Sara Lipshutz, Duke University – Multiple Evolutionary Routes to Building an Angry Bird

Angry birds aren’t just a videogame; they have a biological reason to be angry in real life too.

Sara Lipshutz, assistant professor of biology at Duke University, explores why.

Sara Lipshutz is an Assistant Professor in the Biology Department at Duke University. Her lab’s research focuses on the evolution of behavior across weird and wonderfully diverse species of birds. This work bridges “muddy boots” experimental fieldwork with a variety of molecular and computational approaches in population genomics, transcriptomics, and neuroendocrinology.

Multiple Evolutionary Routes to Building an Angry Bird

One of the most challenging things an animal needs to do, in order to pass on its genetic variation, is reproduce. And for a lot of animals, reproductive success involves competition for important resources, like mates or breeding sites, and aggressive behavior. But what makes an individual aggressive? Why does this differ between females and males, and across species? To understand the evolution of aggression, we need consider the ecological context, like resource availability, as well as its underlying molecular mechanisms.

Our study examined a repeated behavioral target of natural selection in birds: obligate cavity-nesting, the strategy of nesting in natural or abandoned tree cavities. These birds also breed in nestboxes, which makes them relatively easy to study. We hypothesized that competition for limited cavity nest sites generates higher territorial aggression, especially in females, because they need to lay their eggs in the nest cavity. By measuring aggression in over 300 females and males, across 10 different bird species, we found that cavity-nesting is associated with increased physical aggression. In particular, we found that females in cavity-nesting species have higher territorial aggression than their flexible-nesting relatives.

We then wanted to know whether the repeated evolution of cavity nesting has selected for parallel changes in testosterone and neurogenomic patterns across avian families – did the shared aggressive behavior evolved from diverse or similar molecular mechanisms? Interestingly, the more aggressive species did not have higher levels of testosterone in circulation. We then compared gene expression in the brain, in a region including the extended medial amygdala. We found diverse gene regulatory signatures of aggression, suggesting evolution deploys many diverse paths to build an aggressive bird.