|InterJournal Complex Systems, 1844
|Manuscript Number: |
Submission Date: 2006
|Complexity and the evolution of the social brain|
How do social forces shape the brain? While our knowledge is limited, two productive approaches include exploring the relationship between social complexity and brain size and exploring the role of neuropeptides in social behavior (Insel & Young, 2001). The highly visual cichlids of Lake Tanganyika, renowned for their explosive radiation, are the ?gold standard? for exploring how social pressures influence brain evolution and behavior. They allow fine ecological, behavioral and neuronal comparisons within monophyletic groups (Meyer, 1993). We compared brains and behavior of closely-related, but socially diverse, species of the Ectodini clade. Evolution has produced multiple transitions from monogamy to polygamy in this clade. We previously showed that the telencephalon of monogamous species is 15-20% larger than that of the polygamous species, after controlling for phylogeny using independent contrasts (Pollen et al., 2006). What telencephalic structure is responsible for this expansion? We obtained volumetric measures from brain sections and found that area Dm, the amygdala homologue, was 34% larger in the monogamous, pair-bonding X. flavipinnis than the polygamous X. ochrogenys (p = .05). The amygdala is critical for social recognition in mammals. The neuropeptide vasopressin (AVP), found in the amygdala, among other structures, mediates social recognition (Bielsky et al., 2004). The AVP/AVT system (AVT is the homologue in lower vertebrates) may contribute to interspecific and intraspecific variation in social behavior (e.g., Lim et al., 2005a,b; Goodson & Bass, 2001). Using AVT immunoreactivity and controlling for brain size, we found a 2.3-fold increase in the number of AVT-immunoreactive cells in wild-caught, monogamous species compared to polygamous species (p<0.05). How much of the observed species-specific differences in brain/ behavior are developmentally determined by the social environment? Cichlid?s phenotypic plasticity is renowned, having been demonstrated for social behavior and brain structure/function, visual behavior and anatomy, and feeding morphology (e.g., Insel & Fernald, 2004; Kröger et al., 2001, 2003; Liem & Osse, 1975). We compared the visual acuity of the monogamous species raised under two different social conditions: isolated and normal social groups. At 4 months, the visual acuity of the isolated fish was significantly decreased at the higher spatial frequencies, indicating plasticity does play a role. These results suggest some of the key neural changes taking place as a result of social organization. Given the repeated transitions in social organization within this clade, we can now explore whether amygdalar expansion and AVT differences are fundamental to pair bond formation and social recognition in vertebrates. In this talk, we will also review the challenge of quantifying social complexity, the missing link in correlating complexity to brain structure.
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