Osteoderm accretion as proxy for whole body growth in the American alligator (1161.3)

Abstract

Osteoderms are small bones found in the dermal layer of sauropsid scales, form an integral part of integumental armour and may participate in thermoregulation. Whether growth of osteoderms parallels that of the whole animal has not yet been ascertained. We studied histomorphometry of cervical osteoderms of juvenile female American alligator (Alligator mississippiensis) reared at 30°C on ad libitum diet. All animals had undergone a cardiac surgical procedure, which either rendered their circulatory system in‐series (experimental n=24) or retained the original in‐parallel design (sham n=36). In addition, animals were assigned to one of three exercise groups: sedentary, running or swimming (n=20 each) for 18 months. Body mass and length measurements were taken every other week, and animals received injections of fluorescent dyes (calcein, alizarin) during the growth period. We found a positive correlation (ANCOVA, p<0.01) between periosteal bone deposition in osteoderms and body growth. Neither surgical manipulation nor exercise had a statistically significant effect on this relationship. We propose annular growth lines in osteoderms can be used as part of non‐lethal methodology to track growth in populations of wild crocodilians. Further, osteoderm histology can inform our understanding of growth trajectories in extinct archosaurs, regardless of their cardiovascular design or activity patterns.