Developmental arcs of plasticity in whole movement repertoires of a clonal fish

Developmental plasticity at the behavioral repertoire level allows animals to incrementally adjust their behavioral phenotypes to match their environments through ontogeny. Quantifying this plasticity in sufficient resolution across substantial periods of development, however, has been challenging. Here, we use high-resolution tracking to monitor 45 genetically identical Amazon mollies (Poecilia formosa) reared in near-identical environments over their first four weeks of life. We analyze behavior at 0.2-s resolution to assess plasticity across entire behavioral repertoires. Testing a key prediction from Bayesian models-that plasticity should decline in stable environments-we measure plasticity using both individual behavioral metrics and a bespoke "behavioral entropy" approach in a multi-dimensional phenotype space. Surprisingly, and despite closely conforming to model assumptions, we find a consistent initial two-week increase in movement plasticity before a decline. These results challenge expectations about how plasticity unfolds early in life and highlight the importance of continuous behavioral tracking for evaluating developmental theories.