Scaling of major organs in hatchling female American alligators ( Alligator mississippiensis )

Allometric equations represent relationships between morphological/physiological traits and body mass Y = aM b , where Y is the trait, a is elevation, b is the exponent describing the shape of the line, and M is body mass. We measured visceral organ masses in hatchling alligators ( Alligator mississippiensis ) from five clutches from approximately 45 to 500 g wet body mass. The interaction between initial egg mass and clutch identity was significant for initial hatchling mass, but only egg mass, not clutch, had a significant effect on initial snout–vent and head length. Kidney and liver mass showed biphasic scaling with body mass, as determined by “breakpoint” analyses, with the breakpoint at 120 g wet body mass. Kidney and liver wet mass showed slopes b  > 1.0 as animals increased approximately 45–120 g, with significantly lower b approximately 0.8–0.9 for alligators 120–500 g. Within kidney and liver mass, below and above the breakpoint, organ mass slopes tended to be similar across clutches. Lung and heart wet mass did not show biphasic scaling, with b approximately 0.8–0.9. Within lung and heart mass, clutches had statistically identical slopes. Combined clutch data for wet mass showed distinct regressions with b  > 1.4 for approximately 45–120 g alligators’ kidney and liver mass, compared with approximately 120–500 g alligators’ kidney, liver, lung, and heart mass b  < 1.0. Alligators show rapid kidney and liver growth following hatching, with higher rates than lung or heart tissue. Clutch, egg mass, and hatchling size influence organ size, and each factor should be accounted for in future studies exploring reptile morphology and physiology to assess environmental versus clutch contributions.