AGE-DETERMINATION IN MANATEES USING GROWTH-LAYER-GROUP COUNTS IN BONE

Growth layers were observed in histological preparations of bones of k nown-age, known minimum-age, and tetracycline-marked free-ranging and captive Florida manatees (Trichechus manatus latirostris), substantiat ing earlier preliminary findings of other studies. Detailed analysis o f 17 new case histories showed that growth-layer group (GLG) counts in the periotic bone were consistent with known age, or time since tetra cycline administration, but were less reliable in other bones. GLG cou nts were also made in periotic bones of 1,196 Florida manatees of unkn own age found dead from 1974 through 1991. These counts were conducted in order to assess variability and to determine relationships among e stimated age, size, sex, and degree of bone resorption. Resorption can interfere with accuracy of GLG counts. This effect does not occur unt il ages greater than about 15 yr and body lengths greater than 300 cm are attained. GLGs were also observed in periotic bones of Antillean m anatees (Trichechus manatus manatus) but were not validated against kn own-age specimens. Use of GLG counts in the periotic bone is suitable for application to studies of population dynamics and other age-relate d aspects of manatee biology.