Sirens packs control vast territories, often spanning full rivers or swamplands. They are known to travel large distances, leaving when people in an area grow aware of their methods. Some groups have developed novel techniques, such as targeting leisure yachts or producing human-like vocalizations.

A fully grown male may reach up to 5.5 meters in length, with the largest reported individual reaching nearly 6 meters. Their bite force has been measured at 14,031 N, the highest recorded among crocodilians.

Upon reaching maturity, larger male sirens develop a nasal protuberance, which has adapted to look like the silhouette of a human head. Some individuals have been seen decorating it with grass and weeds to imitate hair. This attracts unwary swimmers who are lured into a sense of safety.