SREL Reprint #1847





Growth curve analyses and their application to the conservation and captive management of crocodilians

Brisbin, I.L., Jr.

Abstract. The analysis of growth is an important component of many studies of crocodilians both in the wild and captivity. Although reptilian growth has often been considered to be indeterminate, this is most likely an artifact of abbreviated longevity under natural conditions. In captivity, many reptiles including crocodilians, may live long enough for growth to eventually approach an upper asymptote. The 4-parameter Richards model describes sigmoid asymptotic growth and was fit to data from the literature for Alligator mississippiensis. Growth in alligator body weight could not be analyzed with the Richards model because of a lack of adequate data for larger individuals. The Richards model did however provide an adequate description of growth in total body length. Both the Richards and the widely-used von Bertalanffy sigmoid model described the growth of captive male and female alligators equally well. In the case of wild alligators, however, the Richards model explained significantly more variation than did the von Bertalanffy model. In these analyses, values for the Richards curve shape parameter were lower than that predicted by the von Bertalanffy model and suggested more accelerated growth shortly after hatching under wild conditions. Thus, the description of alligator growth with fixed-shape sigmoid models, such as the logistic, Gompertz or von Bertalanffy curves, may not be warranted because of the failure of the assumption of constant curve shape to hold across treatment groups. Although analysis with a flexibly-shaped sigmoid model offers many advantages over the use of simpler growth models, many growth data sets do not possess the characteristics required for use of them more complex methods. This is particularly true with regard to the need for measurements from older known-aged animals as they approach final asymptotic body size or weight.

SREL Reprint #1847

Brisbin, I.L., Jr. 1990. Growth curve analyses and their application to the conservation and captive management of crocodilians. p. 116-145. In Proceedings of the Ninth Working Meeting of the Crocodile Specialist Group, edited by F.W. King. Vol. 1. SSC/IUCN. Gland, Switzerland.

To request a reprint