The goals of this study were to estimate the stress at failure and the fracture toughness of whole manatee ribs fractured in impact; and to determine whether typical watercraft are capable of generating enough energy to break manatee ribs upon impact. The unique construction of manatee ribs enabled us to apply quantitative fractographic techniques to measure some fracture mechanics parameters. Adult manatee bone behaves more like a ceramic than other types of bone. Due to this, we were able to see many of the features observed for brittle fracture in ceramics. We were able to identify crack origins, and make quantitative measurements of crack size. Failure stress was constant across body size despite the increase in rib size as the animals grow. Similarly, flaw size was the same for all animals regardless of body size. Fracture toughness for whole ribs (measured as the critical stress intensity factor, K (C)) calculated from strain gage data was 8 MPa m(1/2). This value was greater than that reported for small sample specimens, suggestive of R-curve behavior in this bone. There were no differences between the sexes in their ability to resist fracture. Kinetic energy calculations indicated that recreational boats commonly found in Florida waters are capable of generating sufficient energy to fracture manatee ribs upon impact.