Social evolution and behaviour in Bonobos

The bonobo still remains the least understood species of great ape, and seems to hold many clues to understanding our own human evolution. Together with chimpanzees, bonobos represent our closest living evolutionary relatives, and understanding how behavioural differences and similarities in bonobos, chimpanzees and humans may have arisen, can help us understand how humans evolved. The Centre for Research and Conservation has one of the longest outstanding research projects on bonobos in captivity and has studied bonobos in Planckendael for over 20 years. The long-term project continues to yield new insights and add to our understanding of behavioural diversity and flexibility in this species. In 2013 we organised a symposium called “80 years of Pan paniscus” at the 5th Conference of the European Federation for Primatology in Antwerp, bringing together international bonobo researchers to discuss current trends in bonobo research. Following our previous studies in tolerance in this species, we investigated play behaviour in bonobos. We used data collected in six European zoos to see whether female bonobos are indeed more playful than males, as suggested by literature. When considering only play with mature partners or individual play, we did not find a significant difference between time spent playing between male and female bonobos in those two play contexts. Surprisingly we found that male bonobos spent significantly more time playing with infants, compared to female-infant play. Interestingly we found an effect of group on the amount of social play between adults, but no group effects on play including infants or on individual play. These results suggest that female bonobos do not play more than male bonobos, but that levels of playfulness can differ between groups and that care should be undertaken to generalise about the playful nature of bonobo society. In 2013, we started a new longer-term monitoring of infant development in bonobos. Bonobos have been described as a “paedomorphic” species, retaining infant characteristics into adulthood. This has mainly been applied to morphology, but also to behavioural aspects such as cognition and sexuality. However very little is known about the development of behaviour in mother-reared bonobos. Therefore we started a new long-term project on infant development and maternal styles. Master students from different universities will observe behaviour in a large sample of infants and juveniles in European zoos, to test hypotheses about developmental delay in this species. We facilitated research for a number of students. In 2013 we welcomed Sumir Keenan, PhD student from the University of St Etienne (France) and the University of St Andrews (Scotland), who recorded vocalisations of bonobos in Planckendael to identify individual vocal signatures, which will be used in a further experimental play-back study in 2014. The Bonobo Project in Planckendael has collected a large collection of bonobo urine samples over the years. In 2011 we transported this collection of over a 1000 samples to the Max Planck Institute in Leipzig, which resulted in a publication on age-related variation in urinary levels of thyroid hormones in 96 bonobos and 100 chimpanzees ranging between one and 56 years of age. Fresh urine samples were used for hormone measurements with a commercial competitive total triiodothyronine (T3) ELISA. In both species, immature individuals had higher TT3 levels than adults and there was a marked decrease in TT3 levels between age classes. The two species differed in terms of the timing of TT3 level changes, with chimpanzees experiencing a significant decline in TT3 levels after 10 years of age and bonobos after 20 years of age. The decline of TT3 in chimpanzees appears to coincide with the time when somatic growth terminates while TT3 values in bonobos decrease much later. This temporal asymmetry in urinary thyroid hormone levels indicates heterochrony in the ontogenetic changes of the two sister species and developmental delay in bonobos. The prolongation of high TT3 levels in bonobos, which is characteristic of immatures of both Pan species may affect the behaviour of bonobos; namely, the low intensity of aggression they display. Given that developmental studies are often based on post-mortem analyses of skeletons, measures of urinary thyroid hormones offer a non-invasive tool for exploring ontogenetic changes in living wild and captive hominoids. Time line: 2002-ongoing