The Pantanal floods and drains on an annual cycle. As water retreats into channels and pools, fish concentrate, and Caiman yacare gathers at these choke points in numbers rarely seen among crocodilians. Aggregations of hundreds of animals along a single stretch of river are routine in the dry season, and regional population estimates run to millions.
The diet is dominated by fish, including piranha, along with snails and other invertebrates. The lower teeth of large individuals sometimes protrude through the upper jaw, giving the species its old common name of piranha caiman.
The population was hit hard by illegal hide hunting in the 1970s and 1980s, when large quantities of skins moved through unregulated trade. Enforcement, CITES controls and the development of legal ranching operations reduced that pressure, and the species is now Least Concern. Regional population estimates are extrapolations from aerial and spotlight surveys rather than direct counts, and carry wide margins.