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.
- The crooked ways of inheritance
- Multifactorial
It is possible to inherit more than one type of aggression, even if they can be inherited
independently.
Inheritable aggression is often influenced by the environment: without the right (wrong?)
stimulus the problem may go undetected.
- Variable penetrance
Rats with the same type of inheritable aggression may act more or less aggressively
because of other inheritable traits or of environmental differences.
- Probably both dominant and recessive forms
Recessive inheritance has been demonstrated for one case of inheritable aggression in
rats.
Dominant inheritance has so far only been demonstrated for aggression in mice and men.
- Possibly incomplete dominant forms
It is easy to imagine that inheritance of a gene defect from both parents will give a different
result (stronger effect) from inheriting the same gene from one parent only.
- Probably both sex-specific and sex independent forms
SHR (Spontaneously Hypertensive Rats) have a genetic mutation in the Y-chromosome that increases aggression.
- As "side effect" to inheritable, physical disease
Pain can trigger aggression. Rats are prone to inheritable, painful ailments like rheumatism,
And they go to great lengths to hide their pain. It has also been shown that some vira
and hormonal disturbances may cause irritability. Viral disease may be linked to
an inheritable poor immunity system. Hormonal disturbances may have a genetic cause.
- May be due to a mutation in this single individual
Even if the parent animal did not have a genetic mutation causing aggression,
the mutation may have developed in this particular animal. The new mutation may then be inherited.
Aggression and environment
Parallel 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.
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Aggression by environment
- Can be multifactorial, with contributions from inheritable causes
As different kinds of experience can contribute to different kinds of aggression it is
possible for a rat to
develop both anxiety related aggression and isolation aggression.
Impulses for aggression are tempered by experience. Rats that leave a fight victorious are
more likely to initiate fights later on.
- Maternal hormones and food habits during pregnancy/nursing
- Maternal neglect
- Socializing with other rats
Isolation from other rats increase aggression
By the resident-intruder-test the status of the intruder in its old cage predicts the level of
violence it is met with during introduction.
Intruder females give up more quickly than intruder males when met with high levels of
aggression. The status of the intruder can be decided by chance - a rat owner with the
rats divided into small colonies will have more Alphas than one with many rats in one
cage.
- Socializing with humans
- Disease and pain
Infectious diseases accompanied by pain can increase aggression in the sick rat.
As rats are uncannily good at hiding disease, aggression may be the first outward sign of
illness.
Feelings and biochemistry
What in fact happens when a rat becomes aggressive?
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. |
From genetic information to hormone building blocks, enzymes, and receptors
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.
From hormone building blocks, enzymes, and receptors, to coupling
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.
From hormone to signal
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.
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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.
Learned aggression
Conditioning:
With learned aggression at least two mechanisms come into play. One was described by Pavlov and is called
conditioning. He let dogs hear a bell each time they were fed. After a while the dogs made the connection
between eating and the sound of the bell. Even if they were not given food the dogs started salivating
in anticipation of a meal when they heard the bell. A rat who learns that special situations
(introductions, for instance) lead to fighting, will be ready to fight even if its adversary does not
show aggression.
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.
Testosterone, for example, is said to make male rats aggressive. At best that is a hand waving simplification.
at worst it is directly misleading.
The aggression itself stems from hormones synthesized from testosterone:
oestrogen and 5-alpha-hydrotestosteron.
The effect of testosterone itself is not to make the rats angry. Testosterone makes them unafraid. When a rat
is about to attack, clearly it is an advantage that part of the brain is not occupied worrying about loosing.
Seen like that testosterone may contribute to aggression. On the other hand, a fearless rat will not easily
feel cornered and resort to fear biting. For a nervous rat, testosterone may actually decrease the level of
aggression. Accordingly a lowering of the testosterone level may increase the level of aggression in fear
biters. For these rats the world becomes increasingly scary when puberty is over. All rat bucks
experience a lowering of the testosterone level with ageing. Castrating a male fear biter to reduce aggression
may be counterproductive. (Still I've heard of nervous rats finding a new calm after castration, so you never
know...)
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! |
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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.
Possible types of aggression and acting hormones
| 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 |
| Sexual aggression |
5-alfa-hydrotestosteron
testosterone
oxytocin |
? |
Maternal
(against adult rats) |
Oxytocin
Serotonin
Prolactin
Vasopressin |
? |
Maternal
(against the rat pups) |
?
probably reduced by oxytocin and/or progesteron |
? |
Killing of rat pups
(by other than the dam) |
Progesteron |
? |
| Competing |
Dopamin
Serotonin (5HT) |
Inheritance independent of isolation aggression |
| Intruder aggression |
Østrogen*
Vasopressin
Excessive violence: serotonin
* from experiments with mice and birds |
? |
| Fear aggression |
Corticosteron ("stress hormone" in rats)
Testosterone (reduces!) |
? |
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.
Hormonal paths of synthesis
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.
Types of aggression
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.
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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:
- Isolation induced aggression
is about studying aggression in rats that have been living alone for a couple of weeks,
compared to rats living in a group.
i forhold til rotter som bor sammen i en gruppe. (Rats that have been isolated from its fellow rats for
a few weeks, show characteristic behaviour not seen in aggressive, co-habiting rats.)
- Pain induced aggression
commonly uses electric foot shocks to induce aggression in the rat.
- Electric Brain Stimulation aggression
is studying by delivering a current to an area of the brain that has been associated with
aggressive behaviour.
- Sexual aggression
handler om å observere aggresjon mellom en hunrotte i løpetid og en kjønnsmoden hanrotte.
- Maternal aggression
does not mean the same to all scientists. Some researchers use this term for aggression
from a doe (pregnant or with nursing rat pups) against an intruder. Others use this term for dam
aggression directed against her own pups.
- Competitive fighting
is about observing two rats fighting about food after a period of fasting or other scarce resources.
- Restraint fighting
is being carried out by holding the rat tight while waving a target (a pencil, for instance) so that it
touches the vibrissa of the rat.
- Predatory aggression
measures how quickly/how often the rat attacks/kills natural prey (most commonly baby mice).
NB! This method is mostly obsolete for ethical reasons. Most scientist agree that it no longer
has a place in the research on aggression, as killing natural prey probably is unrelated to
aggression.
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-learning
The 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
- Find out what makes a rat aggressive
and try to avoid those situations
- Training by exposure
where exposure is interrupted as soon as the rat shows aggressive tendencies
- Reward good behaviour
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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.
