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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

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:

Perspireing rat

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

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

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?

Pheromones 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

Tear glands Salivary glands, submaxillary Cheeks Harderian's gland behind the eyes with outlets to eyes and nostrils Flanks (mostly males) Other sebaceous glands in the skin Urin outlet via urethra 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 Soles of the feet Faeces (rat rasins!) Testickles (males only) outlet through the urethra Coagulation gland (males only) outlet through the urethra Mammary glands (more in females than males) Salivary glands, sublingual that is, underneath the tongue 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
• The trigeminal nerve
• The vomeronasal sense

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:

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

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:

• If a male rat turns aggressive after mating: borrow a female rat and wait with re-introduction until the day after she has been in heat
  The scent of a female in heat reduced aggression in freshly mated males the day.
• Brew yourself a cup of coffee and let the rats smell it!
  The smell of coffee is supposed to have a calming effect on rats. If nothing else comes of it, you can always relax afterwards with the coffee yourself...
• Keep cool!
  People have short fuses in hot weather. Rats, being more sensitive to smell are probably worse affected: The odour molecules spread more easily when the temperature rises.

Links about smell