Correlation of Neuron Size and Number with Brain Size in Bumblebees

Abstract

Over the past several decades, cell size and its resulting effects on tissue and organ function, as well as on its overall ability of the animal to perform complex tasks, has been studied extensively. Neuronal size (diameter of individual neurons) could have an influence on intelligence, brain capacity, and ability to perform complex behavioral tasks. Furthermore, there appears to be an increase in number of neurons with an increase in brain size in vertebrates. In insects, increased neuron number has also been correlated with more complex behavior. In this thesis, I test the hypothesis that the neuronal number and/or neuronal size correlate with the brain size using an insect model. This may help elucidate the apparent positive correlation between brain size and intelligence. To achieve this goal, I used a species of bumblebee, Bombus impatiens. Bumblebee workers vary extensively in brain and body size and weight, therefore allowing comparison between individuals of the same species. Workers within a colony differ in size and the amount of work a worker does depends on their body size. Larger sized workers have more foraging capability than smaller sized workers and foraging requires a more demanding sensory integration and memory capacity. In my study, it was found that brain volume was positively correlated with bee body size. Three cell body regions of the brain were further analyzed: inside of the mushroom body calyces, a cell body region next to the lobula, and cell bodies associated with the antennal lobe. No significant correlations between neuron number per unit of volume (neuron density) and brain volume were found. Assuming similar neuronal density in large and small brains, increased brain size is thus correlated with an overall increased neuron number.