What drives brain size evolution in insects? Using the “brain soup” to quantify insect brains by cell number
Evolutionary biologists and neuroscientists seek to understand how natural selection acts on brain size through changes in cell number and what those changes mean for cognition. However, these have been elusive subjects, in part because neuron population sizes for entire brains are notoriously difficult to approximate. Traditionally, brains were sectioned and stereological methods applied to count neurons and scale up to brain regions or the entire brain. Neuron cell size and density vary across the brain, leading to difficulty approximating whole brain counts through subsampling. Reliably estimating neuron numbers in vertebrate brains became more practical with Herculano-Houzel and Lent’s introduction of the isotropic fractionator. This method involves homogenizing a particular amount of brain and counting the nuclei in this homogenate, then scaling from a series of subsamples to estimate the number in the entire brain. This technique provided neuron numbers for the brains of many mammals and birds, and allowed powerful insights into comparative evolutionary neuroscience. Here, we modify the isotropic fractionator method for use in insects to provide estimates of brain cell number in species of solitary and social Hymenoptera (wasps, bees, and ants). We plan use comparisons to test whether neuron number scales isometrically with brain mass and body mass across taxa, and ask if behavioral and life history variables contribute predictably to cell number.