Rat Aggression

Jennifer Kroll
From the May/June 1999 Rat & Mouse Gazette

All of us would like to believe that our sweet socialized baby rat will stay as sweet as the day we brought it home, and that should be the reality we end up with. Unfortunately, all too often, we are being shown the clear danger of rat aggression. This is a very tough thing for us breeders to deal with for several reasons.

  1. Most aggressive rats that we encounter do not become aggressive until they hit a certain age as determined by their genetic background.
  2. Most of our rats have produced offspring that have been placed into new homes by the time they show any aggressive tendencies.
  3. Some of us may be dismissing rat aggression altogether or feel it can be bred out, or are possibly even sacrificing temperament for color, type, etc. No one should attempt to breed out rat aggression!

I am not excluding myself from the breeders who have let rat aggression be passed on from one generation to the next, but through this article I hope to inform people who may not know about it, and convince other breeders to be much more careful in what they are doing. If I could turn back the clock and remove rats from my breeding program, I would do it in a flash.


From my own experiences, aggression is a much harder thing to rid our breeding programs of than anything else out there. With health or color, or whatever you can breed one with good color to another with okay color and good type and get good results. But, with aggression this does not work.

For instance, a female named Opal that I gave to a friend. Opal was out of my sweetest female, Cherub, and her father was also very sweet at the time. She was not aggressive when I gave her to my friend, and I did not yet know her father was aggressive. The friend of mine bred Opal to the sweetest, kissiest, most lovable male in the entire world, Logan. Opal was about seven months old when she had her litter (from my own experiences rat aggression shows up in a rat between 6 and 12 months of age), and it was at that time that she started showing her aggressive tendencies. She attacked her own babies and soon attacked her owner to the point that she had quite a bit of damage done to her hand.

Opal came back to live with me at my request. Every day that her babies grew older I cringed at the thought that they might inherit her behavior. Sure enough when one of her children was about six months old, I received word that she had suddenly turned on her owner. Up until then, she had been very sweet and affectionate. The attack, which included the rat shaking my friends hand with her teeth, came without any warning. I still get chills thinking about it. I have not heard yet if anyone else in that litter has ended up the same way, but having one in a litter is far too many.


Rat aggression may also be shown on other rats, not just their owners. Male rats may get into brawls, but when one male escapes from his cage, finds a way into another maleís cage and savagely rips him to pieces, something is not right! This, again, is another lesson learned too late.

Monty ended up killing Gabriel, and several other males, but before he showed any strange behavior he sired two litters. I ended up placing all of the first litter except one, and he was one year old when he died from a nasty bout with mycoplasma. Monty's first son, Templeton, never showed any aggressiveness in his life. He got along well with others, and even won a prize in the Kissiest rat contest at one of the shows. Templeton also sired one litter. Monty's second litter contained 13 babies who were all vicious to me and other rats, and eventually killed each other off by the time they were six months old. Templeton's only litter contained aggressive males who by age five months had killed themselves off, and females that I have had no problems with to this day and are all living peacefully in a community cage, and they are about eight months old now.


It's tough sometimes to put aside some rats that we would like to use in a breeding program, but please, we need to make temperament our number one goal! Yes, even above health, I believe. After all what good is a rat who lives to be three or four if it lives out its life locked alone in a cage because its too unpredictable!?! This is another reason why records of pedigrees and contact information on every rat you place is essential! How can you notify your clients of a potential temperament or any problem if you canít get a hold of them? What happens if you canít get a hold of them and they breed a rat without knowing its background? Then the whole problem starts over again!


As rat buyers, we need to let breeders know what our priorities are. I often have people emailing me or calling me interested in rats that are, for instance, Blues or Minks. I realize that most of us at one time or another do get into that phase where we want something different in our rat cages, but also realize that by not doing your homework about temperaments, and a breeders own philosophy on them that you may be setting yourself up for trouble down the line. There are far too many people out looking for that certain color or variety, than are asking questions about a rat's temperament. I would be delighted to get requests for sweet socialized babies from lines with excellent temperaments, regardless of color or variety. Or if you really are looking for a good pet in one of those varieties, please ask a million and one questions about the temperaments of the others in the line, especially those in its pedigree over six to eight months of age. Remember, a good breeder won't breed what he/she can't place. This works two ways. Either we all forget about temperament and ask for the exotic colors and the lines of awesome temperaments die out or become rare. Or, we ask the questions we need to and get only the rats with exceptional temperaments and eventually some day it will go without saying that all rats make excellent pets. With so many rats with temperament problems out there, it's certainly hard to say that with out choking on the words.


I know there will never be a time when every rat has a wonderful temperament, but it is our responsibility, to the rats we love, to remove these "bad" lines from the breeding pool.