The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 19 No. 6 • July 2016


SOCIAL BEHAVIOR: AGGRESSIVE OR PLAYFUL GRAY MATTER MAY MAKE A DIFFERENCE

Aggression is a character trait that is more commonly expressed by males. Not surprisingly, then, it is males that have always been the instigators and perpetrators of war. Sex, on the other hand, can also involve aggression (the extreme end of it being rape, which is also nearly exclusively carried out by men).

Sex, however, can take the place of violent combat by hijacking the aggressive behavior, a form of displacement activity. This is seen when you compare the sexual behavior in bonobos and chimpanzees. A recent paper (Rilling, 2012) studied the differences in the neural systems of bonobos and chimpanzees, trying to find what sophisticated brain imaging could reveal about why bonobos showed little aggressive behavior between males and females (who engaged in sex instead—the same is true for female-female interactions as well), while chimpanzees were inclined to engage in violent interactions, even including the killing of members of other chimp groups.

This is a very desirable type of research. We need to know how the energy of aggressive behavior can be used for peaceful activity, such as sex, and diverted from destructive ends. In the chimp-bonobo study, the results showed that “bonobos have more gray matter in brain regions involved in perceiving distress in both oneself and others, including the right dorsal amygdala and right anterior insula. Bonobos also have a larger pathway linking the amygdala with the ventral anterior cingulate cortex, a pathway implicated in both top-down control of aggressive impulses as well as bottom-up biases against harming others.” (Rilling, 2012) The authors suggest that this can help explain the relative non-violence of bonobos compared to chimpanzees, but “also behaviors like sex and play that serve to dissipate tension, thereby limiting distress and anxiety to levels conducive with prosocial behavior.”

The involvement of the amygdala, a brain area intimately involved in the sensing and regulation of fear and aggression, suggests that the switch to sexual behavior takes place in the case of bonobos with the active participation of this brain area. It would be very useful indeed to find out more about this “switch.”

THE SWITCH

The switch from aggressive behavior to sexual playfulness may be related to the following phenomenon. In studies of animals, such as rodents and non-human primates, the response to a threat involves a release of dopamine. You may be surprised at this, as dopamine is usually thought of as a response to a reward or the expectation of a reward. However, there is a subset of dopamine neurons that specifically respond to stress perceived as a threat. When an animal is exposed to a threat, its brain releases reduced amounts of dopamine in the nucleus accumbens in response to danger, but when the animal perceives that the danger is CONTROLLABLE, more dopamine (rather than less) is released in that area. “...whether an increase or decrease in [nucleus] accumbens dopamine levels is observed in response to stress depends on whether the stressor is appraised as controllable (increase) or not (decrease).” (Lloyd, 2016)

One hypothesis is that when a threat is perceived as controllable, it switches from representing a danger to a signal for possible safety and this makes all the difference to the dopamine releasing neurons in the nucleus accumbens. “Thus, provided the animal has the expectation that the situation is controllable, observation of the aversive stimulus should predict this future appetitive [rewarding] outcome...” (Lloyd, 2016)

References

Rilling et al. Differences between chimpanzees and bonobos in neural systems supporting social cognition. Soc Cogn Affect Neurosci. 7:369-79 (2012).

Lloyd and Dayan. Safety out of control: dopamine and defence. Behav Brain Funct. 12(1):15. doi: 10.1186/s12993-016-0099-7. (2016).

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