Homing and feral pigeons differ in hippocampal formation neuron size: a Golgi study

Artificial selection for behavioural traits can significantly affect the anatomy of brain regions related to the behaviour under selection. The homing pigeon (Columba livia) is a prime example of how anatomical changes can arise from artificial selection. Compared with feral and other pigeon breeds, homing pigeons have a much higher density and number neurons in the hippocampal formation, a region important for spatial memory and homing. Neuron numbers and density are, however, only one component of a brain region's processing capacity and whether hippocampal formation neuron size and morphology also differ remains unknown. Using Golgi staining and virtual microscopy, we reconstructed and quantified the size and morphology of neurons within the dorsomedial and dorsolateral regions of the hippocampal formation in homing and feral pigeons. While no significant differences were found in the size or morphology of dorsolateral neurons between the two breeds, homing pigeons had significantly smaller neurons (approximately 30% reduction in total volume and soma volume) in the dorsomedial region compared to feral pigeons. These findings suggest that smaller dorsomedial neurons in homing pigeons may facilitate increased neuronal packing density. How these differences in neuron size reflect behaviour in homing and feral pigeons has yet to be determined, but our results suggest that there may be behavioural and physiological differences in spatial cognition between the two breeds.