CASTLE: a training-free foundation-model pipeline for cross-species behavioral classification

Accurately and efficiently quantifying animal behavior at scale without intensive manual labeling is a long-standing challenge for neuroscience and ethology. Keypoint-based tracking emphasizes simplicity and efficiency but loses the richness of posture and context, while emerging foundation models capture pixel-level details, yet often require nontrivial efforts of retraining and can be more sensitive to backgrounds or lighting. Here, we present CASTLE, a training-free pipeline that addresses all these issues by synergistically combining foundation models for segmentation, tracking, and feature extraction. By isolating regions-of-interest (ROI), CASTLE first generates "focused (ROI-masked)" and orientation-invariant latent features, capturing rich postural details in zero-shot, fine-tuning-free manners. Following ROI isolation, CASTLE, through an interactive "Behavior Microscope" module, supports hierarchical clustering, for progressive, human-in-the-loop embedding and clustering. This enables raw-image-assisted discovery of behavioral classes without predefined categories. Across mice, Drosophila and C. elegans, CASTLE matches expert class annotations (>90%), reveals disease-relevant phenotypes in Parkinsonian mouse models. By eliminating purpose-specific model training and providing a raw-image-informed accessible workflow, CASTLE offers a scalable framework for interpretable, cross-species behavioral phenotyping.