Breeding for Temperament. It's one of the main things every breeder strives to do- is breed happy, well adjusted rats. 

But how do you do that? 

I have to state that this is in no way the "only" way to breed for happy rats, but this is the way that I breed for happy rats; I have come up with this process from over a decade worth of experience, in combination with my mentor's (RIP) experience of over two decades, and then a lot of peer reviewed studies!

As with anything, I always encourage others to seek out their own information and come to their own conclusions, it is the only way the fancy will grow. 


Lets define a few terms before we get into everything else.

What is Temperament?

Here's the definition straight from the Cambridge Dictionary:
"the emotional character or state of mind of people or animals, as shown in their behavior" [LINK}

What is a Gene/ Genetics?

"A gene is the basic physical and functional unit of heredity." [LINK]
So, pretty much the study of genetics is figuring out what will be passed down to the next generation.

What is Epigenetics?

"Over the following years, with the rapid growth of genetics, the meaning of the word has gradually narrowed. Epigenetics has been defined and today is generally accepted as ‘‘the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail a change in DNA sequence." [LINK] 
To put simply, the environment you were raised in can turn genes "off" and "on" and change who you are and those changed can be passed on to your offspring.


We Nurture Nature

Now that we defined a few of the terms that will be used in this writing, lets get to how we breed for happier, friendlier rats!

So, what determines temperament? What things even go into making a rat that has a better temperament?

Genetics and Epigenetics work together to make the animal you see in front of you. "Scientists estimate that 20 to 60 percent of temperament is determined by genetics." [LINK] 

For example, an outgoing animal bred to an outgoing animal will likely result in outgoing animals! BUT, this isn't the complete story. We arnt sure how exactly how much of an animal temperament is ONLY genetic. This is where epigenetics steps in! This is based on how an animal is raised! 

"Environmental factors also play a role in temperament by influencing gene activity. In children raised in an adverse environment (such as one of child abuse and violence), genes that increase the risk of impulsive temperamental characteristics may be turned on (activated). However, a child who grows up in a positive environment (for example a safe and loving home) may have a calmer temperament, in part because a different set of genes is activated" [LINK]

What does this mean? This means raising a rat in a loving environment will actually make rats friendlier! And with those genes switched "on", their offspring will also be born with that gene switched "on" and be friendlier as well.

So as you can see, we nurture nature! Nature cannot make friendly rats by itself, we must be involved to create a loving, happy environment for our animals.

There are even studies showing that us petting our rats makes them happy!

"In summary, our results indicate that post-weaning stroking procedures facilitate affiliative responses in rats possibly via activation of oxytocin neurons in the caudal PVN. Our results provide an important animal model for elucidation of affiliative relationships between humans and other animals, which should contribute to the enhancement of health of both humans and companion animals." {LINK}

Which is pretty cool if you ask me!

We've learned that while there are some parts to a rats temperament that ARE genetic, a lot of it can be changed through epigenetics. 

A shy rat can be raised in a loving, trusting environment and the offspring born will be an improvement to the parent rat. Socializing your animals genetically changes their temperament to be friendlier! 


Is aggression genetic? It seems that 50% of aggression IS genetic, while the other 50% is environmental. [LINK]

I must also say that handling a rat will not "mask" its temperament. You cannot mask genetics. A genetically aggressive animal will be aggressive no matter what you do. Handling and socializing your animals will only improve their temperaments.

A question that is always asked is, how do you breed AWAY from aggression? It seems a lot of the newer varieties have issues with aggression and people are not sure how to improve them. 

Spotting an actual genetic aggressive animal is easy, it will bite you and cannot be helped even with socialization.


If this animal is the only one of its variety, it would need to be bred to a friendly, outgoing animal, then the offspring raised with the mindset of knowing that aggression can be passed down. You'll know pretty quickly which of the offspring are aggressive or not towards you. With the rest of the offspring you'll want to raise them in a loving environment so they can learn what "love" is on a genetic level. I would continue to pick out the friendliest animals and inbreed them, along with outcrossing to the most friendly, stable animals/lines.

I want to add here that I will not be talking about culling in this article. My stance is that an animal that is miserable should not be forced to live it's whole life like that. I also do not think any animal that is genetically aggressive should be bred. But sometimes it cannot be helped.

This goes for fearful/anxious animals as well. [LINK]

Maternal aggression and intermale aggression seems to be a combination of both genetics and epigenetics. [LINK]

I will not be getting into the Maternal aggression debate (which is some breeders are fine with MA as it is natural, and others believe that since there are no predators in captivity, that MA should not exist). I believe breeders at this time should focus on making loving, friendly rats first, then worry about Maternal aggression. The best way to breed out intermale aggression would be to breed males later in life so you can see how they behave and not breed the "bullies" in the group.

I will make a separate article about Maternal Aggression at a later date, along with a more in-depth article about aggression since it all deals with Neurogenetics, and I'm not trying to confuse people with a bunch of scientific terms right off the bat! Haha.

A Mothers Love

It's been shown that the way a mother rat raised her young can have an affect on their temperament. [LINK] A mother rat that takes care of her young raises calmer, happier rats, while a mom that ignores her young tend to be more anxious. [LINK] A good mom rat makes a huge difference in a babies personality.


This article is meant to be a starting point for new breeders. While it is in no way shape or form a complete scientific study, it will hopefully help new breeders who are unsure as where to even begin when it comes to improving the temperament of their rats.

Lets do a quick recap!

If you have two rats that have decent personalities but want to improve on them, be sure to provide the parent rats a loving, nurturing environment, and then do the same for their offspring. Within the first generation you should see a lot of improvements in the temperaments of your rats!

A little socialization goes a long ways! 

Always remember that your rats are animals, with their own thoughts and feelings! Do not expect them to be limp beanbags all the time.

Sometimes line and inbreeding can also bring out hidden genetics that may involve aggression and fear, simply choose the offspring that either do not show those traits, or at that point you could outcross to a much more outgoing rat to bring in a better set of genetics!

I always recommend getting at least some of your rats from a breeder who has a solid line of friendly, happy rats! So if you decide to work on a variety that needs a bit more love, you will have the proper rats to help improve it!

I really hope this helps! There is a LOT that I did not touch on simply because I do not want to confuse people. There are many branches of genetics (like epigenetics, neurogenetics, and even nutrigenomics) that affect breeding, but those will be for another time!