Declining rates of evolution and limited convergence in bat sternum shape can be explained by a ratchet of specialisation

Strong clustering in animal trait variety between subclades can be explained by evolutionary bursts early in clade history followed by subsequent decay in rates of change. However, the mechanisms underlying evolutionary burstiness and the unevenness of diversity across the Tree of Life are still debated. Suggested explanations appeal to founder effects and constraint release followed by stabilising selection. Bat evolution, which evokes founder effects given bats’ innovation of features that resemble developmental pathologies, is a model system for helping to disentangle whether and how evolutionary burstiness relates to stabilising selection and adaptive constraint. We examine the evolution of the bat sternum, a component of the skeleton integral to bat flight. We investigate the relationship between sternum shape, flight-style and roosting ecology across a representative dataset of 70 bat species and find that sternum shape differs strongly between taxonomic groups pursuing distinct flight-style ecologies. Further, variance within adaptive zones is reduced in species that roost in enclosed spaces. We explore further, estimating the history of roosting ecology across bats, and find that initial bursts in sternum evolution may decelerate as ecological constraints from roosting accumulate. We hypothesise that roosting adaptation constitutes a ratchet of specialisation, explaining bursty rate variation and perhaps bats’ ultimate settlement within adaptive zones structured by flight-style ecology.