Stereotyped behaviors: behavioral markers of brain dysfunction and welfare impairment in animals

Stereotyped behaviors are repetitive and invariant actions that appear to lack an immediate goal or function and are commonly observed in animals maintained under captive or intensive husbandry conditions. Within animal welfare science, such behaviors are widely regarded as sensitive behavioral indicators of compromised welfare states [1].Stereotyped behaviors are repetitive and unchanging behaviors that occur frequently in specific contexts, appear to lack immediate purpose or function, and are commonly observed in animals housed in captivity or under intensive rearing conditions. In animal welfare science, stereotyped behaviors are widely recognized as external manifestations of compromised welfare in adverse environments [1,2]. Among domestic animals, pregnant sows are a valuable model for studying stereotypic behavior because of their apparent sensitivity to feeding patterns, spatial constraints, and environmental complexity. Under conditions of confinement or barren housing, sows frequently develop oral and locomotor stereotypies, including sham chewing, bar biting, and repetitive rooting [23]. Notably, stereotyped behaviors often persist despite partialeven after improvements in housing or management conditions, indicatingsuggesting that their expression cannot be fully attributed to short-term behavioral responses to adverse environments. Experimental evidence suggestsonce established, these behaviors may become partially decoupled from the original eliciting environment. Empirical studies indicate that environmental enrichment alone is frequently insufficient to abolish establishedmay reduce the frequency of stereotypy, whereas interventions targeting neural modulation can produce more robust reductions in but often fails to completely abolish wellestablished repetitive behaviors when combined with environmental improvements , implying longerterm neural or behavioral stabilization mechanisms . In parallel, human neuroimaging studies have demonstrated strong associations between alterations in cortico-basal ganglia white matter connectivity and restricted repetitive behaviors, particularly in neurodevelopmental disorders [48]. Collectively, these observations indicate that stereotyped behaviors reflect more than superficial behavioral abnormalities. In this Opinion article, we contend that stereotypy represents a cross-species neurobehavioral phenotype arising from convergent dysfunction within cortico-striatal-thalamocortical (CSTC) circuits. From this perspective, stereotyped behavior may be interpreted as a behavioral manifestation of impaired neural plasticity and altered brain function, with direct implications for the assessment of animal welfare. Stereotyped behaviors are repetitive and unchanging behaviors that occur frequently in specific contexts, appear to lack immediate purpose or function, and are commonly observed in animals housed in captivity or under intensive rearing conditions. In animal welfare science, stereotyped behaviors are widely recognized as external manifestations of compromised welfare in adverse environments.Cross-species stereotypy and convergent neural circuitryStereotyped behaviors have been documented across a broad range of species, including rodents, farm animals, and humans, suggesting that this phenomenon reflects conserved vulnerabilities in neural systems governing behavioral control [5]. [9]. Experimental models of stereotypy encompass environmentally induced, pharmacological, genetic, and pathology-related paradigms [6]. Despite substantial heterogeneity in induction methods and behavioral expression, these models largely implicate dysfunction within CSTC-related neural circuitry. The CSTC circuit plays a central role in behavioral selection, inhibitory control, and the regulation of the balance between goal-directed and habitual actions. [10]. Despite heterogeneity in induction methods, converging evidence from pharmacological, genetic, and neurodevelopmental models implicates dysfunction within CSTC circuitry. The CSTC loop plays a central role in behavioral selection, inhibitory control, and the balance between goal-directed and habitual action systems. Disruption of prefrontal regulation over striatal output biases behavior toward inflexible, habit-dominated patterns [11][12][13][14]. When top-down control from the prefrontal cortex over the basal ganglia is compromised, behavior becomes increasingly automatic, repetitive, and insensitive to environmental feedback [7]. [11]. Cross-species evidence indicates that the severity of stereotyped behaviors is often associated with structural and functional alterations in cortical-striatal connectivity, a relationship that has been observed both in animal models and in humans exhibiting restricted repetitive behaviors [48]. Table 1 summarizes representative animal models of stereotyped behavior across species. Although the overt expression of stereotypy varies-from oral movements in pigs to repetitive locomotion in rodents-these models converge on shared neurobiological characteristics, including dopaminergic dysregulation, impaired cortical inhibitory control, and maladaptive engagement of habit-related striatal pathways. This convergence supports the interpretation that stereotyped behavior constitutes a unified neurobehavioral syndrome rather than a collection of unrelated abnormal actions. of plasticity. Under chronic adverse conditions, synaptic and circuit-level remodeling may progressively stabilize rigid behavioral patterns, thereby favoring the emergence of persistent stereotyped actions [14]. [21]. At the circuit level, stereotyped behaviors are consistently associated with excessive engagement of sensorimotor striatal loops accompanied by diminished regulatory input from the prefrontal cortex. This imbalance biases behavioral control toward habit-dominated processes and substantially reduces behavioral flexibility [15]. [6]. Alterations in glutamatergic transmission and changes in dendritic spine densityremodeling have been reported across multiplein several models of stereotypy, leading to attenuated responsiveness to novel stimuli and impaired updating of behavioroutcome associations.repetitive and compulsive behavior. Dysregulated corticostriatal glutamatergic signaling is thought to impair behavioral updating and flexibility, while structural spine changes in the striatum may stabilize maladaptive habit circuits [22][23][24][25]. Farm animals subjected to long-term confinement may provide a valuable useful long-term naturalistic modelcontext for examining pathological neuroplasticity. In pregnantmechanisms parallel to those observed in laboratory models, especially as certain behavioral features such as persistence and resistance to change have been documented across diverse species. While direct neurobiological investigations in farm animals remain limited, physiological and behavioral data from long-term confined sows, suggest chronic housing restriction is stress-associated with persistent stereotyped behaviors accompanied by elevated cortisol concentrations and immunesystemic alterations, consistent with a state of sustained physiological stress with potential that may plausibly extend to neural involvement circuit remodeling[2, 103, 17]. Furthermore, high levels of maternal stereotypy have been linked to long-term emotional and behavioral alterations in offspring, suggesting that the neural consequences of stereotyped behavior may extend across generations, potentially through epigenetic mechanisms [16]. [26]. Taken together, these findings support the interpretation that stereotyped behavior reflectsmay reflect a relatively stable neuroplastic state rather than a transient or reversible stress response, reinforcing its significance as an indicator of enduring brain dysfunction and compromised welfare.adaptation that persists beyond the initial stressor, although accumulating evidence from epigenetic and circuit-level studies suggests that such changes may retain a degree of plasticity and potential reversibility under targeted interventions [21,27].From a behavioral standpoint, stereotyped actions are characterized by high degrees of automaticity, reduced sensitivity to feedback, and marked resistance to change-features that closely resemble habitual and compulsive behaviors. Within the continuum extending from goal-directed actions to compulsive or addictive behaviors, stereotypy appears to occupy an adjacent position [17,1813,28]. Although stereotyped behavior does not depend on exogenous substances, it may emerge from persistent abnormal activation of neural circuits mediating reward processing and habit formation. Dopaminergic signaling within the striatum plays a critical role in this process by facilitating the transition from flexible action selection to rigid habit expression [19]. [29]. In pigs, pharmacological blockade of dopamine receptors using agents such as haloperidol significantly suppresses environmentinduced stereotyped behaviors, supporting a contributory role for dopaminergic mechanisms in their maintenance [20]. [30]. Similarly, opioid receptor antagonists reduce the frequency and duration of stereotyped behaviors, suggesting that repetitive actions may generate endogenous reinforcement via intrinsic reward pathways [2131].At the molecular level, the transcription factor ΔFosB has been identified as a key mediator linking repeated neural activation to long-term neuroplastic adaptation. Sustained accumulation of ΔFosB within the striatum is a hallmark of substance-related and behavioral addictions, where it promotes compulsive behavior and diminishes behavioral flexibility [2232]. The widespread occurrence of abnormal repetitive behaviors across species raises the possibility that similar transcriptional mechanisms may contribute to the stabilization of stereotyped behavior [2333]. We propose that elevated ΔFosB expression may represent a molecular signature of long-term stereotypy, a hypothesis that warrants targeted experimental investigation.Stereotyped behaviors can be conceptualized as cross-species behavioral indicators of CSTC dysfunction and impaired neural plasticity rather than as simple responses to suboptimal environments. This framework has important implications for animal welfare assessment, which has traditionally emphasized observable behavior while insufficiently considering underlying brain function. Recognizing stereotypy as a manifestation of brain impairment underscores the importance of preventive strategies-particularly early and sustained environmental enrichment-to preserve neural flexibility.Recognizing stereotyped behavior as a manifestation of corticostriatal dysfunction and maladaptive neuroplasticity carries important implications for intervention strategies. Beyond conventional environmental enrichment, targeted approaches aimed at restoring behavioral flexibility and circuit balance may be required. Firstly, early-life environmental complexity during critical developmental windows may preserve corticostriatal plasticity and prevent the consolidation of maladaptive habit circuits. Secondly, combined environmental-pharmacological strategies targeting dopaminergic or glutamatergic modulation may enhance the reversibility of established stereotypy by facilitating circuit rebalancing. Thirdly, future welfare assessment frameworks may benefit from incorporating neurobiological biomarkers-such as measures of behavioral flexibility, corticostriatal connectivity, or molecular indicators of plasticity-to complement purely behavioral observations. Such integrative strategies may shift the field from reactive symptom management toward proactive preservation of neural function and adaptive capacity.