Stereotypic behaviors across species: From neurobiological mechanisms to farm animal welfare.

Stereotypic behaviors, characterized by repetitive, fixed, and seemingly functionless behavioral patterns, are critical indicator of compromised animal welfare and neuropsychiatric disorders. While stereotypic behaviors have been extensively studied in humans and laboratory animals, the neurobiological mechanisms of farm animals and their cross-species commonalities remain to be systematically elucidated. This review summarizes the progress in the research on stereotypic behaviors, with a particular focus on comparing the phenotypic similarities among humans, laboratory animals, and farm animals. It delves into key biological principles underlying stereotypic behaviors, including neurotransmitter system imbalances, basal ganglia circuit dysfunction, and the modulatory effects of environmental enrichment. Although these mechanisms have been thoroughly investigated in humans and laboratory animals, corresponding evidence in farm animals is still limited. Furthermore, this paper systematically analyzes the current limitations in farm animal stereotypic behaviors research, such as the lack of standardized quantification tools, inconsistent phenotypic descriptions, high economic costs, and insufficient interdisciplinary communication. However, several emerging opportunities promise to advance future developments, including the use of physiological and structural similarities across species for translational research, the application of computer vision, a focus on the potential regulatory mechanisms of the microbiome-gut-brain axis, and the integration of other advanced methods such as multi-omics and genetic marker screening. This review aims to provide new perspectives for a deeper understanding of the mechanisms underlying stereotypic behaviors in farm animals, offer a theoretical basis for developing precise behavioral welfare assessment and intervention strategies, and ultimately promote the advancement of farm animal welfare.