The authors in this review analyze research involving aggressive behavior in rodents (mice and rats), nonhuman primates (Old World Monkeys and Chimpanzees) and humans and attempt to categorize the aggression as either adaptive to evolution or pathological. They divide adaptive aggression into two broad categories: competitive aggression, involving competition for resources, and protective aggression, involving the protection of the animal or its offspring from predators or potentially dangerous conspecifics. They question whether extreme cases of aggression, such as the deadly Chimpanzee “killing parties” and the “sexually selected infanticide” in primates and rodents would be considered pathological; while they remain inconclusive in regards to the Chimp killing parties, they believe the killing of infants to gain access to mates and/or resources is an adaptive form of aggression that selectively began as a form of competitive aggression. Through further reading in the article, “Lethal intergroup aggression leads to territorial expansion in wild chimpanzees” (Current Biology, Vol. 20 No. 12), I discovered that other researchers recently found that the killing parties are adaptive in the form of competitive aggression, as they lead to territorial and resource expansion and, therefore, greater reproductive success.
The authors are assuming, of course, that aggression, as a species-specific behavior, must have inherited characteristics in order to be shared by all members of the species. In their analysis of the aggression found in the social organization of rodents, the authors find that the dominant rat or mouse displays aggressive behavior that is highly adaptive since the territorial male has the access to reproduction. In both rats and mice, the dominant, more aggressive rodent dominates a group of breeding females, therefore having greater reproductive success compared to subordinate or non-territorial males. While, in mice, the subordinate males are kicked out of their territory by the aggressive, dominant mouse, at the age of puberty, rats form more complex dominant/subordinate hierarchical relationships that are maintained by aggressive behavior. The authors suggest that, based on studies measuring aggressive behavior and cerebrospinal fluid levels of the serotonin neurotransmitter metabolite 5-HIAA, that perhaps low levels of 5-HIAA correspond to an aggressive trait in rodents that is adaptive to evolution.
The social organization and use of aggression in Old World monkeys are much different than those of rodents; rhesus monkeys, for example, live in large social groups called “troops” which are headed by three or more generations of females with immigrant adult males. Aggression is a small portion of a monkey’s total daily activity (2-5%) and is usually at its highest levels when monkeys attempt to attain a higher social status: male monkeys emigrate from their natal group at the time of puberty, and both males and females aggressively attempt to prevent the emigrating monkey from entering their group (30 to 50% of emigrating monkeys are killed off or disappear). Competitive aggression can also become quite intense between groups when territory and resources need to be defended. The authors view the aggression that is correlated with low levels of 5-HIAA as maladaptive to evolution since it is related to those forms that are at excessive levels and are injurious and persistent. Similarly, in human studies, low CSF 5-HIAA levels are seen to be correlated to forms of aggression involving impaired impulse control, which are also excessive and lead to negative social consequences. Therefore, based on these studies, the authors suggest that in humans and nonhuman primates, low levels of 5-HIAA in the CSF may correlate to an impulsivity trait that is maladaptive to evolution. The impulsivity and excessive, injurious aggression are predicted to be maladaptive since they will lead to social isolation from the other group members and few opportunities to reproduce successfully.
The authors also discuss how genetic polymorphisms can be influenced by environmental factors, such as early life experiences, resulting in various aggression phenotypes. For instance, the polymorphism that results in low monoamine oxidase A activity is influenced by early life experiences, or “rearing” experiences, of the rhesus macaque. (Monoamine oxidase A degrades and inactivates serotonin and other neurotransmitters, so in MAO-A knockout mice, in which the MAO-A gene is not expressed, serotonin levels are increased. Despite this serotonin level increase in MAO-A mice, they exhibit increased aggression. In other words: low MAO-A activity equals increased aggression.) Rhesus monkeys who were normally reared by the mother showed higher levels of aggression when in competition for food only when they carried the low-activity MAO-A allele. Rhesus monkeys with the high-activity MAO-A allele and impoverished infancy showed species-typical levels of competitive aggression. Therefore, even when negative environmental factors come into play, high MAO-A activity results in a species-typical phenotype, showing the complex environment between genes and the environment.
This paper’s presentation of aggression did not focus on the physical structure of the communities, but rather the production of serotonin neurotransmitter metabolite 5-HIAA. Evolution may have brought about different levels of expression in various organisms, and this expression is vital to understanding the representation of forms of aggression in life-forms. This topic is one that we normally do not research nor read about in our everyday biology courses, so the exposure to a topic that is not common was very intriguing. I am still not sure whether social constraints are imposing specimens to express different levels of serotonin, however the constraints that these specimens have been exposed to could be resourceful in examining these levels of expression. This paper’s identification of in-vitro sampling is very significant to these sorts of studies and I would be interested in further research examining other protein expressions in life situations. As biologists, we often measure heart and breathing rate in stressful situations, but there are many other secretions that may be connected to other physical attributes such as the one examined in this paper, aggression. Further, evolution/mutation may be contributing to these levels of aggression by further enlarging the source for the secretion of serotonin neurotransmitter metabolite 5-HIAA in select individuals whereby enhancing the level of aggression found in certain samples. We didn’t have much time to discuss this paper or the relationship with evolution of a behavior or production of serotonin, so I would enjoy further discussing some of the topics that we touched upon. As a result of reading this paper I would be very interested in finding out more concerning the research in measuring brain size and what these sizes are in correlation with and how that research may be leading to a further understanding in aggression between populations. I further believe that in this study we have to examine inbreeding of populations, as this may be a leading cause in aggression between individuals. This impeding of population growth and diversity may also be a cause in the rate/expression of aggression of individuals. The gene flow between the population(s) may also be a contributing factor to examine with the rate of aggression in the sample populations. Overall, this was an excellent topic concerning evolution of a behavior and will enjoy reading more in the next paper.
When viewing sexually selected infanticide vs. killing parties it seems odd that one would an adaptive form of aggression while the other pathological (since they were slightly leaning that way in the paper). From a biological point they both free up habitat and reproductive possibilities. Taking into account that the chimps make “pleasurable noises” during the killing parties, I can understand why these particular authors may think the killings are more than adaptive aggression. You mention that later authors do place the killing parties in the adaptive aggression category, I wonder if they mention anything about the noises that are made? One addition to this paper that I think would have been neat is including Bonobos. This is somewhat overdone in the world of evolution (comparing the aggressive chimp to the friendly Bonobo) but I would be interested what differences this paper would have focused on and what difference in 5-HIAA levels would be present.
This paper was a very interesting read, but I felt as if it had a lot of personal input. As humans we at times have the luxury to view life beyond the pure survival stand point, and the authors really included this in the paper. They at times let their feelings show through a little; how the pleasurable noises during killing parties were somewhat disturbing or eating the young is viewed as wrong. While in reality these animals are doing what they must in order to ensure their survival and increase their chances to pass on their genes. Having said that I can imagine it would be hard not to apply feelings and human characteristics to these situations. As we talked about in class they mention how being impulsive is maladaptive. We all discussed that as humans sometimes being impulsive can increase your reproductive chances. This creates a characteristic that is unique and stands out, but again this is dancing across the previous line of involving feelings into a scientific study.
Thank you both for your interesting comments. I think it’s an interesting point that you brought up Matt, that, “evolution/mutation may be contributing to these levels of aggression by further enlarging the source for the secretion of serotonin neurotransmitter metabolite 5-HIAA in select individuals.” I think that an interesting way that I could test this idea in my study would be to breed the male observer rats that show an increase in aggressive behavior after chronic exposure to passive aggression with non-aggressive females and to observe whether their future offspring also show increased aggression levels. I am leaning more towards the idea that chronic exposures to passive aggression may result in an aggressive phenotype due to epigenetic modification of the genes that encode serotonergic or dopaminergic neurotransmitters and/or specific serotonergic or dopaminergic receptor subtypes.
I think that the difficulties that lie with behavioral studies is trying to find an objective, rather than a subjective way to measure the behavior. As I stated in my discussion, I believe the best way to do this is to develop a consistent way to score aggression among studies. Still though, that won’t rule out all the inconsistencies among these types of studies. Even with the same categorization, subjectivity lies in determining at what point stereotypical behavior can be seen as psychopathological. The lines are blurred in animals such as the rodent or rhesus monkey, where killing of offspring may be a survival tactic or defending territory commonly results in injury; in humans however, with higher levels of cognitive ability, these acts are obviously psychopathological.
For example, in my aggression study, I place an “intruder” male rat into a cage with a larger “resident” male rat who was previously housed with a breeding female. This “resident/intruder” paradigm results in an aggressive fight for dominance/submission, which the “observer” rat in the same cage (separated from the resident and intruder rats inside of a smaller aquarium cage) witnesses for ten minutes a day. Sometimes, the resident rat, though 100 grams larger, would become overly aggressive with the intruder rat even after dominance was clearly established. In these few instances, the intruder rat was injured so badly that we had to euthanize it.
Is this extreme, injurious case of aggression simply to defend territory, even though the cage they were placed in was a new cage with new bedding? Or is this aggression psychopathological? Dominance was already established, so the serious injuries inflicted on the smaller intruder served no purpose. Even when the researcher’s feelings or emotions aren’t involved, (which is rare unless you become desensitized to euthanizing rats or are a sociopath) complete objectivity can’t be acheived when behavioral scoring/categorizing is a judgement call.
Having never taken a neuro class during my undergraduate career (which I regret), this paper was a bit of an eye opener for me. I had no idea about these studies showing that aggression could be genetically-linked. We hear a lot about the nurture vs. nature debate and also hear people choosing one side over the other as a cause for different behaviors. It was really interesting to read about the experiments that created genetic lines of mice specifically selected for aggressive behaviors, and how, within a few generations, they developed animals with different levels of aggressive behavior. It would be interesting to know how much this holds true for humans as well. In mice and non-human primates, the authors suggest that aggression can be evolutionarily advantageous. But aggression isn’t currently advantageous for humans since society punishes most aggressive behavior. Obviously, medical ethics wouldn’t allow you to do as much with humans in this type of research as it would with mice or monkeys. But it would be interesting to look at the levels of aggression in humans using twin studies or adoption studies (I’m not sure that’s the correct term for it)…