Models, morphology and modesty

We have recently published an article (Druelle et al., 2018) showing that the overall body proportions of our two closest relatives – chimpanzees and bonobos – are rather similar. To the uninitiated this probably sounds very unspectacular, but surprisingly a large body of work has in fact been based on the assumption that chimpanzees and bonobos have fundamentally different body proportions. This includes a lot of my own work.

It was such a beautiful story. Bonobos had already been labelled the “Apes from Venus”, with female dominated societies in which conflicts were solved by means of sex. Chimpanzees, on the other hand, were the “Apes from Mars” and lived in male-dominated societies where infanticide was rife, and conflicts were solved through aggression. Morphologists happily joined in, stressing that bonobos had body proportions which were quite different from chimpanzees, with longer legs and shorter arms. Briefly: more like us. Therefore, bonobos were the prime stand-in models to help understand our own evolutionary background.  The data underpinning this idea were either on very small sample sizes or did not actually show this. But, it was such a beautiful story! However we now know, this is mostly wrong.

We measured bonobos and chimpanzees in Belgium and the Netherlands when they were anaesthetised for routine medical interventions. Based on simple external measurements, we applied a geometric model to yield masses and centre of mass locations for each body segment (such as the head, trunk, thigh, shank, etc.) Comparing the two species showed that there are only subtle differences between the species in most regards. Crucially, bonobos do not have longer legs and neither species approaches human-like leg lengths. So, in this respect bonobos are not better models than chimpanzees.

emb_blog_nov18_Kris_fig_small

Figure 1: Comparison of bonobo and chimpanzee body proportions with average segment length and proximal, medial and distal diameter in the frontal plane (in cm). Red dots indicate the position of the centre of mass (in brackets) from the proximal joint as a % of segment length (from hip joint for the trunk). Body mass distribution is given as a % of total body mass on the morphotype in the middle.

Should we forget about bonobos then? Certainly not. But let us be modest about our models. We have learned a lot from studying bonobos. And chimpanzees, gibbons, baboons, and many others that each help us answer specific questions (D‘Aout et al. 2014). Or, as statistician George Box famously said in 1978: “All models are wrong, but some are useful”.

Kris D’Août

References

Druelle F, Schoonaert K, Aerts P, Nauwelaerts S, Stevens JM and D’Août K (2018). Segmental morphometrics of bonobos (pan paniscus): Are they really different from chimpanzees (pan troglodytes)? J Anat. DOI: 10.1111/joa.12894

D’Août K, Aerts P and Berillon G (2014). Using primate models for the evolution of human locomotion: Concepts and cases. Bull Mem Soc Anthropol Paris 26, 105-110. DOI 10.1007/s13219-014-0102-5

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