Rattus Biologicus:
Aggression in Rats

Caryl Hilscher-Conklin
From the March/April 1997 Rat & Mouse Gazette

Peering through the glass window at the adorable brown hooded rat stretched out in his plastic tube, I had no idea what I was getting into. I would soon claim him as my own, adopting the homeless animal after being notified by one of the Society for the Prevention of Cruelty to Animals (SPCA) shelter volunteers that he was available. Being naive and optimistic, I envisioned him sharing the rat condo with my other two male rats, Branta and Mykiss.

I'd had the two albino siblings for 1 1/2 years now, and they were very sweet, gentle beasts. I just knew they would welcome the poor, timid new rat into their home.

Not long thereafter, my world was shattered when I tried to introduce the boys using the tried and true vanilla method (where one swabs vanilla extract onto the rats' noses and genitalia in order to mask their offending scents from one another). The new rat, whom I had named Tricki Woo, immediately attacked unsuspecting Branta and Mykiss. When I tried to separate them, Tricki sank his teeth into my hand, inflicting a deep wound which caused grotesque swelling and required a late-night visit to the clinic. My husband admonished me that I must respect the behavior of my rats, and I begrudgingly agreed. Like all animals, much of their behavior is driven by instinct, hardwired and controlled by hormones and neurotransmitters... something beyond human control.

SALLY shows LOUIE who's boss in this photo of an introduction;
a good example of rat boxing. Photo by Kathy Bakken

Rat Boxing Aggression is that fighting instinct in beast and man which is directed against members of the same species. It is theorized that in natural conditions aggression helps to ensure survival of individuals (and thus the species), by preventing too dense a population from developing and exhausting all of its food sources. Fighting is generally considered to be a trait characteristic of males and uncharacteristic of females. However, many but not all female mammals exhibit aggression during the postpartum period in defense of their young.

The great animal behaviorist Konrad Lorenz observed that in every individual the readiness to fight is greatest in the most familiar place: in the middle of its territory. As the distance from territory increases, readiness to fight decreases proportionately as surroundings become stranger and more intimidating. This was not the case with my Tricki Woo; he was ready and willing to beat up Branta and Mykiss the minute he stepped through their door. What causes some rats to be more aggressive than others? There is certainly evidence that aggression has a genetic basis, at least in some animals. Some strains of rats are inherently more aggressive than others.

Some studies show that there are also differences in circulating testosterone levels between strains of rats, the more aggressive animals having higher testosterone levels and vice-versa. Interestingly, it has also been observed that individually-housed rats attack intruders more readily than rats that are housed in groups. In rats, exposure to androgens (the male steroids testosterone and dihydrotestosterone) during fetal development determines the number of cells produced in a part of the brain termed the sexual dimorphic nucleus.

Patterns of nerve connections within this area are also different in male and female rats. Therefore, androgen exposure during this period of development influences brain structure in rats. These changes could lead to differences in brain function and, thus, behavior. It has been shown that female rats that are situated between two males in utero (termed "2M" rats) are more aggressive towards other female rats, compared to those that lie between two females (0M), or a female and a male (1M). Apparently, prenatal exposure to androgens that diffuse into the amniotic fluid can have effects on fetuses in close proximity to the androgen source.

The olfactory sense is the primary means of social communication in rodents. It has been said that mammals think through their noses and it is not surprising that marking of a territory by scent can play a big role in eliciting aggression in rats. Pheromones are chemicals synthesized by a living cell that are released into an animal's external environment, where they induce a species-specific reaction. Sex steroids stimulate production of urinary pheromones that alter the frequency and intensity of fighting in rodents. Based upon behavioral observations, the urine of castrated rats lacks the aggression-provoking pheromone, and the urine of female rats may contain an aggression-inhibiting pheromone

After much deliberation and consultation, my husband and I decided that neutering Tricki Woo was our only option. We had learned the hard way that we needed to keep him away from the other rats for their safety, but now he was beginning to bite us whenever he caught a whiff of their scent on our clothing or hands. Removal of the major source of sex steroids generally suppresses the incidence and intensity of social aggression in rats. It should be noted that the response to this operation is very dependent on the animal's prior experience. For example, male rats that kill pups will continue to do so after castration, and dominant rats will always be dominant, even when the source of testosterone (the testes) is removed. Overall, it appears that neutering younger rats will yield more successful results than castrating older, street-smart rats. It is not clear just how experience exerts its effects on behavior, but it is likely due to the formation of permanent neural connections in the brain.

We didn't notice a difference in Tricki's behavior right away, but within a week or two, he was like a different rat. Our repeated attempts to introduce him to the other boys still failed, but we don't have to worry about him biting us anymore. He has become a sweet cuddly rat that comes running to the sound of his name, and he sits on my lap every day and lets me rub his fur and scratch his chin. We've accepted the fact that he'll always live as a loner, and we make an effort to give him extra attention to make up for that. We are so happy that we will be able to enjoy his presence for many, many more months!

This column is dedicated to the memory of my beautiful boy Branta (1-6-95 ~ 2-24-97)

Questions or comments can be directed to Caryl at Branta1@aol.com.