Environmental enrichment affects growth and behaviour differently in head-started critically endangered freshwater turtles

Freshwater chelonians face severe extinction risk, and head-starting programs increasingly rely on captive rearing to offset poor recruitment. However, evaluating success by growth and release numbers overlooks welfare and post-release competence. We assessed head-started Critically Endangered Batagur dhongoka and Batagur kachuga under five enrichment treatments, combining growth data with individual-level behavioural scans and 24-hour group activity budgets over 367 days. Final body mass was highest under mixed enrichment (141.1 g) and lowest under no-enrichment (108.1 g). B. kachuga were heavier than B. dhongoka under all treatments. Enrichment-related behaviour was highest under natural enrichment (52.5%) and lowest under no-enrichment (11.9%), while stress-related behaviour was lowest under seminatural enrichment (1.6%) and highest under no-enrichment (10.7%). Mixed enrichment produced the highest basking response and most balanced group activity budget; natural enrichment the highest shelter-use response. Integrating growth and behaviour at the same month × species × treatment scale, natural enrichment ranked first, mixed second, and no-enrichment last. Environmental enrichment shapes growth and behaviour differently; growth and behavioural metrics should therefore be treated as complementary rather than interchangeable indicators of captive-rearing success. Each species-treatment combination comprised a single chamber, with individual-level behaviour from one marked animal; results are therefore descriptive rather than independently replicated.