Differential serotonergic innervation of the amygdala in bonobos and chimpanzees

Abstract

Humans' closest living relatives are bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), yet these great ape species differ considerably from each other in terms of social behavior. Bonobos are more tolerant of conspecifics in competitive contexts and often use sexual behavior to mediate social interactions. Chimpanzees more frequently employ aggression during conflicts and actively patrol territories between communities. Regulation of emotional responses is facilitated by the amygdala, which also modulates social decision-making, memory and attention. Amygdala responsiveness is further regulated by the neurotransmitter serotonin. We hypothesized that the amygdala of bonobos and chimpanzees would differ in its neuroanatomical organization and serotonergic innervation. We measured volumes of regions and the length density of serotonin transporter-containing axons in the whole amygdala and its lateral, basal, accessory basal and central nuclei. Results showed that accessory basal nucleus volume was larger in chimpanzees than in bonobos. Of particular note, the amygdala of bonobos had more than twice the density of serotonergic axons than chimpanzees, with the most pronounced differences in the basal and central nuclei. These findings suggest that variation in serotonergic innervation of the amygdala may contribute to mediating the remarkable differences in social behavior exhibited by bonobos and chimpanzees.

Positive and balancing selection onSLC18A1gene associated with psychiatric disorders and human-unique personality traits

“…Stimpson et al. () has identified an evolutionary change in the serotonergic innervation of the amygdala between chimpanzees and bonobos, and suggested that it has led to differences in the aggressive behaviors, cautious temperaments, risk preferences, and performances on “theory of mind” tasks even between these closely related species. Raghanti et al.…”

A Comparison Between Bonobos and Chimpanzees: A Review and Update

“…Bonobos have more gray matter than do chimpanzees in brain regions thought to be concerned with empathy and social cognition . In bonobos, the amygdala, an area of the brain that regulates emotional responses, social decision making, memory, and attention, is larger and has more serotonic innervation than that in chimpanzees …”

Unraveling the evolution of uniquely human cognition

“…This hypothesis is supported by data revealing a syndrome of behavioral, morphological, and psychological traits in bonobos that are similar to those found in artificially domesticated species. The cooccurrence of these traits in bonobos and domesticated species is thought to result from selection (natural or artificial) against aggression, which has led to changes in developmental timing and neurophysiology, including alterations to the hypothalamic-pituitary-adrenal axis, androgen levels, and the serotonergic system (40,53).…”

“…2). Comparative brain imaging studies with chimpanzees and bonobos also reveal that bonobos have more gray matter in regions implicated in empathy and more robust neural pathways relating to the inhibition of aggression (86), with bonobos having approximately twice the serotonergic innervation of the amygdala in comparison with chimpanzees (53).…”