The cerebellar anatomy of red junglefowl and white leghorn chickens: insights into the effects of domestication on the cerebellum

Abstract

Domestication is the process by which wild organisms become adapted for human use. Many phenotypic changes are associated with animal domestication, including decreases in brain and brain region sizes. In contrast with this general pattern, the chicken has a larger cerebellum compared with the wild red junglefowl, but what neuroanatomical changes are responsible for this difference have yet to be investigated. Here, we quantified cell layer volumes, neuron numbers and neuron sizes in the cerebella of chickens and junglefowl. Chickens have larger, more folded cerebella with more and larger granule cells than junglefowl, but neuron numbers and cerebellar folding were proportional to cerebellum size. However, chickens do have relatively larger granule cell layer volumes and relatively larger granule cells than junglefowl. Thus, the chicken cerebellum can be considered a scaled-up version of the junglefowl cerebellum, but with enlarged granule cells. The combination of scaling neuron number and disproportionate enlargement of cell bodies partially supports a recent theory that domestication does not affect neuronal density within brain regions. Whether the neuroanatomical changes we observed are typical of domestication or not requires similar quantitative analyses in other domesticated species and across multiple brain regions.

Selection for Reduced Fear of Humans Changes Brain and Cerebellum Size in Red Junglefowl in Line with Effects of Chicken Domestication

“…However, although the more fearful line of birds had a smaller cerebellum relative to total brain mass, the neuron density was in fact higher, contrary to our hypotheses. This is not in line with comparisons between present-day domesticated egg layers and ancestral Red Junglefowl that found a higher number of neurons in the larger cerebellum of the domesticates [36], which in turn is consistent with studies of mice which have shown that increased number of cerebellar neurons is related to a decrease in fear-related behaviour [37]. Of course, neural processing capacity is a function of both the number of neurons and synaptic networks, and in this study, we were not able to estimate synapse densities.…”

“…Of course, neural processing capacity is a function of both the number of neurons and synaptic networks, and in this study, we were not able to estimate synapse densities. It is noteworthy that the size of the cerebellum relative to the rest of the brain increases substantially during the first four weeks of life of chicks, unlike all other brain parts, indicating that ontogenetic factors may play an important role in the development of this part of the brain [36]. In rats, extensive synaptogenesis occurs in the cerebellum during the early neonatal period [38], and for future studies, it will be important to include this aspect in comparisons such as ours.…”

Proportional Cerebellum Size Predicts Fear Habituation in Chickens

“…We have previously demonstrated that the cerebellum has increased during domestication, both proportional to the brain and in absolute size ( Henriksen et al, 2016 ). Additionally, this increase in cerebellar size in domestic chickens is associated with an increase in the foliation of the cerebellum, granule cell layer size, granule cell size and number, and Purkinje cell number ( Racicot et al, 2021 ). The intercross population used in this study therefore have the potential to vary in all these brain parameters.…”

“…Here, to test if the link between cerebellum size and fear habituation is consistent across development, we performed repeated behavioral testing in a fear inducing test (the emergence test) early in life and after sexual maturity (eight trials per age group) in a population of 36 chickens bred from a unique advanced intercross between domestic chickens (White Leghorn) and their wild ancestor, the red junglefowl. Domestic chickens have a much larger cerebellum (both absolute and proportionally to the rest of the brain, Henriksen et al, 2016 ) with proportionally more neurons ( Racicot et al, 2021 ) than red junglefowl, and differ significantly in their response to fearful stimuli ( Campler et al, 2009 ). By interbreeding these two chicken populations, we were able to generate a population with large phenotypic variation in (1) cerebellum size (both absolute and proportional to the rest of the brain) and (2) fearfulness.…”

Domestication effects on social information transfer in chickens

“…The latter may be related to the fact that brain size and composition have been altered in domesticated birds. While total brain mass relative to body mass has been reduced, some parts, primarily the cerebellum, in fact, increased in relative size (Henriksen et al 2016 ), the latter mainly through increase and enlargement of granule cells (Racicot et al 2021 ). For a long time, the cerebellum has been considered to mainly control motor activity, but later research has shown its much broader involvement in social cognition and memory in many species (Van Overwalle 2009 ).…”