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.