The mammalian brain under domestication: Discovering patterns after a century of old and new analyses

Abstract

Comparisons of wild and domestic populations have established brain reduction as one of the most consistent patterns correlated with domestication. Over a century of scholarly work has been devoted to this subject, and yet, new data continue to foster its debate. Current arguments, both for and against the validity of brain reduction occurring in domestic taxa, have repeatedly cited a small set of reviews on this subject. The original works, their sampling, methodological details, and nuances of results that would be key to establishing validity, particularly in light of new data, have not been investigated. To facilitate and encourage a more informed discussion, we present a comprehensive review of original brain reduction literature for four mammalian clades: Artiodactyla, Perissodactyla, Carnivora, and Glires. Among these are studies that generated the most cited brain reduction values in modern domestication literature. In doing so, we provide a fairer stage for the critique of traits associated with domestication. We conclude that while brain reduction magnitudes may contain error, empirical data collectively support the reduction in brain size and cranial capacity for domestic forms.

Cranial volume and palate length of cats, Felis spp., under domestication, hybridization and in wild populations

“…An interesting recent example of the effect of specific breeds is presented by Balcarcel et al . [ 52 , 53 ] regarding cattle. The authors compare an impressive dataset of various cattle ( Bos taurus ) breeds with the ancestral aurochs ( Bos primigenius ), finding that all domestic cattle have smaller brains compared with their ancestors.…”

“…We further present new data regarding palate length, showing a lack of snout length reduction in domestic cats. Many studies documenting morphological changes foundational to domestication research are more than 40–50 years old, and in need of replication and further study in accordance with current scientific knowledge and standards (see overview across species by [ 52 , 53 ]). The current study is a step in this direction and helps to solidify the database for an increased understanding of domestication and its effects on morphology.…”

Self-Domestication: From Analogy to Homology via Genomics

“…Lord et al 39 , in their reply to Wright et al 10 , calls for more data regarding the association between domestication and brain size reduction. The juxtaposition of Balcarcel 67 and Hecht 68 provides a powerful illustration of the more complex nature of this issue. Nevertheless, recent detailed work point to a genuine association between domestication and brain size reduction for at least some species 69,70 .…”

The expensive-tissue hypothesis may help explain brain-size reduction during domestication

“…Domestic animals typically have relatively smaller brains than their wild ancestors [ 1 , 2 ]. Together with several morphological traits like white patches, floppy ears and curly tails, these changes are referred to as the ‘domestication syndrome’ [ 3 , 4 ].…”

“…A well-documented and nearly ubiquitous trait across domestic animals is a significant reduction in brain size [ 1 ], which also occurs in cats [ 8–10 ]. While the (cranial) neural crest does have a connection to the development of the fore- and midbrain, the causal connection to brain size itself remains hypothetical [ 6 , 11 ].…”