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Aggressive scent communication

It was a hot summer's day in the city of Oslo. First my holiday guest bit me badly enough to send me to the emergency room. Now Mimer stood wondering why his five friends all of a sudden sported an afro and were rolling around in a tight wrestle. While Mimer was puzzeled, I knew. Faster than you could say "ratfight" I'd separated the five fighting cocks and press ganged my children to carrying one each - those that could not have a cage to themselves. Then I scrubbed the wax tablecloth, and carried an unconspicious brown paperbag outside. A paper bag with scent marks from an unfamiliar, stressed out and aggressive male...

It was a hot summer's day in Oslo. Well, the parking lot was full, and swimming with perforated fingres was not brilliant idea anyway. A shadowed porch, iced water with lemon, quiet contemplation and a portable PC with an internet connection ensured the holiday spirit.
If you want to know what I discovered about smell and aggression, keep reading! And under Squeek's story you can read about the otherwise peacful Squeek who attacked anyone smelling of baby rats.

Perspireing rat, hidden dragon

Our guest had a cage of his own, and for a reason. Nor did I have any plans to introduce him to my group. But out there on the porch it seemed a perfect idyl. He showed no sign of stress by being put in a cage half a meter from six strange rats. A couple of hours after arrival my guest lay prone in my lap in the 27 degree C heat. Labben ("Paw"), my wearied ex-alpha trundled over to the cage door. Decisions, decisions... Sooner or later I had to find out how The Stranger reacted to fresh rat smell. As drowsy as he was now, I probably could not find a better opportunity? I leaned over, opened the cage door and patted Labben on the back. On my lap, The Stranger moved. I pushed back the cage door and wanted to reassure the guest rat before he escaped my hospitality. Whatever substance triggered it, the attack was quick as lightening. As the hand with rat scent on it approached him, The Stranger put both teeth and claws deeply into the hand restraining him. Then he lept 20-30 cm at the offending smell and chomped down on my other hand again and again before I knew what was happening. I arrested him in my skirt and put him behind bars. Labben was hanging from the bars of his cage, and he'd sensed it all. A sweet and cloying scent of flowers whafted towards me...

Let's fast-forward to the next day. It was a fairly hectic one, and suddenly it was 'way past rat playtime. My youngest had let out Mimer, the alpha, for a while. He got to rummage in the play pen for half an hour before it was our guest's turn. This was, uh, instructive. The Stranger widdled (urine marked) everywhere, scraped his feet, huffed and puffed. After a while he got up on two legs at the table's edge, as close as he could get to the cage with my rat bucks. He looked ready to make a leap and conquer the world. A pair of thick working gloves saved me from more bites while I herded him back into the cage. By now my own rats were clamoring at the cage door to have their freedom. I thought I'd have a quiet time with rats that knew how to behave... A bit of clearing up, and the table was ready.

To my astonishment all my rats except for Mimer seemed edgy from the moment they set foot in the play pen! Seconds later I had to break up a fight involving five out of six cagemates. This from a gang who had recently been through several introductions to adult males, with an open mind and closed mouths!

Actually, reconstructing the events was not hard at all. Because we were on a holiday, I had not brought only a limited supply of toys, which the rats had to share. The Stranger had metodically marked everything by peeing on them. A paper bag, an Aladino travelling cage and a ferret pipe got special attension, and the brown paper bag most of all. He stomped around inside while snorting.

In between the free-range periods I had cleaned the waxed table cloth where the rats could walk. I could not clean the toys. The first rat stayed to sniff the wax cloth. This was Mimer, who seemed as unprepared for the change in his cagemates as I was. The second rat crawled into the ferret pipe with poor air circulation. He had piloerection and was feeling grumpy when he got out. The two rats who jumped into the open traveller cage with textiles in the bottom also had piloerection before they climbed out in a terrible mood. When the last three rats crawled inside the urine marinated and paw scent marked, stuffy paper bag, at least two of them went crazy. I heard a snort and then a loud squeak. The bag was upended and out slid Birk, and Frodo who had connected to his butt end. Birk got on his feet and jumped one of the black brothers, and suddenly I had five rats doing a passable impression of a comics fight. The black brothers, of course, were a team, and whoever wanted to bully one of them would have to deal with both. Incredibly, none of the rats got even a scratch! I breathed a sigh of relief.

Ten minutes later the entire living room reeked of perfume. And we had sticky fingers from patting the rats to calm them down...

Which led me to perch on the porch, wondering if I could find support for my ideas in scientific litterature:

Assumption 1: Male rats produce a fragrant, fatty secrete on their back and rump which signalises identity and status (as the secrete did not seem to make my rats aggressive towards eachother). Internal conflicts can cause a rapid increase in the secreteion.
Assumption 2: Male rat urine contains a substance that can cause aggression in other rats. The effect is concentration dependant. The concentration of this substance in the urine varies between rats and is probably affected by the rat's state of mind.

Perspireing rat

I have no clear-cut evidence, but I do suspect that the "rat perfume" is partly due to androstenon. Androstenon is commonly produced by mammals, and in greater amounts by males than females. This scent is described by most people as floral or musky. Others describe the scent as "cats piss", and these individuals are able to detect it at lower levels than the rest of us. If the concentration is high enough, everyone find the former flowery scent unpleasant. A third group cannot describe the smell at all. Even so their brain may well be able to register it. The recognition of the smell of androsteron may be detectable through the trigeminal nerve, not the "main olfactory sense", which is responsible for most of the sense of smell in man. (You can read more about the sense of smell further down the page.)

What I am absolutely certian of, is that this smell which I recognize as sweet and flowery when I bury my nose in the fur of my rat bucks is generated on their back side. This is where it is at its strongest, and sometimes you can even see the secretion! The rat fur becomes moist on the rump and hips. This typically happens in the "hormonal" stage around 3-6 months, or when the rat undergoes an introduction. The rat appears to be sweating. But the sweat does not always smell the same. Twenty-two different scent molecules have been identified in this secretion. I cannot tell you what each of them mean to another rat!

Fig.1: The picture on the right is by Anita Sjřlie. It clearly shows a "sweating" rat. Its only connection to the story is that I was allowed to use the picture for illustration. A moist stripe runs down its right side. The owner described the smell from the cage "as from a boar". Androsteron, by the way, is one of the chemicals that make the meat of intact male pigs unpalatable... None of mye male rat sweated like this, but I could still feel the stickiness on my fingers when I touched their backs.

With all of 22 different scent molecules in varying concentration the back secrete could go a long way towards identifying a rat. Still, normally it is the urine marking which is used to recognize other individuals between rats. The production of the flank secrete is so copious and sudden during a conflict that maybe this secretion is a way of communicating stress level and "macho male"? Appearantly female rats signal their fertility through the same glands. This indicates that the secretion could be rich in male hormones, and related substances.

Widdleing rat

Urine marking in rats have had a good deal more attention than the oily secretion from sebaceous glands on their back. We know that male rats can recognize one another as individuals by the smells of urine. What they are actually sensing is the smell generated by bacteria in and on the their penis! Genetically similar rats from a sterile enviromnent do not recognise individual siblings by smell. As status within the group affects hormone levels in the urine, the rats can also smell roughly which rank another rat holds. Stress leads to rising level of other hormones. The urine even tells of what kind of food a rat has eaten, of disease (which can both change the bacterial flora and which end products are excreted.

By the urinary tract several other glands empty into the urine. Male baby rats secrete one or more scent molecules which make the dam pay special attention to them. Male baby rats need more stimuli to evacuate the faeces than the females do. This particular secretion is formed in the preputial gland. Even in adult rats the secretion signalize the sex of the rat. Most likely other information can be encoded. We simply haven't got the key to decipher it.

I did not find a scientific paper showing that urine marking from strange male rats increases the conflict level between bucks living together. I did find it for mice, though! In mice the smell of the preputial secretion from an intact male increases aggression towards castrates. Having witnessed the ruckus above I am in no doubt that the same applies to rats. I must admit the strenght of the effect, and the fact that different strange males had different effect, raised the hair on my neck! I wanted to learn more. Two or three months later I cannot claim to "understand" how the instincts take over. But I have learned a great deal more about the sense of smell than I started with - and I will try to pass it on.

The fantastic rat nose

The human sense of smell is more acute than most people give it credit for, bur the rat nose is more than a tad better. Rats with a normal olfactory sense can absorb all this information, and probably more, just by sniffing the footprint of a fellow rat:
Fig.2: Nosy animals
Apart from sex and fertility, stress level and food habits, we know that rats can pick out closely related humans based on smell. I recon it is a safe bet they know as much about related rats. One rat tracking another knows if the one ahead found food, danger, or nothing of interest. Rats actually smell in stereo! They hardly need to think to determine where a smell comes from.

People too can be influenced by chemical signals through the nose. Some smells we find so revolting we feel an urge to throw up. The smell of good food, however, increases the sale of the same foodstuff. Most of us know how the odours from a bakery can break down self control... So how are rats different from us?


An instinct is an inheritable, species specific tendency to act in a particular manner. The instinct is released by one or more signalling events.
There is a name for chemical substances used for communication or which trigger stereotypic behaviour within a species. They are called pheromones. Pheromones can be sub-divided into those that trigger an immediate reaction, and those that elict more subtle effects. The aggression above was obviously of the immediate reaction kind. But changes in aggression level by smell can also be a long time affair. A female rat that has been exposed to the smell of a pregnant fellow rat for a couple of days is less likely to attack than one who has not had this olfactory stimulation.

All rat molecules detectable by the olfactory sense can, on principle, be a pheromone. This includes both small, fat soluble molecules and up to medium sized water soluble molecules. In actual fact the pheromones seem to represent chemicals connected to the needs of the rat: finding food, avoiding predators, managing in the rat community and reproduction.

There may be more reasons than one why we become uninterested in our surroundings when we suffer from a stuffed up nose...

Some rat pheromones have readable names like androstenon, while others are known by the name of tongue twisters 6,11-dihydro-dibenz-b,e-oxepin-11-one. According to a former theory they were produced in the different kinds of scent glands that the rats have. Now it seems as if hormones and their natural waste products, present in urine and other body fluids, and even breath, can act as pheromones. The hormones, naturally, are produced by internal glands, but the degraded hormones ready for elimination are not. Nor is sulphur dioxide, produced by bacteria in the gut or oral cavity.

Fig. 3: Scent glands and sources of pheromones on a (hemaphrodite) rat
  1. Tear glands
  2. Salivary glands, submaxillary
  3. Cheeks
  4. Harderian's gland
    behind the eyes with outlets to eyes and nostrils
  5. Flanks (mostly males)
  6. Other sebaceous glands in the skin
  7. Urin
    outlet via urethra
  8. Preputial gland
    on the belly between the skin and body wall, in front of the clitoris in female rats and the penis in male rats with an outlet under the forskin/clitorial folds
  9. Soles of the feet
  10. Faeces (rat rasins!)
  11. Testickles (males only)
    outlet through the urethra
  12. Coagulation gland (males only)
    outlet through the urethra
  13. Mammary glands
    (more in females than males)
  14. Salivary glands, sublingual
    that is, underneath the tongue
  15. Exhaled breath

Some pheromones are common to several species. The smell of male human urine can make male mice aggressive just like the smell of mouse pee from a strange male. Several insects react with interest to one of the pheromones emitted by elephants in heat. Hopefylly some of the other pheromones from the elephant signalize: "Stay away, I am not an insect!" From a developmental point of view there are good reasons for being able to separate between pheromones form different species. In mice, for instance, the whiff a strange male can be enough to make a pregnant doe reabsorb her litter at an early stage. (If you're wondering: rats don't!) If the mice reacted to all strange males regardless of species we'd have far fewer mice in this world.

Hidden dragon

(or the physiology of smell)

But even if the rats could sense eachother's olfactory signals, how did it come to pass that their normally peacful interactions turned into fighting and the inner dragon awakened? All my rat boys reacted, but some more strongly than others. The two attackers were among the three that had received the strongest concentration of Strange Rat Smell. Still, it may not have been purely random which rats sekt out the strongest stimulant. The two attackers were definitely the ones I already rated "least stable" in my group. Two months later Frodo was euthanized because of nervous disposition and aggression against other rats. Shortly afterwards Birk ended up in a solitary cage after cutting open at least one cagemate. The third rat to enter the marinated paper bag was Ulf. And Ulf too must be classified as "abnormally excitable", even if he was not particularly aggressiv. So it seems as if the olfactory signals enhance an already present tendency, more than it causes a complete change in personality. In spite of the aggressive impulse, none of my rat boys actually did anything worse than tear off a mouthfull of fur.

Concerning my guest rat, I did not know much about his "usual" behaviour. Still, when one of my food fanatics miss trying to grap a morsel from my fingers, he usually stops short of breaking the skin. The Stranger rat had seemed perfectly content being petted in my lap. He did not react when I leaned away to sneak a hand into the cage next to me. but when that hand came close with the scent of my frail ex-alpha which I thought would be the least threatening of them all - my guest just turned a switch. The fact that I also smelt and tasted of human just did not register. In his frenzy he managed to bite two or three times in each hand, and those bites meant buisness. Whatever signalled the change in attitude, it clearly overruled all other senses. There is no way he conciously believed I was a rat. Actually, male mice can be fooled into attacking a rubber mouse smeared with the saliva of a strange male. Rats are smarter than mice, but mice are not that dumb! It seemed much the same mecanism.

So why do specific olfactory signals (like our guest rat) cause otherwise peaceful rats to fight? "An unconcious urge" is not a proper answer. Actually, the question is the wrong one too! Our senses are bombarded with impressions all the time, so it is of vital importance that some sort of filtering takes place. We all sense a lot more than we actively remember and think about. There's nothing special about having mecanisms for immediate responding to danger signals either. Aggresion somtimes figures as a mean to survival. The question is rather why smell has a much more overwhelming effect than sight or hearing. Besides, why does smell influence rat aggression much stronger than human aggression?

Odours and feelings do connect differently from sound and feelings or light and feelings. This is because of how the nerve cells are organized in the brain. Sight and audatory information is channeled through the thalamus. Olfactorial information is sent through the olfactory glomerula. This organ contains a kind of relay station, re-directing some (or no) information to the thalamus, and some information to other parts of the brain. These other parts could be the autonomous nervous system or hormone producing glands.

As a matter of fact we have more than one sense of smell up our nose! We humans have two kinds. The rats have three! Those three senses are more different than just sporting differen sensors. The localisation is different, and the neural paths involved are not the same:

The main olfactory sense has the biggest scope in rats as well as humans. It has both the gratest number of sensors and the greatest number of different sensors. (olfactory = having to do with smell) But if you poke your nose close to the surface of an acid, or alcohol, or a number of poisonous or irritating substances, you'll feel something that can be described as a "sharp, stinging" smell. This stinging smell is actually the trigeminal nerve being irritated. The organ said to be the most sensitive of the three though, is the vomeronasal organ. One molecule is enough to trigger a nerve signal.

The vomeronasal sense is probably not well developed in man, if it even exists. The scientists do not agree to how large a fraction of the population is actually endowed with one. Nor do they agree about its ability to send signals to the brain. In most mammals this organ is not only intact, but probably cruical to the survival of the species... The VMO sends signals which trigger instinctive behaviour. It is a "make love or war" organ. So is the main olfactory sense, at least by pigs and mice. Even humans have instincts triggered by smells: A sudden interest for sexually mature individuals of (usually) the opposite sex. A strong drive to care for babies. Generosity. Sometimes, or perhaps even most times, (a frightening thought!) we do not know what makes us think or act the way we do. It is not likely that this is due to an active vomeronasal organ in man.

People generally do not experience smells as a trigger for violence. We have other triggers, and they are probably more of a conglomerate. As a functional vomeronasal organ seems to be a likely difference between most other mammals and us, it is pempting to ascribe triggering to the extra sense of smell.

Fig. 4: Main olfactory sense

The olfactory receptors send their message of a "hit" to the nervous system. The nerves pass on impulses to the olfactory glomeruli, which is an area of the brain fairly squarely behind the eyes. The olfactory glomeruli acts as a kind of relay station, sending coded messages to the brain cortex and the limbic system. The limbic system receives signals from the olfactory area of the cortex too. This rat recognizes the collection of olfactory molecules as "apple" and "banana", and suddenly recalls it's first experience with an apple...

One theory has the crucial difference between the main olfactory sense and the vomeronasal-sense - with respect to behaviour - is due to how the signals are propagated to the cortex and the limbic system after exposure to olfactory stimuli. If the cortex does not "know" that a scent is influencing it, there will be no effort to resist the impulses from the limbic system. Another theory holds that the difference is due to a lack of communication between the cortex and the limbick system when the smell primarily is sensed through the vomeronasal organ.

Based on common knowledge about the sense of smell, this is how I imagine the interplay between smell and aggression:

  • Scents can affect production of hormones and nerveous connections, and thereby behaviour, even if the rat is not concious of their influence.
  • Scents the rat is subconciously aware of have a greater potential to affect the rat than conscious smells. Noone has the same defence agains impulses they cannot explain, or may not even be aware of.
  • The concious smelling is partly dependant on concentration, partly of which olfactory system is activated, and partly by the rat's ability to recognize the smell. Time, too, is a factor. Known odours are recognized before new ones, but odours surrounding a rat all the time are not conciously noticed.
  • The ability to produce and sense odours triggering instinctive behaviour is inheritable.

Just an interesting thought: are some rats born particularly sensitive towards the body odour of other rats, while others give off a B.O. that other rats find REALLY offensive? Both types of rat will get in trouble, and be trouble makers in their groups. One can imagine that these rats really do have a B.O. problem.

Smell and introductions

How do theory and practise match with respect to advice on introductions?
  1. Wait until the rats have grown accustomed to eachothers' scentin
    Probably sensible. Aggression between male mice is reduced by exposure to the new mouse smell one hour a day for ten days.
  2. Mask the rat smell with vanilla essence or other strongly smelling substances
    Maybe. The concious sense of smell in man can be affected by other scents. Some odours strengthen eachother, others mask, and some have no effect on other scents. The fact that vanilla essence can overpower the smell of rat to a human nose does not ensure that the rats feel the same, or vice versa. Still, a number of people swear by it, and they may have a point. In man, vanilla is reputed (not proven) to have pheromone like quality. It is supposed to make men more interested in women. I cannot say I have seen this effect, and if it did, it might not have the same effect on rats!
    Rats can be bathed to reduce smell, but the effect is uncertian. As previously mentioned it will only take a few minutes for the rat to start producing new scent molecules if stressed. Lots of rats feel bathing to be stressful. You may not want the rat to smell stressed-out at the start of an introduction...
  3. Find a neutral place for the introduction - use the tub if you have one.
    A no-rat's land is sensible. Particularly if you do not bring toys that have been peed on already! But I am not sure about the bathtub. When tha rats mark the place with their urine, the smell will stay much longer in a small volume of air because of the relativly high walls of the tub.
  4. Get into the outdoors
    Reduces smell by a dilution effect. Sensible as long as the weather allows. A well ventilated room could help when the climatic conditions are unfavourable.

Leafing through a number of research articles have given me a few more ideas:

Links about smell

Overview article Odor-guided behavior in mammals
R. L. Doty
Expcrientia 42 ( 1986). Birkhiiuser Verlag. CH 4010 BaselSwitzerland ~57
Full texst Norway Rats’ Communication About Foods and Feeding Sites
Bennett G. Galef Jr.
Jana Schrock and Dominic Costanzo. Muskingum College .
Educational material AMYGDALOID BODY,
Full text The dog and rat olfactory receptor repertoires
Pascale Quignon, Mathieu Giraud, Maud Rimbault, Patricia Lavigne, Sandrine Tacher, Emmanuelle Morin, Elodie Retout, Anne-Sophie Valin, Kerstin Lindblad-Toh, Jacques Nicolas and Francis Galibert
Genome Biology 2005, 6:R83
Full text Disruption of the Fifth Melanocortin Receptor Alters the Urinary Excretion of Aggression-modifying Pheromones in Male House Mice
Heather K. Caldwell and John J. Lepri
Chem. Senses 27: 91-94, 2002
Full text Sixth Sense: The Vomeronasal Organ
Jason Bernstein
Biology 202 1999 Final Web Report, Bryn Mawr College
Abstract The olfactory bulbectomised rat as a model of depression
Cai Songa and Brian E. Leonard
Neuroscience & Biobehavioral Rewiews
Vol. 29 Issuses 4-5, 2005, p. 627-647
Abstract The mother rat's vomeronasal organ is involved in detection of dodecyl propionate, the pup's preputial gland pheromone.
Brouette-Lahlou I, Godinot F, Vernet-Maury E.
Physiol Behav. 1999 May;66(3):427-36
Full text Chemistry of clitoral gland secretions of the laboratory rat: Assessment of behavioural response to identified compounds
J. Biosci. Vol. 26 No. 2 June 2001 pp.247–252
Abstract Neural encoding of olfactory recognition memory
Gabriela Sánchez-Andrade, Bronwen M James, and Keith M Kendrick
J Reprod Dev
2005-Oct; vol 51 (issue 5) : pp 547-58
Overview article The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells
Rebecca Elsaesser and Jacques Paysan
BMC Neuroscience 2007, 8(Suppl 3):S1doi:10.1186/1471-2202-8-S3-S1
Science News Wake up and smell the java
Tia Ghose
Web edition : Friday, June 6th, 2008
Undervisningsmateriell AMYGDALOID BODY,
Educational material A tutorial on the sense of smell
Tim Jacob
Full texst Jones R (2008)
How Quickly Can a Rat Perceive Novel Odors?
PLoS Biol 6(4): e94
Full text Olfactory preferences for the rat preputial gland
Paul J. Orsulak and Anthony M. Gawienowsky
Biology of Reproduction 6, 219-223 (1972)
The preputial gland in male and female rats contain pheromones attractive to the opposite sex. These pheromones are not secreted in saliva or from the foot sole.
Abstract Prim B. Singh et. al.
Rearing rats in a germ-free environment eliminates their odors of individuality
J.Chem. Ecol. Vol. 16 No. 5/May 1990 pp.1667-1682
Abstract Odor from rats tasting a signal of illness
Journal of experimental psychology. Animal behavior processes 1990, vol. 16, no2, pp. 193-199 (20 ref.)
Abstract Rats assess degree of relatedness from human odors
Erin M. Ablesa, Leslie M. Kaya og Jill M. Mateo
Physiology & Behavior Volume 90, Issue 5, 23 April 2007, Pages 726-732
Abstract Ovulatory pheromone shortens ovarian cycles of female rats living in olfactory isolation
Physiology & behavior ISSN 0031-9384
1997, vol. 62, no4, pp. 899-904 (17 ref.)
Full text Corticosterone controls the developmental emergence of fear and amygdala function to predator odors in infant rat pups
Stephanie Moriceau, Tania L. Roth, Terri Okotoghaide, and Regina M. Sullivan
Full text Developmental Changes in Olfactory Behavior and Limbic Circuitry
Regina M. Sullivan

Matmor original 2008.10.02
translated 2009.10.06
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