Group Size as a Trade Off Between Fertility and Predation Risk: Implications for Social Evolution

Cluster analysis reveals a fractal pattern in the sizes of baboon groups, with peaks at ∼20, ∼40, ∼80 and ∼160. Although all baboon species individually exhibit this pattern, the two largest are mainly characteristic of the hamadryas and gelada. We suggest that these constitute three pairs of linear oscillators (20/40, 40/80 and 80/160), where in each case the higher value is set by limits on female fertility and the lower by predation risk. The lower pair of oscillators form an ESS in woodland baboons, with choice of oscillator being determined by local predation risk. Female fertility rates would naturally prevent baboons from achieving the highest oscillator with any regularity; nonetheless, hamadryas and gelada have been able to break through this fertility ‘glass ceiling’ and we suggest that they have been able to do so by using substructuring (based partly on using males as ‘hired guns’). This seems to have allowed them to increase group size significantly so as to occupy higher predation risk habitats (thereby creating the upper oscillator).

Gossip, Conversation and Group Size: Language as a Bonding Mechanism

Humans share social skills with other primates, but at the same time they possess cognitive capacities that make them unique as a social species. One explanation that could be advanced is that language and verbal communication might play an important role in differentiating humans from other primates (Tomasello 1999; Dunbar 1993, 1996). Different arguments have been provided to explain how language intervenes as supporting species-specific social skills. One is that language could have an important role as a bonding mechanism. The approach here is based on Robin Dunbar's (1993) claim that language is a bonding process related to social group formation. Language as a bonding mechanism contributes to the extension of group size. The paper provides a language-based interpretation of social networks conceived as a complementary approach to the one propounded by Dunbar. Another claim is added to Dunbar's argument by considering the fact that language must generate social ties and networks if it is to increase group size. The main argument is that language creates virtual networks by projecting social ties as public representations available to be reported and assessed (Livet and Nef 2009).

Antimicrobial strength increases with group size: implications for social evolution

We hypothesize that aggregations of animals are likely to attract pathogenic micro-organisms and that this is especially the case for semisocial and eusocial insects where selection ultimately led to group sizes in the thousands or even millions, attracting the epithet ‘superorganism’. Here, we analyse antimicrobial strength, per individual, in eight thrips species (Insecta: Thysanoptera) that present increasing innate group sizes and show that species with the largest group size (100–700) had the strongest antimicrobials, those with smaller groups (10–80) had lower antimicrobial activity, while solitary species showed none. Species with large innate group sizes showed strong antimicrobial activity while the semisocial species showed no activity until group size increased sufficiently to make activity detectable. The eusocial species behaved in a similar way, with detectable activity appearing once group size exceeded 120. These analyses show that antimicrobial strength is determined by innate group size. This suggests that the evolution of sociality that, by definition, increases group size, may have had particular requirements for defences against microbial pathogens. Thus, increase in group size, accompanied by increased antibiotic strength, may have been a critical factor determining the ‘point of no return’, early in the evolution of social insects, beyond which the evolution of social anatomical and morphological traits was irreversible. Our data suggest that traits that increase group size in general are accompanied by increased antimicrobial strength and that this was critical for transitions from solitary to social and eusocial organization.