Oral stereotypic behaviors in farm animals and their causes

High stocking density and suboptimal conditions limit animal behaviors in modern livestock farming. This is particularly evident in captive animals, in which the motivation for foraging behavior is often thwarted. Oral stereotypic behaviors are common in farm animals. Ruminants (e.g., cattle and sheep) show oral stereotypic behaviors such as tongue-rolling, self-sucking, and inter-sucking. Captive pigs exhibit oral stereotypic behaviors such as bar-biting, sham-chewing, and ear-biting. Chickens peck at drinkers, feeders, and pens. Stereotypic behavior in livestock can be reduced by selecting a specific diet composition that prolongs their eating time and increases their satiety. Furthermore, reducing stocking density and enriching the farming environment encourage livestock to explore and reduce stereotypic behavior. It is important to note that stereotypic behavior is also influenced by organismal physiology. Stereotypic behavior was considered an indicator of poor animal welfare. However, recent research has revealed that animals engage in stereotypic behavior as a response to external stimuli, aiming to alleviate the negative impact of these stimuli on their well-being. Animals that frequently show stereotypic behavior may have higher levels of stress. Certain stress indicators also affect the expression of stereotypic behavior, such as 5-hydroxytryptamine and dopamine. Consequently, further investigation is necessary to understand how stereotypic behaviors affect the physiological state and metabolic processes of animals. This paper discusses the research progress on the oral stereotypic behaviors of farm animals. The objective is to establish a foundation for enhancing livestock feeding conditions and optimizing feeding practices, ultimately reducing stereotypic behaviors.