The Consequences of Encephalization: Testing Trade-offs in Cranial Evolution

Evolutionary changes in brain anatomy underlie some of the most significant evolutionary transitions, including the origins of birds, placental mammals, and hominids. Enlarged, globular forebrains are thought to have facilitated the evolutionary success of these groups by enabling new cognitive, sensory, and behavioural repertoires, thereby unlocking new ecological niches. For this reason, understanding the timing, rate, and cause of evolutionary changes in brain shape and size in birds and mammals has been a major focus in evolutionary biology for decades. But brain expansion comes at a cost: not only is the brain metabolically expensive, but the rest of the head must also physically accommodate and protect the large brain. When brain size increases, what are the evolutionary consequences for the structure and function of other closely associated anatomical systems such as the skull, eye, and cranial muscles? Conversely, what influence do these structures exert on the brain's anatomy and evolvability? The aim of this project is to quantify how the morphological diversity and macroevolutionary dynamics of cranial structures have been influenced by the tradeoffs among these structures. To achieve this goal, we will go beyond simply quantifying correlations between traits and instead analyse the cause-and-effect interactions between traits by developing new evolutionary causal modelling methods. We will examine the evolution of the head as an integrated whole, quantifying the cranial anatomy of hundreds of modern and fossil birds and mammals using advanced 3D imaging and morphometric methods pioneered in my lab. By interrogating the trade-offs among cranial structures across scales of evolution, from macroevolutionary to microevolutionary, and developing new phylogenetic comparative methods, we will provide new evidence and tools for understanding how the interactions among complex traits affect the evolution of diversity through time.