Allometric and Biphasic Allometric Growth of Major Organs in Hatchling Female Alligators ( Alligator mississippiensis )

Allometric equations represent relationships between a morphological size (e.g., organ size) or physiological rate (e.g., metabolic rate) and body mass using the equation Y=aM b , where Y is morphological size or physiological rate, a is elevation or y‐intercept, b is slope and M is animal body mass. Vertebrate organ growth relative to body mass varies between allometric ( b <1.0 or b >1.0) and isometric (b=1.0), depending on the organ and species. In mammals and reptiles, heart and lungs have been shown to grow isometrically, whereas in mammals, the kidneys and liver have been shown to grow with negative allometry. Most allometric studies of organ growth focus on changes that span orders of magnitude achieved between juvenile and adult life stages or between various species of widely divergent size. This may ignore or possibly ‘wash out’ trends that occur during rapid growth in very early life stages. Large avian species and reptiles grow most rapidly during their early life history, and there can be significant genetic or ‘clutch’ effects for reptiles with different parentage. Large crocodilians, like the American alligator, can increase in size by a factor of >10 during their first year post‐hatching and show physiological differences between clutches that persist at least through the first year post‐hatching. We measured organ growth in hatchling female alligators from 5 clutches at 8–10 time points during their first order of magnitude of growth (~45 g to ~500 g). We tracked each individual alligator (N>90) from their original egg identity (clutch and egg mass). Animals were anesthetized and sacrificed, and their organs carefully dissected. Organ and animal wet masses (±0.1 mg and ±0.1 g, respectively) were measured upon sacrifice, and dry masses were measured after organs and the animal's carcass had fully dried (2d ‐ several weeks at 70°C) so that mass did not change by >1% between >18‐h spaced measurements. Slopes and intercepts for regressions of organ mass against body mass were compared among clutches using ANCOVA. Breakpoint analyses determined the intersection with minimum residual sum of squares or the intersection where the differences of the residual variances were smallest. Preliminary data showed that dry kidney and liver size increased in relative proportion to body mass ( b ~1.0), and heart size ( b ~0.9) and lung size ( b ~0.75) decreased in relative proportion. Dry masses showed less variation compared to wet masses. Within the first order of magnitude of growth, breakpoint analyses revealed that kidney and liver mass grew more quickly during the hatchlings first weeks‐to‐months post‐hatching, and our study is the first to document biphasic organ growth in hatchlings of a large, long‐lived reptile. Hatchling alligators appear to undergo a very early period of rapid kidney and liver growth following hatching, and these two organs grow with different kinetics than lung or heart tissue. Reasons for this disparity will be an area of further research. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .