Gut–Brain Axis and Stress Physiology: Linking Microbiome Diversity with Behavioral Adaptations in Captive and Free-Ranging Mammals

Abstract

Background: Stress physiology and behavior are influenced by the gut–brain axis, a bidirectional communication network that connects the central nervous system with the gastrointestinal bacteria.  Environmental factors, especially in captivity, may change the richness of microbiomes, which may have an impact on animals' stress reactions and adaptive behaviors.

Objective: This study aimed to investigate how gut microbiome diversity influences stress physiology and behavioral adaptations by comparing captive and free-ranging Rhesus macaques (Macaca mulatta) and Indian crested porcupines (Hystrix indica).

Methodology: A 12-month longitudinal comparison research was carried out on 40 individuals from both species (20 in captivity and 20 in the wild).  Fecal samples (n = 480) were taken every month for glucocorticoid assay and 16S rRNA sequencing.  Stereotyping, exploration, social interactions, eating, grooming, and mobility were all evaluated by weekly behavioral observations utilizing species-specific ethograms.  Microbial richness and composition were measured using alpha- and beta-diversity measurements, and statistical analyses included correlations, multivariate regressions, redundancy analysis, and mixed-effects models.

Results: Free-ranging mammals exhibited significantly higher gut microbiome diversity than captive animals (Shannon index: macaques 4.12 ± 0.18 vs. 3.45 ± 0.21, porcupines 3.85 ± 0.20 vs. 2.98 ± 0.19; p < 0.001). Captive individuals had elevated cortisol levels (macaques 28.5 ± 3.2 vs. 18.2 ± 2.7 ng/mL; porcupines 22.7 ± 2.9 vs. 14.5 ± 2.4 ng/mL; p < 0.01) and displayed more stereotypic behaviors and reduced exploration and social interactions. Microbiome diversity negatively correlated with cortisol and stereotypy but positively with exploration (p < 0.01). Regression and mixed-effects models confirmed that microbiome diversity and stress levels were significant predictors of behavioral outcomes.

Conclusion: Gut microbiome diversity is a key determinant of stress physiology and behavioral adaptability in mammals. Captivity reduces microbial richness, increases stress, and constrains natural behaviors, highlighting the need for enriched environments and dietary management to promote welfare.