International Rescue dog Group

5311AD, Gameren, The Netherlands.




This note is written in order to bring more clarity in the discussions about scent discrimination among dog handlers who train dogs for various kinds of SAR work. It represents the result of a study of the existing literature and of the discussions we had with other dog trainers as well as our own experience in training search dogs. We trained dogs for disaster and area search as well as for tracing. Also we have experience in working with wolf-dog hybrids (wolf x german shepherd). The theory presented here is not a final stage but a reasonable frame work to use in training dogs for search and rescue work.

We will restrict ourselves to the work of search dogs which are trained to search for humans, living our dead. The training of all these dogs has one thing in common:


They learn to work on a collection of human scents.


It is this collection of scents which will be dealt with very thoroughly in this note. The discussions about the discrimination problems of rescue dogs can be reduced to the discussion about what scents the dog is working on.

In order to deal with this subject we choose for the following approach.

In Section 2 we tell something about the scent discrimination of the wolf and wolf-dog hybrids we trained. Next, in Section 3, we discuss some general aspects of scent discrimination of all kinds of search dogs such as dogs trained to locate explosives and drugs. In Section 4 we introduce the concept of the scent complex and give some complexes that are of great importance to the search and rescue work. Also we deal with some problems we met in training SAR dogs.

In Section 5 we will deal with the differences in scent complex between dead and living victims. This also turns out to be of great importance to training of the SAR dog. In the last section we will give some conclusions that follow from the theory developed in this note and the consequences this theory has for the training our SAR dogs.



This section is introduced in this note because we all know that according to the actual state of science, scientists believe that wolves in general are the ancestors of our domestic dogs. For the wolf-dog hybrid we know for sure that it has a certain percentage wolf blood. Studying the hunting behavior and scent discrimination behavior of these animals can learn us things about the dog which can be of great importance in the training of rescue dogs. Lets first have a closer look at the hunting and scent discriminating behavior of a wolf. Of course we will restrict ourselves to those aspects which are of importance for the problems we are dealing with. For more extensive discussion see D. Mech [3] and Bekoff and Wells [2].

Wolves start to hunt when they are hungry. They search for live animals mainly although they also eat dead prey in case no live prey is available. When they search for prey and there is a wind blowing, they mostly scan the wind in a ±900 angle or search strait on the wind for traces of scent from the prey. In this scanning they can easily recognize whether the scent is coming from the prey or from another wolf or from man. You can see this directly from the body language of the wolf (excitement, suspicion). If there is no wind blowing they search for traces on the ground and on bushes or little trees. In doing this they can also recognize what kind of animal left the trace. From this we can learn two things. Wolves can discriminate between scents coming from man or animal and they can discriminate between scents coming from dead or living prey.

From our own experience we have seen wolf-dog hybrids that could work out a really old (10 hours) track which was left by a human. Also we noticed that the hybrid was still recognizing the tracer. This recognizing could be seen from the friendly tail wagging when the tracer was familiar and from the suspicious behavior when the tracer was not familiar. Also we have seen wolf-dog hybrids discriminating very sharply between a living or a dead prey. In case of live prey they prepare directly for the hunt and the kill (great excitement), whereas in case of dead prey they only prepare to secure and eat the prey. The difference in behavior can directly bee seen from the body language.

From all this we drew the following conclusion.

The wolf and the wolf-dog hybrid can discriminate between scents produced by different humans and he can discriminate between living and dead prey.


Here we will give a small survey of all kinds of search dogs and tell something about the scent the are trained on.

We start with the dogs that are trained very strongly on one isolated vapor produced by one particular substance, as for instance the dogs working at the customs for locating drugs or the dogs trained for locating explosives. The dogs for this kind of work are trained to locate one particular vapor out of a collection of vapors he is smelling.

The police in Holland has dogs which can identify whether an object found at the scene of the crime which was handled by the criminal (gun) carries the same collection of scents as one of the persons (suspects) lined up. See also De Bruin [1]. So what these dogs are trained for is to compare a particular collection of scents with other collections and select the matching ones.

Hunting dogs are trained to discriminate between various kinds of prey. In general these dogs do not work on an isolated vapor but on a scent out of a collection of scents.

Rescue dogs are trained for all kinds of work. For instance, searching for humans (dead and living) in a disaster area, searching for humans (dead and living) in a forest, searching for dead bodies in general, or searching for drowned people. Also they are trained for tracing and tracking in order to find a missing person. Mostly rescue dogs are trained to perform more than one task and sometimes a SAR dog is trained to do all the possible jobs mentioned above! From the discussion of the search dogs other than SAR dogs we come to the following conclusion.

In order to work properly the SAR dog must be trained to work on a particular scent or some scents out of a collection of scents.

What scent(s) we choose depends on the kind of rescue work the dog is trained for. For instance a dog which is trained to search only for dead bodies it will work on a different scent, than the dog which is trained to search only for living only. Our experience is that the rescue-dog trainers and handlers do often not realize the fact that a dog learned to search for drowned persons should work on a different scents than a dog trained to search for living persons in a disaster area. Combining these two types of work without paying special attention to the possible overlapping scents, can thus lead to problems which are not properly recognized and consequently not properly dealt with (false alerts in a disaster area on recently worn clothes and recently used beds).

It is the purpose of this note to bring more clarity to the possible problems a trainer of rescue dogs can meet.



In this section we define the most important concepts used in the sequel. Many misunderstanding is caused by bad definition of the words used. It is therefore of major importance to focus the discussion on essential points and not to let it get stuck in a discussion about the meaning of words.

At first we will tell what we actually mean by scent and give a definition. Then we will define the important concept of scent complex and give tables of the most important scent complexes met by a SAR dog. Some of the definitions given in this Section may sound formal but for a good understanding of the discrimination abilities of a dog it is absolutely necessary to know the facts dealt with in the sequel.

We can now pose the question what a dog actually smells. What he actually smells we do not know, what we however do now is what our dogs inhale. Our dogs inhale scents.

From the physiology of the dog nose it is known that scents inhaled come in touch with slimy layer inside the nose of the dog. It is this slimy layer that brings about the contact between the substances inhaled and the nerves in the nose and consequently cause the detection. How this contact is actually established is not yet known. We do know that the substances getting in contact with this layer are solved. However how this solutions brings about detection is not yet known. The last theory about this contact is that it works as a lock and corresponding key. This means that only a limited amount of substances can be smelled, namely only those substances for which a lock is available. This could also explain why several substances are not smelled although they produce scents.

We now give the formal definition of scent.


Scent is a mixture of substances in a gaseous state (vapors).


As all substances this mixture consists of molecules, which are the smallest parts of a substance still having the physical and chemical properties of the particular substance. Because we are talking about a mixture it will be obvious that there are several kinds of molecules in scent.

Very obvious examples of scents are for instance the smell of sweat, perfume, soap, the smell of rubber (rain) clothing etc.

We all know of course that not all substances are producing equal amounts of scent. Several substances we can smell very easy whereas other substances we can hardly smell.

The ability of smelling a scent depends not only on the amount of scent produced but of course also on the nose capacity of the creature smelling. Making for instance a comparison between man and a dog we see that a dog has 225,000,000 nerve cells on a surface of 150 cm2, whereas a man has 5-7,000,000 nerve cells on 5 cm2. From these figures it should be obvious that a dog has a far greater capability in smelling scents than man. For more details see De Bruin [1]. We now give a formal definition of a scent complex.


A scent complex is the collection of scents on a particular location.


This definition contains a restriction regarding the location. This is for the training of the rescue dog of great importance as we will see in the sequel. An example of a scent complex is for instance the collection of scents met by a rescue dog trained for water search.

After this rather abstract definition we will turn to a more appealing presentation of the scent complex.

In table 1 we give a quite general scheme of a human scent complex. This scheme includes the scent complex of the most common types of work done by rescue dogs, i.e., rubble search (living and dead), area search (living and dead) and tracing.


Table 1














Biological scents from victim


Scents attached to the skin




Scents from the surroundings





breath, vaporized urine, anal gases, sweat drops, hair, skin pieces, scents from bacterial metabolism, etc


Perfume, soap, shampoo


Rubber, plastics, synthetics, etc.


Bricks, concrete, iron, stones, grass, sand, forest, water, dead animals



The complex given in table 1 is quite general. The distribution of the categories I-IV over the scent complex varies a lot depending on all kind of circumstances. For instance the complex belonging to the search for a dead person will be different to the complex belonging to the search for a living person. Also the surroundings where the dog has to perform its work are of great importance! One has to think about the differences between the scent complex belonging to an area search and a rubble search! Hence it is necessary to make a subdivision of the complex presented in table 1. We start with a subdivision between tracing and rubble search.

The specific scent complex of a trace left by a human is given in table 2 below.



Table 2














Sweat drops, sweat from the feet,

hair, dandruff skin pieces, etc


Only small amounts of soap, perfume, shampoo


Rubber, plastics,

synthetics, leather pieces, etc.


Broken grass, disturbed sand,

broken leaves, dead animals, etc



The general scent complex of a victim (dead or living) in a disaster area (earthquake) is given in the table 3 below


Table 3














Sweat- vapor, breath

vaporized urine, anal gases, hair, dandruff, skin pieces, scents coming from bacterial metabolism, etc


Perfume, shampoo, soap


Rubber, plastics,

synthetics, leather pieces, etc.


Bricks, iron masonry

food, toilet odors,

dead animals, etc


The first conclusion that can be drawn from these tables is that in order to do his job properly a rescue dog, no matter what his specific discipline is, should be trained on scents from category I, the biological scents from the victim.

To make this more plausible we will discuss some problems we met in training search dogs. We will start with a problem we often met in the training of tracing for the Schutzhund or IPO program in Europe.

Problem 1. A dog that is trained to do tracing only on grass, which occurs very often in Holland with its nice meadows, turns out to work quite bad in Hungary on the European championship where it has to work on bare sand!


Analysis. Obviously this dog is so often trained in the meadow that it works on the scent of broken grass of category IV. If this dog is trained on the biological scents from the tracer (category I), which are always present (under the circumstances of the schutzhund program!) no matter what the soil conditions are, then this problem would not have occurred!


Also these tables give us already an indication what problems can occur if we train a dog for two or more disciplines even if we train him to work on scents from category I.


Problem 2. A dog that is trained to do tracing by training him on the biological scents from category I of table 2, is giving false alerts on for instance clothing or recently used beds.


Analysis. Obviously this dog is trained on those biological scents that both table 2 and 3 have in common, i.e., dandruff, skin pieces, sweat from feet, fatty acids. It will be obvious that these biological scents also can be found on clothing and recently used beds etc. To solve this problem one should search for those biological scents that table 2 and 3 have not in common. In case the dog is trained to search for living victims this is not to difficult, however in case the dog is trained to search for dead victims this is not easy but not impossible. We will discuss this later in more detail.


Last but not least we will as a matter of introduction of the next section discuss a problem very often occurred in the work of rescue dogs when they work on both dead and living victims!


Problem 3. A dog that is trained to do water search (dead victims) by training him on the biological scents coming from hair and fatty acids, is giving false alerts on object that were touched recently by persons, clothing or recently used beds.

From this it follows that in order to solve this problem we should make a distinction between the scent complex belonging to the search for dead victims and the scent complex coming from living victims. Therefore it is necessary to divide the scent complex of table 3 into two scent complexes, one belonging to living victims and one belonging to dead victims!!


It is obvious that the problem stated in Problem 3 is a serious one! This can cause that rescue workers start digging on a place where there is no victim and perhaps squeezing the real victim because of the heavy machinery used or because of the movement of heavy rubble! We will analyze problem 3 in detail in the Section 5.



Before we give the tables with the scent complexes, we will start by stating some facts about a living victim. Substances which on the body of a living victim are heated depending on the temperature of the body. The temperature on the outside of a living body is constant and ± 270 C. Due to this body temperature the victim will constantly generate a particular biological scent. At this temperature certain substances on the body will vaporize and others will not. Of course volatile substances vaporize sooner that non volatile substances. Also belonging to this temperature there is a particular bacterial metabolism on the skin of the victim.

Last but not least a living person is breathing and sweating.

Table 4 summarizes these facts. We only give the categories I and II as these are of major importance and categories III and IV are the same for both living and dead victims under the condition that they are located in a similar situation.


Table 4 The scent complex from a living victim




I Biological scents from victim


II Attached to the skin of victim


More volatile substances


Sweat, breath, vaporizing urine, bacterial metabolism!


Perfume, strong vaporizing shampoo, chemical substances that are on the victim and that vaporize because of the body temperature.


Less volatile substances


Fatty acids (skin fat), saliva (spit), skin pieces, dandruff, hair, nails


Soap, other less volatile, chemical substances that are on the victim


Turning to the discussion of the scent complex produced by a dead victim we see immediately that there is one very important difference between a dead and a living victim: A dead victim is not breathing nor sweating!

If the outside temperature is lower than 270 then the temperature of the dead body is decreasing at a constant rate of about 0.5 degree C per hour (of course this strongly depends on outside temperatures!). Due to this temperature change, the victim will generate a changing scent complex until a stable temperature is reached. This is the temperature of the surroundings. At this temperature fewer substances on the body will vaporize.

After two days the destruction process of a dead person starts. Again this depends strongly on the outside conditions such as temperature, humidity, etc. In this process bacteria and insects play an important role. There will be a different bacterial metabolism on the victim compared with the bacterial metabolism on a living victim. Anal gases are produced.

If the outside temperature is higher than 270 then the process of destruction will start almost directly again causing the scent complex to change rapidly!

Of course during the process of destruction the scent complex does not remain the same. It will undergo some major changes in time. However to clarify our major point of discussion it is sufficient to restrict ourselves to the facts stated above. For more detailed discussion see N. Sharp [4].

Table 5 summarizes these facts. Again we restrict ourselves to categories I and II.


Table 5. The scent complex coming from a victim that is dead for three days.




I Biological scents from victim


II Attached to victim


More volatile substances


Anal gases, Bacterial metabolism of destruction. Scents produced by insect metabolism.


More volatile, chemical substances that are on the victim.


Less volatile substances


fatty acids (skin fat), skin pieces, dandruff, hair, nails


Less volatile, chemical substances that are on the victim.


Of course as the scent complex follows a trajectory from table 4 to table 5 from the moment of death to three days after death. But one thing is directly discriminating between tables 4 and 5 and that is the fact that at moment of death breathing stops and sweat production stops!


Analysis problem 3. Returning now to our problem 3 of Section 4 it will be clear that a dog trained for water search (searching dead persons) and for rubble search (searching living persons) should be trained during the rubble training on breath, sweat vapor and bacterial metabolism produced by living victims in the rubble. Also this dog should be regularly tested and corrected if it gives false alerts on fatty acids coming from clothing and recently used beds. This requires special attention. The following questions often arise after the previous discussion.


Question 1. Is it possible to train a dog on the search for dead persons and not give false alerts on recently used beds and clothing?  

Answer. In order to give an answer to this question we must look at table 5 in order to find those scents coming from a dead victim that are not on clothing and beds. This leads us to the conclusion that training on the scents produced by bacterial or insect metabolism is a possibility. Another possibility is training the dog on anal gases. However the production of these gases stops after a period of time. 

Question 2. Is it possible that a dog that is trained to search for living persons gives alerts on dead victims whereas it is trained to give no alerts on clothes, beds and other objects carrying human scents?

Answer. In order to give an answer to this question we must look at tables 4 and 5 in order to find those scents coming from a living and a dead victim that are not on clothing and beds. We see that if the victim has died recently, then the bacterial metabolism has not yet radically changed. We already stated that the change in the scent complex follows a trajectory from table 4 to table 5 from the moment of death to three days after death. If the dog has picked the scent produced by the bacterial metabolism out of the complex itself or when it is trained to work on this scent, then it is possible that he gives alerts on recently died victims still producing this scent. This leads us to the conclusion that training on sweat vapor and breath does not exclude this possibility. The only way to be absolutely certain that this will not happen is to test the dog to make a selection between a living victim and human tissue! This human tissue should be of a varying age.



It is my opinion that this theory is not a final stage. However during the years I have been training search and rescue dogs I noticed that in contrast with other disciplines of search dogs such as drugs or explosives the rescue dog handlers do not always realize on what scent complex they actually train their dog! This can cause serious problems such as false alerts in a disaster area. As rescue dog handlers often have to make decisions with great responsibility and it is of major importance that they realize exactly for what kind of work their dog is trained for! Consequently we come to the following conclusions.

-It has strong preference to train a SAR dog for a specific task, with its specific scent or scent complex, and not for a large scale of tasks! This will avoid unnecessary confusion and problems.

-If a SAR dog is trained for more disciplines with a possibility of confusion then we have to pay much attention to this problem in the training of this dog!

-If a SAR dog is trained for a specific task it should not be used for tasks it is not trained and tested for! If the authorities call for help the dog handlers should inform these authorities about the work the dogs have been trained for and can reliably perform!

-SAR dogs trained to search for living victims should be trained with living victims only! Furthermore they should be tested regularly if they do not give false alerts on object carrying human scents.

-SAR dogs trained to search for dead victims should be trained on human tissue only! Also these dogs should be tested if they do not give false alerts on object carrying human scents.



[1] De Bruin J.C., 1988: De Speurhond en de Wetenschap. Rotterdam.

[2] M. Bekoff M. C., Wells.,1980: The social ecology of Coyotes. Scientific American. Vol. 242 no 4, april 1980.

[3] Mech L. D.,1970: The Wolf, The Ecology and Behaviour of an Endangered Species. Minnesota. University of Minnesota Press. Fifth printing.

[4] N. Sharp.,1993: Research into Dead Body Detection. Notes presented on the fifth International Symposium on Rescue Dogs, Sweden 1993.