Aggression in fancy rats
Aggression in fancy rats is not a pleasant theme. Nobody likes to think that their wonderful baby rat
soft fur can become a monster with teeth and claws. At the same time aggression is one of the two traits that
serious Norwegian breeders are most concerned about. They wish to breed aggression out,
(and breed health in).
This just shows how lucky we are to have rats for pets! Rats are among the most studied animals for aggression.
There are hundreds of scientific articles, and some are even available for free on the Internet. There are good
articles written for the layman too, about rat aggression. Unfortunately the popularized articles did not answer all
my questions. That is why I started gathering information from original sources. Many of the answers I was looking for
do not exist yet. Even so I found exciting news that made me sit up straight and take notes.
Aggression and inheritanceImagine taking a gene test to learn if your rat was carrying inheritable aggression! What a difference it would make for breeding! Alas, that gene test does not exist. In spite of all the rat research there are still so much we do not know yet. Among mice and men, gene mutations have been found which cause aggressive tendencies. With mice there are even models for creating these gene mutations in the laboratory. The rats, however, have not been as complacent. Even so it is quite evident that some gene mutations can lead to aggression in rats. Actually, we know that different gene mutations can lead to different kinds of aggressive behaviour. For that reason any hope of taking just one gene test must be rudely crushed from the start. One blood sample might do, but there would have to be a whole array of tests performed.
One rat can live harmonically with its kin, but attack humans again and again. Another rat is the perfect, lovable pet in the arms of its owner, but is turned into a killing machine when presented with baby rats. Some does are only aggressive when pregnant or while nursing. Some rats get along well in their own group but go bananas over strangers. There are rats that mainly show aggression in a feeding situation, or in a particularly stressful situation. Other rats are selectively aggressive against their own or the opposite sex. Then there are those who will pick on a particular victim, and those that simply cannot stand any other rats. It is possible that some of these forms of aggression have the same genetic cause. Some are unquestionably unrelated. Aggression, however, is not always (or perhaps even as a rule) inherited. Environmental factors will be discussed further on. This chapter focuses on current knowledge of inheritable aggression in rats.
As we do not know the genes that cause aggression in rats, we cannot study the effects of gene manipulation directly. Evidence for inheritance comes from two types of trials. One is breeding for low and high aggression. The other type of evidence comes from comparing existing lines of laboratory rats. The results can hardly have come as a surprise to the scientist. People experienced in laboratory animals do not need a controlled study to discover that with regard to aggression, Sprague-Dawly rats are different form Fischer-rats... For the benefit of less experienced rat handlers the difference has now been documented. There is also evidence that the tendency to bite when subject to pain is independently inheritable from the tendency to bite when restrained. Restrain aggression have no direct link with aggression when competing for food. And aggressive behaviour because of social isolation can be inherited independently of all the three above.
Aggression and environmentParallel to the documentation of inheritable aggression runs a trail of evidence to show aggression can be brought on by the environment. Even during pregnancy the rat foetus is influenced by hormones in its environment. This influence continues through the nursing period. Apparently a female rat pup that lay between two brothers in the womb (an M2-rat) will become more aggressive and more sexualised as an adult, than a female rat between two sisters (F2-rat). Not all researchers have found the same trends, though. After birth the aggression level of the (foster-)mother affects the pups. Rats born to an aggressive mother become milder if raised with a peaceful foster mother. It is also possible to make rat pups more aggressive by fostering the pups of a calm rat to an aggressive doe.
Early influence on aggression is not limited to the temperament of the mother/foster mother. Lack of maternal care is associated with later aggressive behaviour. Nor are grown rats spared the influence of the environment: Eating habits matter. A diet free of the amino acid Tryptophan lowers the level of serotonin in the brain. Serotonin is a hormone and a neurotransmittor which affects mood and aggression. A strictly vegetarian diet introduces plant oestrogens to such an extent it affects behaviour. Because rats are intelligent animals it should come as no surprise that they learn by fighting experience. The rat who emerged the winner of a fight will tend to be more aggressive than the one who lost. The age of her rat pups will affect the level of aggression in a dam or the foster dam.
The instruction in the genes are read and lead to the production of the specified chemical substances.
Those chemical substances can be the building blocks of
a hormone, an enzyme, or a receptor protein which relays a signal from the hormones.
If there is a fault in the production somewhere, the
intensity or the duration of signals to the brain is changed.
The signals are the information that makes the rats react.
Instructions that are repeated frequently get a VIP-status.
Instructions from the DNA are copied/read and followed. Occasionally the instructions are mis-read, or something goes wrong in carrying them out. Here the clown in the bottom line has made his own personal variety of the receptor. If the clown represents copying of the DNA the misunderstanding causes an inheritable mutation. It the clown's antics concern the carrying out of instructions, they only become a problem for that particular rat.
Here are the results of a poorly constructed receptor. The enzyme for coupling the hormone to the receptor depend on a fit between the two parts. If the hormone cannot be connected to the receptor normally, the signal from the receptor to the rat brain will be disturbed.
Errors of construction in receptors are fairly common. Actually, a number of receptors in the brains of rats (and men) do exist in several versions. Sometimes the differences are so small it cannot be discerned. At other times the difference can be a matter of life and death. Rats that have an abundance of one special serotonin-receptor in a particular are of the brain are prone to depression. These rats give up sooner in situations where they have to struggle for survival.
The norm, and different kinds of errors that may occur
1: Both enzymes and the receptor have been made according to the normal instruction.
2: The enzyme needed to connect the hormone to the receptor has a defect. Most likely the signal will decrease in strength.
3: The receptor has a defect. The signal may be either weakened or strengthened.
4: The speed of production of enzymes, hormones, or receptors is not right. The signal may become weakened or strengthened.
In rats we know that case 4 may lead to inheritable aggression. Experiments in taming wild rats through selective breeding show greater amount of serotonin receptors of a particular type, in a particular area of the brain. These special receptors lead to a less aggressive behaviour.
From hormone to behaviour
The signals that cause to emotions and behaviour are ruled by hormones. Different kinds of aggression are connected with different kinds of hormones. Not unexpectedly cortisol ("the stress hormone") of rats is raised in rats who become aggressive by stressful situations. Oxytocin, released by nursing for instance, affects aggression in rat dams against anything that can be perceived as a threat to the rat pups. Serotonin seems to be a key factor for rats lacking normal inhibition against inflicting serious damage to their adversaries.Naturally things get a bit more complicated than that... The same hormone can bind to different types of receptors, and each type of receptor can trigger different signals. Some receptors can receive several types of hormones. The signal will be the same but the signal strength will depend on the hormone.
Behaviour is not ruled by one hormone and one signal only. Rats like people must handle more than one type of information simultaneously. The rat is never merely irritated. It is irritated and afraid, irritated and hungry, irritated but friendly towards cage mates. Some signals put a damper on aggression while others cause an increase. Others appear to override competing signals. Unfortunately we do not know exactly what these "basic feelings" are, nor how the hormones affect them. Maybe it is something akin to the figure underneath. NB! The rat brain does have a different shape than the human.
Hostile and friendly disposed, frightened and secure are opposites in the sense that they generally do not surface at the same time. The needs (Signal 4) probably follow a hierarchy so that the rat does not have to consider all needs at the same time. This hierarchy was first described by Maslov, and is usually represented as Maslov's pyramid of needs. The needs that have to be met first tie in with survival in the short term. For instance, not being killed by a predator, adversaries or a deluge. Then comes the need for survival at slightly longer time span: water, food, sleep etc. Higher up are the needs concerning survival of the genetic code of the individual, in the shape of offspring and caring for the young.
With the explanation above in mind it is not difficult to see the connection between behaviour and inheritance. Nor is it difficult to imagine that environment can have short-time effects. The question is: How do the long term changes arise? How can the hormonal level inside the womb of the rat dam affect the level of aggression in the offspring as adults? How can maternal neglect lead to aggression for the rest of the rat's life? Well, there is a reason why rats are popular subjects in brain research. They are a lot like us... they learn by experience, and they are creatures of habit. Long term environmentally induced changes seem to be tied in with processes of learning. Basically all the brain can sense are the signals. Whether the signals are caused by a factual experience or artificial stimuli (or understimulation!) matters little to the outcome. Signalling paths that are much used will become signalling roads or highways. Precisely how this "road improvement" occurs is not clear. What we do know is that hormones affect each other. The rate of one hormonal synthesis depends on the concentration of other hormones. Nor are the levels of enzymes and the concentrations of receptors static. The brain has mechanisms for regulating signals up or down.
Re-enforcement: The other mechanism, reinforcement, was described by Skinner. Skinner was actually researching rat behaviour when he formulated his idea. He stated that learning occurs as a consequence of an action. If the reaction is positive the tendency to act in this way will be strengthened. A behavioral pattern is formed. If the result is negative the tendency to repeat the action will decrease. We have come a bit further than that in the understanding of learning. Concerning aggression in rats it has been shown that winning releases a hormone cocktail that makes the animal feel good! Even rats that do not expect to win can learn an aggressive behaviour pattern because of re-enforcement. In a dominance situation the dominate rat is usually not looking for a fight. An anxious rat can learn that it will be left alone if it attacks first...
Imitation: We also believe that rats can learn by imitating others. Watching an action releases the same kinds of hormones as being involved, only to a lesser degree.
Hormones and behaviour
Hormones do not always have the same function as popular opinion has it.
Oestrogen is reputed to have a dampening effect on aggression in women. Estrogen and rat bucks are a different matter. Their own production of estrogens actually helps increase aggression between strangers!
Below is a table of different hormones that have been associated with aggression. The list is anything but complete. What the researchers find depends on what they are looking for, and which methods are in their arsenal. Not to mention which rats are at their service.
|Type of aggression||Hormone(s)||Inheritance|
|Isolation aggression||Serotonin (lowers!)||Inheritance independent of pain/restriction/competitive|
|Pain aggression||Probably stress related, corticosteron||Inheritance independent of isolation/restriction aggression|
(against adult rats)
(against the rat pups)
probably reduced by oxytocin and/or progesteron
|Killing of rat pups
(by other than the dam)
|Inheritance independent of isolation aggression|
Excessive violence: serotonin
* from experiments with mice and birds
|Fear aggression||Corticosteron ("stress hormone" in rats)
Looking at the types of hormones involved it is evident that some types of aggression may be related. Others do not appear to have a common hormonal base. The question is: do they still have the same genetic cause? In some cases the answer is yes. The regulation of hormonal production could fill a chapter (or many!) alone. The concentration of a hormone is influenced by the concentration of other hormones. With testosterone it is quite easy to understand how this happens, as testosterone is the building block of hormones like estrogens and 5-alpha-hydrotestosterone. When the amount of testosterone decreases it is only logical that less estrogen and 5-alpha-HT is produced too. But testosterone and 5-alpha-HT interacts in more ways than one. The substances compete for some types of receptors, which sends signals to the brain. The elimination of both hormones is probably facilitated through the same enzyme systems in the liver. Suddenly there emerge at least three "places" where a single genetic mutation can affect two hormone - and have impact on several types of aggression. As a matter of fact, the number of possible places of interaction is far greater than that... This explains why it can be hard to determine if the reaction to a mixture of hormones are due to one or more mutations.
Three paths of synthesis showing how production of one hormone is dependant on other hormones
Cholesterol is the starting point for testosterone, which is modified to make estrogens and 5-alpha-hydrotestosterone.
Tyrosine is the starting point for dopamine, which is modified to make noradrenalin (norepineprine), which again is modified to make adrenalin (epineprine)
The amino acid tryptophan is the starting point for serotonin, which is modified to make melatonin.
As mentioned, several types of aggression can be inherited independently of eachother. Aggressive is not something the rat either is, or is not. It all depends on the circumstances and how it is being measured.
Just to make things clear: there is no single, universally accepted definition of aggression! My understanding of aggression with respect to rats goes something like this: "Consciously acting out violence against live beings, or threatening behaviour, but not including hunting for natural prey". This division is more practical than philosophical. Naturally hunting is a violent affair, but hunting instincts have little impact on the suitability of rats as pets. Researchers have discovered that hunting instincts are independent of violence towards one’s own species, or against humans (with the exception of large predators. They might start seeing a human as prey.) As a fancy rat owner I am much more concerned that my rat will bite people or rats, than if it takes a spider when the opportunity arises. Researchers of violence between humans are generally more concerned with intent than animal behaviourists. (It is somewhat harder to ask the animals if they knew someone was going to get hurt by their action.) Even so I have put "consciously acting out violence" in my definition. My youngest son once ate scrambled eggs and smoked salmon with his fingers before he ran to the rat cage to let his rat Squeek have a morsel. I do not call Squeek aggressive even if he quite effectively taught my son one of the advantages of using knife and fork...
|It is a regrettable fact that much of the research on aggression is not particularly animal friendly.
If you do not want to know how this research is carried out, feel free to skip to the part called Unlearning.
The most common methods for studying aggression in rats is either to observe rat encounters, or by target biting. Target biting measures how quickly/often/hard the rat bites a movable object such as a piece of hide on a stick. This method is sometimes used as a measure of tameness against humans, for instance combined with body restraint (see below). As researchers have respect for rat teeth it is common to wear thick gloves during the experiments! In rat-against-rat aggression it is possible to look at how quickly/ how often/ how seriously the rats do damage to eachother. Often threatening behaviour, vocalisation, marking, dominant and evasive/submissive behaviour is observed too. These experiments can utilize either an established colony or by letting an intruder meet a resident rat. Actually, the intruder method starts by putting a male rat with a female in a cage for a while before introducing a(n) (usually) male stranger. The stranger may be let into the cage, or made to meet the resident rats in the open.
Apart from this a number of other types of aggression are being studied:
As is natural, the rat fanciers are highly concerned that rats with inheritable aggression should not be bred from. Disputes over using relatives of aggressive rats are frequent. For the time being there are no genetic tests which will put an end to the discussions. What I believe is important is to consider the probability that genetics or environment are the cause of aggression in the relatives. Were the cases of aggression in this rat's family tree comparable? If all the aggressive rats showed aggression towards towards humans when handled, and not to other rats, it is likely that they are connected. If one rat turns aggressive after mating, another displays food aggression, a third - the alpha of the cage - showed aggression when being introduced to a new group in adulthood, the fourth was euthanized because it hit its owner and the fifth became generally aggressive after living alone for a month, it is a lot more difficult to see how inheritance fits the picture!
Un-learningThe downside of having pets with learning ability is that aggression can be learned. This is where you as a rat owner have a mission! You can
and try to avoid those situations
where exposure is interrupted as soon as the rat shows aggressive tendencies
The easiest by far is to start with a pair of low aggression rats, and make sure they are never exposed to undue harassment by other rats. On the other hand, there is a limit to how many cages one rat owner can keep if one ends up with rats that do not get on. The next step up is to play rat psychologist. Are there special situations that make the rat see red? By food aggression it may be a good idea to have more than one food bowl so that the rats can feed without competition. If a rat cannot stop bothering rat babies you can put off the introduction for another fortnight until the young ones are a little bigger and braver. Do not isolate a rat that gets along well with some cage mates, but not the others! Isolation is a situation that causes aggression in rats. It is better to put the difficult animal together with a rat it is comfortable with.
Training by exposure is time consuming. It does not always work, but when it does it is a fantastic experience for you and the rat. It is all about finding out how much the rat can take before reacting with aggression - and take it to its limit, but not beyond. If this is about an anxious rat, the experience of overcoming fear may be a reward in itself. If this is about a teenage rat exploding in a puff of hormones, plenty of encouragement is required after good behaviour. Reward in the shape of tidbits work well. If the rat is fond of human contact, scritching may work even better. At any rate it is healthier if the rat has a tendency to put on weight.
An important point to consider during exposure training: When the rat becomes agitated it needs a long break to calm down. Maybe as much as an hour. That is how long its hormonal level is raised after a fight, for instance. If the rat is exposed again while the hormonal level is still soaring, the threshold for aggression is lower than before.