Using External Vertebral Growth Rings to Assess Longevity in the Bengal Monitor (Varanus bengalensis)
An attempt was made to assess longevity using growth lines on bone surfaces from five, known-age, captive-raised Varanus bengalensis specimens. These monitors ranged in size from 22–52 cm SVL and spanned hatchling to adult ontogenetic stages. Growth rings were found to be prevalent within the glenoid cavities of vertebrae from all specimens. Total counts of these lines required removal of calcified cartilage deposits beforehand. Following such preparations, one to 11 growth lines were revealed. These counts strongly correspond with the known age of the specimens. It appears these structures form annually and can be used to age this taxon late into ontogeny. The methods developed here show great promise for providing a useful means to age skeletal material from the wild; however, follow-up field studies are recommended prior to such implementation.Abstract
Glenoid cavity of a Varanus bengalensis proximal caudal vertebrae showing peripheral external growth lines and calcified cartilage deposits that have formed in its center. (A) The specimen (UF 404856) is shown in cranial view. Several prominent growth zone ridges separated by thinner incised annular furrows are visible. Note the calcified cartilage ridge nearest the innermost unobstructed growth line. (B) Cavity from same specimen in which the cartilage has been partially removed. The cartilage ridge in this specimen was found to overlie a growth zone. An innermost vertebral core composed of trabecular bone was also revealed
Vertebral glenoid cavities from a growth series of known-age Varanus bengalensis specimens showing growth-line counts. Each specimen has been photographed at an angle that best shows the relief of the growth lines, some of which were revealed only after removal of overlying calcified cartilage. (A) First-year individual (UF 40387) shown in cranio-ventral view. Note the trabecular bone core below the interface, an opaque “birth line” along the interface, and a single annulus (denoted by a 1). (B) Four-year-old specimen shown in cranial view (UF 40486). Note the four definitive growth annuli (denoted by numbers) and pronounced zonal ridges between. (C) Eight-year-old specimen in cranio-dorsal view (UF 56285). Note the eight definitive growth lines (denoted by numbers) that become decidedly thinner beyond the fifth annulus. (D) Cranio-lateral view of 10.5-year-old V. bengalensis specimen (UF 76529). Eleven definitive annuli (denoted by numbers) are visible. (Note: single, weakly incised, partial laminations were also found in each of the animals shown in B–D that were four years and older but were not used in the author's total count and presumably those of the “blind observers” [see Materials and Methods]. Such lines are not counted during skeletochronologic analyses since they are likely the result of noncyclical physiological disruptions to osteogenesis [Castanet and Smirina, 1990])
Contributor Notes
Department of Biological Science, Conradi Building, Florida State University, Tallahassee, Florida 32306–1100. gerickson@bio.fsu.edu