Material properties of manatee rib bone

Watercraft-related mortality of the Florida manatee Trichechus manatus latirostris accounts for 24% of all deaths from 1974 to 2006. Of these, the proportion of deaths due to impact accounts for 66% of all watercraft fatalities. To establish safe boat speeds for manatee protection, an estimate of the material properties of their bone is needed to quantify the biomechanical effects of boat strikes on manatees. Manatee ribs are unique in that they are pachyostotic. Machined specimens from ribs were tested in three point flexure. Strength, modulus, work of fracture and fracture toughness were determined for three size classes, by sex. The mean flexural strength ranged from 62 to 160 MPa, elastic modulus from 4 to 18 GPa and work of fracture from 3 to 6 MJ m⁻³. Fractographic analysis was used to calculate toughness, which averaged 1.4-2.9 MPa m¹/². The results show that manatee bone material is less strong and tough than other mammalian bone. Although the bone increases in static strength as the animals grow, it is not able to absorb more energy. In fact, the ability to absorb impact energy appears to decline with increasing size in animals over 265 cm body length. This decline is due in part to the bone's high density and mineral content. There was little change in mineral content with size, suggesting that material properties are likely correlated with bone quantity. There were no differences in mechanical variables between the sexes. The pachyostotic nature of the bone, which makes the manatee well adapted to its environment, also leaves it highly susceptible to fatal injuries from boats.