vital). This means that knowledgeable investigators would never use a scent article to acclimate the dog. There are several challenges to this type of search. The main problem is that the dog is looking for any decomposing human scent. This means that a dog will alert on any human decomposition fluid or tissue. So in addition to dead bodies they will also alert on things like urination spots, feces, and baby diapers. This is a huge concern in rural areas and trash dumps because investigators may have dozens of "alert" areas to investigate. The scent can persist for months or even years in some cases.
Another very interesting phenomenon is that dogs can not differentiate between the decomposing scent of humans and that of pigs. Pigs have been used for years as human surrogates in a variety of scientific studies. Pigs have a very similar biochemistry, skin to tissue ratio, and are large (relatively hairless) mammals like people. So if a body is believed to have been buried at a pig farm we have to use other methods to detect the body. This conflict isn't very common but it happens and police need to be aware of the issues surrounding these searches.
If you are writing a scene of a dog search it would be wise to understand the basic difference between these searches. The easiest way to remember is that living persons can be searched for specifically. In those searches your character will need some kind of scent article like a piece of clothing, hairbrush, or bedding (if they sleep alone). If your characters are looking for a dead body they will likely find a lot of red herring alerts. These are frustrating because the dog will give a "positive" hit but excavation of the soil may not reveal anything (such as in the case of public urination). You may be able to use that to your advantage in the story so think of how dozens of these "false hits" may affect morale. There is only so many baby diapers you can dig up before questioning the wisdom of getting out of bed that morning :).