The role of vasopressin (Avpr1a) and oxytocin (OXTR) receptor gene variation as a proximate base for inter-and intraspecific differences in personality in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes)

Date: 12 January 2016

Venue: Evenementenhal Planckendael - Leuvensesteenweg 582 - 2812 Mechelen

Time: 3:00 PM

Organization / co-organization: Department of Biology

PhD candidate: Nicky Staes

Principal investigator: Marcel Eens & Jeroen Stevens

Short description: PhD defense of Mrs. Nicky Staes - Faculty of Science, Department of Biology


Behavioral differences in bonobos and chimpanzees have been highlighted throughout the past century, ever since bonobos were discovered. The purpose of this study was to investigate in more detail how bonobos and chimpanzees differ in their personality and which proximate mechanisms may be shaping both within and between-species differences.

First, we measured personality in bonobos using two approaches: a personality psychology approach and a behavioral ecology approach. The psychological approach is based on ratings done by people who are familiar with the individual bonobos that rate them on a list of 54 personality adjectives. Dimension reduction analysis of ratings of 154 individuals (~80% of total captive bonobo population worldwide) yielded a six factor structure, very similar to the personality structure earlier described in chimpanzees. The factors were labeled AssertivenessR, ConscientiousnessR, AgreeablenessR, OpennessR, AttentivenessR and ExtraversionR. Validity of the resulting personality dimensions was tested and AssertivenessR, OpennessR and ExtraversionR dimensions showed good construct validity, as assessed through appropriate associations with sex, age and behaviors. Conscientiousness showed several significant behavioral associations but requires further investigation as factor item loadings differ between bonobos and chimpanzees, which impedes interspecies comparison of this factor. Agreeableness and Attentiveness showed weaker validity in this study and require further testing.

The behavioral ecology approach used frequencies and durations of coded observed behaviors to determine personality dimensions. Dimension reduction analysis for a total of 46 bonobos yielded a four factor solution: SociabilityB, OpennessB, BoldnessB and ActivityB. Validity of all four dimensions was high as assessed by their associations with sex, age and dominance rank. Our results also indicate that combining behavioral variables from naturalistic and experimental contexts into one factor analysis reduces the number of extracted personality dimensions, as variables from both contexts can cluster together. This approach helps in gaining better insight to distinguish which personality factors are robust, independent dimensions and should therefore receive different labels. Comparing the four dimensions resulting from observational data to six previously identified dimensions derived from ratings filled out by human raters, only revealed similarities between the Openness dimensions assessed in both methods. As the other dimensions showed fewer similarities, we conclude that dimensions from both methods should be considered complementary as they appear to capture different facets of bonobo personality. Furthermore, we found that these bonobo personality traits collected using both codings and ratings have heritabilities ranging from 0.13 (ExtraversionR) to 0.53 (SociabilityB), showing that variation in in all personality traits can at least partly be attributed to genetic variation.

Therefore our second goal was to determine the proximate mechanisms shaping variation in personality both between and within bonobos and chimpanzees, by looking at variation in two candidate genes: the vasopressin 1a (Avpr1a) and oxytocin (OXTR) receptor genes. In humans, a single nucleotide polymorphism (SNP) in the third intron of OXTR (rs53576 SNP (A/G)) is linked with social behavior, with the risk allele (A) carriers showing reduced levels of empathy and prosociality. As bonobos and chimpanzees differ in these same traits, we hypothesized that these differences might be reflected in variation at the rs53576 position. We sequenced a 320 bp region surrounding rs53576 but found no indications of this SNP in the genus Pan. However, we identified previously unreported SNP variation in the chimpanzee OXTR sequence that differs from both humans and bonobos. Unfortunately no further evidence was found for associations of these SNP variations with personality in chimpanzees.

Length differences in the promoter region of Avpr1a influence behavioral traits primarily related to sociability in different animal species ranging from voles to primates, including humans. Interestingly, bonobos show more similarity to humans in this polymorphic region compared to chimpanzees, as chimpanzees are often missing a ~350bp region in the promoter region of the gene (called the DupB region), which is suggested to explain part of the behavioral differences found in these species. Our study provides support for this claim, as associations were found for Avpr1a with personality differences both within and between bonobos and chimpanzees. We found an association between Avpr1a and observed Sociability in chimpanzees, and with rated AttentivenessR and observed OpennessB in bonobos. Furthermore, chimpanzees with two DupB+ alleles, who therefore resemble more the Avpr1a genotype found in bonobos, show more bonobo like behavioral profiles. These results highlight the importance of candidate genes with large effects on behavioral variation, with variation in Avpr1a possibly underlying both within and between species differences in social personality traits in Pan.