The evolution of behavioural systems: a study of grooming in rodents

Abstract

In the present paper we focus on the study of complex behavioural systems, within an explicit phylogenetic framework. We reconstruct the phylogeny of rodents using grooming sequences from 12 terminals. Using a method derived from graph theory, we decompose complex behavioural systems into strings of behavioural units (behavioural routines) which are then used as behavioural characters to compose the phylogenetic matrix in addition to three mitochondrial markers as molecular characters (the cytochrome b gene (cytb), the 16S ribosomal RNA gene and the 12S ribosomal RNA gene). Our results point to a highly structured behavioural morphospace: only a few characters have been selected for, within the total space of possibilities. The optimization of hundreds of non-homoplastic routines onto three distinct phylogenies (behavioural, combined data and the molecular supertree of Fabre et al., 2012) reveals the same evolutionary trend from simple to complex: while simple behavioural routines (zero- or first-order sequences) are synapomorphies at basal levels of the phylogeny, progressively more complex behaviours evolve later, appearing closer to the tips of the phylogeny. Also, the optimization shows that the organisation of units into modules of coordinated action patterns first evolved around large body parts, namely the head and the trunk, modules that were later fused into one single organising module among rodents. We support the use of complex behavioural systems as a promising tool in the study of evolutionary scenarios and discuss the role of routines length and microstructure to provide phylogenetic information and elucidate evolutionary processes.