Abstract
BACKGROUND: Aggressive behaviors are an integral part of competitive interactions. There is considerable variation in aggressiveness among individuals both within and among species. Aggressiveness is a quantitative trait that is highly heritable. In modern humans and macaques (Macaca spp.), variation in aggressiveness among individuals is associated with polymorphisms in the serotonergic (5-HT) neurotransmitter system. To further investigate the genetics underlying interspecific variation in aggressiveness, 123 wild individuals from five baboon species (Papio papio, P. hamadryas, P. anubis, P. cynocephalus, and P. ursinus) were screened for two polymorphisms in promoter regions of genes relevant for the 5-HT system (5-HTTLPR and MAOALPR). RESULTS: Surprisingly, despite considerable interspecific variation in aggressiveness, baboons are monomorphic in 5-HTTLPR, except for P. hamadryas, which carries one additional allele. Accordingly, this locus cannot be linked to behavioral variation among species. A comparison among 19 papionin species, including nine species of macaques, shows that the most common baboon allele is similar to the one described for Barbary macaques (Macaca sylvanus), probably representing the ancestral allele in this tribe. It should be noted that (almost) all baboons live in Africa, but within Macaca only M. sylvanus lives on this continent. Baboons are, however, highly polymorphic in the so-called 'warrior gene' MAOALPR, carrying three alleles. Due to considerable variation in allele frequencies among populations of the same species, this genotype cannot be invoked to explain variation in aggressiveness at the species level. CONCLUSIONS: This study provides another indication that 5-HTTLPR is not related to aggressiveness in primates per se, but may have been under differential selective pressures among taxa and potentially among populations in different geographic regions. The results on MAOALPR alleles in Papio indicate that variation in the metabolism of monoamine neurotransmitters and associated behaviors is more important among populations than among species. We, therefore, propose to compile behavioral data from additional populations of Papio to obtain further insight into the genetics underlying behavioral differences among primate species.