CLIFTON, K.B.*; MECHOLSKY, J.J., JR.; YAN, J.-H.; REEP, R.L.; University of Florida, Gainesville; University of Florida, Gainesville; University of Florida, Gainesville; University of Florida, Gainesville: Toughness Determination of Manatee Rib Bone

Watercraft-related mortality of the Florida manatee (Trichechus manatus latirostris), caused by propeller wounds or impact, accounts for 24% of all deaths from 1976-2001, and comprises 80% of human-related deaths. Reducing watercraft-related mortality is identified as a high priority in the manatee recovery plan. In order to establish safe boat speeds for manatee protection, an estimate of the forces required to fracture manatee bone is needed. Fractographic analysis was used to calculate fracture toughness of manatee rib bone. Apparent fracture toughness is the ability to resist fracture, measured as the critical stress intensity factor (K_C). This is an estimate of the amount of energy required to propagate a macrocrack that can lead to breakage. Parallelpipeds were machined from ribs in the anterior, middle, and posterior body regions, and fractured in three-point bending. Measurements of fracture surfaces were taken under light microscopy to calculate K_C. Average toughness was 2.9 MPa�m^1/2 (±0.9 SD). In comparison, human and bovine bone ranges from 2 – 6 MPa�m^1/2, indicating that manatee bone is on average less tough (i.e. fractures more easily). Preliminary data indicate that toughness increases with body size in calves and subadults, but appears to peak in large subadults or young adults. This may be correlated to mineral content. Within individuals, toughness increases caudad, corresponding to a caudad decrease in mineral content of ribs reported by others. Fractographic analysis can be applied to fracture surfaces of whole bones to calculate the load that led to failure.