Smaller brains in laying hens: New insights into the influence of pure breeding and housing conditions on brain size and brain composition

Abstract

During domestication, many different chicken breeds have been developed that show many alterations compared with their wild ancestors and large variability in parameters such as body size, coloring, behavior, and even brain morphology. Among the breeds, one can differentiate between commercial and noncommercial strains, and commercial strains do not usually show variability as high as noncommercial breeds but exhibit a high production rate of eggs (or meat). The breeding of high-performing laying hens, including the housing conditions of hens, is often a focus of concern for animal welfare, and to date, little is known about the correlation between housing conditions and artificial selection on brain structure. Based on an allometric approach, we compared the relative brain sizes of 2 inbred strains of laying hens (WLA and R11) with those of 7 other noncommercial chicken breeds. In addition, we examined the brain composition of laying hens and analyzed the relative sizes of the telencephalon, hippocampus, tectum opticum, and cerebellum. Half of WLA and R11 lines were kept in floor-housing systems, and the other half were kept in a single cage-housing system. Both strains of laying hens showed significantly smaller brains than the other chicken breeds. In addition, there was a significant difference between WLA and R11 hens, with R11 hens having larger brains. There was no difference in the relative brain sizes of floor-housed and cage-housed hens. WLA and R11 hens did not differ in their brain composition, but floor-housed hens showed a significantly larger cerebellum than cage-housed hens. Apparently, pure breeding over a long time and strong artificial selection for a high production of eggs is accompanied by (unintentional) selection for smaller brains. Further studies may also reveal differences in brain composition and the influence of housing conditions on brain composition.

The Effects of Domestication on the Brain and Behavior of the Chicken in the Light of Evolution

“…In the nidopallium caudolaterale, the housing system affects the serotonergic innervation pattern with the highest fiber densities being found in freerange hens. Mehlhorn and Petow [2020] showed such a correlation between housing conditions and brain morphology as well. Floor-housed hens have a significantly larger cerebellum than cage-housed hens.…”

“…Energy expenditure may also be redirected away from specific behavioral traits to sustain the energy costs of greater egg production, and such energy trade-offs may change the cognitive abilities of these hens [Dudde et al, 2018]. This could be reflected morphologically as smaller brain volumes and is in line with the finding of smaller brains in laying hens [Mehlhorn and Petow, 2020]. This could be a reason why such hens have difficulty adapting to these new housing systems.…”

“…There is only one study that describes the macroscopic and microscopic morphology of broiler brains [Gupta et al, 2016]; however, this study was mainly descriptive in nature, with no quantitative comparisons with other chickens. Mehlhorn and Petow [2020] compared two lines of laying hens with seven fancy chicken breeds. In an allo-Brain Behav Evol 2020;95:287-301 DOI: 10.1159/000516787 metric approach, it was shown that laying hens had significantly smaller brains than fancy breeds.…”

Data mining-based discriminant analysis as a tool for the study of egg quality in native hen breeds

“…In this regard, Araucana’s egg group differed from the rest of the Mediterranean and hybrid lines. Araucana geographic isolation may not only have promoted genetic and phenotypic distancing of this breed from the rest but also caused clear differentiation of its product 48 , 49 .…”

Nature calls: intelligence and natural foraging style predict poor welfare in captive parrots

“…Furthermore, recent evidence suggests that parrot species which commonly breed in captivity tend not to be invasive if released [ 102 ], consistent with them being smaller brained, and that birds become less encephalized with domestication (e.g. [ 108 ]), consistent with large brains being selected against in captivity. We therefore suspect that the lower SB of smaller-brained species does indicate that they adjust better to captivity than do larger-brained species.…”