Determinate Growth and Reproductive Lifespan in the American Alligator (Alligator mississippiensis): Evidence from Long-term Recaptures

The perception that crocodilians exhibit indeterminate growth is common in the general reptilian literature. However, this assumption is frequently based on observations of immature and young adult animals and therefore lacks a complete understanding of adult growth patterns. Long-term mark-recapture studies appear to be the most certain method of determining growth patterns of adult crocodilians. From 1979–2015, we conducted a mark-recapture study of an American Alligator (Alligator mississippiensis) population on the Tom Yawkey Wildlife Center (YWC) in coastal South Carolina to examine long-term growth patterns and the influence of age on multiple reproductive parameters. We found no discernible linear growth in 19 of 31 adult female and 7 of 19 adult male alligators over periods of 5–33 years. The mean maximum reproductive lifespan for female alligators on the study site was 46 years, and females continued to reproduce for an extended period of time after reaching maximum size. The Schnute growth model predicted that male alligators grew at a faster rate and attained a greater estimated mean terminal snout–vent length (SVL) than females (males = 186.9, CI_0.95 = 184.5, 189.3 cm; females = 135.9, CI_0.95 = 134.1, 137.8 cm) at the hypothetical age 75. In addition, the model predicted that males exhibited a greater estimated mean size (SVL = 182.0, CI_0.95 = 179.6, 184.4 cm) and age (43 years) at which growth essentially ceased when compared to females (SVL = 131.4, CI_0.95 = 129.5, 133.2 cm; 31 years). However, actual growth records of individual alligators suggested that the growth model may have overestimated the age at which male alligator growth ceased. The estimated mean earliest age at sexual maturity was 11.6 years (CI_0.95 = 10.5, 12.8) for males and 15.8 years (CI_0.95 = 14.5, 17.1) for females. We also documented that alligators on the site commonly live to 50 and can possibly live to >70 years of age. This study provides evidence that both male and female American Alligators in a population in coastal South Carolina exhibit a pattern of determinate growth and adds to a growing list of studies suggesting crocodilians as a group exhibit this growth pattern rather than indeterminate growth. Our findings are important for modeling population growth and determining sustainable harvest rates, particularly for alligators living near their northern distributional limit where growing seasons may be shorter and onset of sexual maturity later than in more southern portions of their range.