Here are five possible reasons why dogs sometimes like to roll around in filth such as rotten organic matter. They are essentially theoretical reasons because even the experts have to speculate as to why dogs are compelled to behave in this apparently strange way. You have to relate the behaviour to that of the dog’s ancestral wild animal: the grey wolf. I’m relying on two experts namely (1) Dr Desmond Morris the world renown ethologist and zoologist who has written many books on animal behaviour including in this instance Dogwatching and (2) Dr Stanley Coren PhD DSc FRSC who writes for Psychology Today. Dr Coren provides the biggest list of possible theories.
The picture below
Before I list the five theories, it’s worth noting that in the picture below the dog is rolling around in a sandy/muddy earth which is not quite the same thing because it is hard to find photographs of dogs rolling around in filth. It seems to me that when a dog rolls around on this sort of substrate it is more to do with the pleasurable feeling that it induces rather than a throwback to grey wolf behaviour as discussed below.
He starts off with the theory that rolling around in filth deters skin and coat parasites that live on the body of a dog. It doesn’t really work as a theory if the dog is rolling around in poop or the carcass of a dead animal. The theory might carry some water if it was referring to a dog rolling around in dust. Dust might help to remove parasites. But the smell of a rotting carcass does not deter but may attract insects.
As an aside, my cat likes to roll around on a textured surface such as my driveway which has become dusty during the summer. I think he does that to (1) remove parasites (he has no parasites!) and (2) because it feels good. Also elephants roll in mud to to protect against insects and the sun.
The second theory concerns how rolling around in filth or smelly objects might be pleasurable. In other words it feels good because it smells good. Dr Coren refers to the fact that humans rely predominantly on their vision while dogs rely predominantly on their sense of smell. Because of this canines roll around in obnoxious smelling organic matter because it provides a huge olfactory experience in the same way that humans can wear overly loud clothes. This theory seems to be the most promising because Dr Desmond Morris refers to laboratory experiments in which dogs rolled on a wide variety of strong-smelling substances including those that they would not find in the wild if they were grey wolf such as perfume, tobacco, lemon rind and so on. In doing this it negates the other theories about camouflage and a hunt-incitement theory. He refers to dogs going into a sort of “odour ecstasy”. It doesn’t matter that the nature of the substance is nothing to do with survival. This supports Dr Coren’s theory under this heading and is perhaps the favoured theory.
Masking body scent for hunting purposes; camouflage
A third theory is that dogs are masking their natural body odour with the smell of a rotting carcass so that they can approach prey animals without notifying the prey animal of their presence. A form of olfactory camouflage. If a wolf rolls around in the dead carcass of the prey animal of a wolf then the wolf can approach very closely. Therefore the attack will be made more efficiently and more successfully.
Masking the smell of the object
A fourth theory suggest that a dog wants to deposit their scent onto the object in which they are rolling around to mask its smell. Something similar takes places between cats and humans called scent exchange. In rubbing against us a cat deposits their scent and receives the scent of the person against whom there rubbing. This creates a reassurance but I don’t think it has any relevance in respect of this aspect of dog behaviour. It is a poor theory because the smell of rotting organic matter is much stronger than the scent of the dog and therefore it cannot be masked in this way.
Bringing a message that there is food out there; hunt-inducement
The fifth theory suggests that it’s a throwback to grey wolf behaviour when one wolf rolls around in the dead carcass of a prey animal, brings that smell back to the pack to inform them that there is food to be eaten. The wolves of the pack smell the returning wolf covered in this ghastly smell (and delight in it) and decide that the carcass in which the wolf rolled around is edible and therefore the pack goes off to eat it. It may also signify the presence of living animals which they can prey on. In other words the individual wolf is bringing back a message that there is food to be eaten. And as far as I know there is no empirical data to support this theory.