Baboon travel progressions as a "social spandrel" in collective animal behaviour

How individuals in a group move relative to one another can influence both their survival and fitness. Spatial positioning has been well studied in baboons (Papio spp.), which travel collectively in line formations or "progressions." Early studies of baboon progressions presented contradictory findings on the progressions' order - some reporting random positioning of individuals, while others reporting non-random positioning, thought to protect more vulnerable group-members. Here, we revisit this topic and use high-resolution GPS tracking data to study travel progressions in a group of chacma baboon (Papio ursinus) on Cape Peninsula, South Africa. We identify 78 progressions over 36 d and find that progression orders are not random. We test four non-exclusive hypotheses to explain progression orders: vulnerable individuals position themselves in the middle (risk hypothesis), subordinate individuals position themselves at the front to gain better access to resources (competition hypothesis), dominant individuals assume leading positions (group decision-making hypothesis), or progression order is an emergent outcome of underlying social bonds (social spandrel hypothesis). We find no evidence that progression orders are adaptive responses to minimize an individuals' risk, maximize their resource acquisition, or are the result of decision-makers leading the group. Instead, we find that individuals' positions are predicted by pairwise affiliations, resulting in consistency in order, with more dominant individuals occupying central positions in progressions. This non-random structuring of individuals during progressions can be considered a side-effect or outcome of underlying social forces acting among individuals, providing an example of a "social spandrel" in collective animal behaviour.