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.