Food consumption and growth in three species of water snakes of the genus Nerodia (Colubridae)

Several factors that can have an influence on the rate of growth were investigated in the water snake genus Nerodia. The study centered around the question of how these snakes attain the different adult body sizes that can be discerned between the sexes and between species. The three species used to investigate this question were Nerodia fasciata confluens, N. c. cyclopion, and N. r. rhombifera. In addition to the question of attainment of adult body size, the effect of different feeding regimens on growth, and the amount of individual variation in growth among members of the same litter were investigated. The Bertalanffy, logistic-by-length, and logistic-by-mass growth models were also tested against one another using growth data from N. fasciata to ascertain which model was the best predictor of the pattern of growth in this species. Both linear and non-linear regression models were used in the analysis of sexual, species, and individual variation in growth. Linear regression models were used in the analysis of the variation in growth due to the feeding regimen.

Changes in the feeding regimen had a more pronounced effect on the females than on the males. For increases in weight, the larger the meal size, the larger the increase in weight per gram of food consumed. For increases in length, the intermediate meal size was more effective in promoting growth on a per gram of food consumed basis than the large or small meal sizes. These conditions resulted in significant differences in the weight per unit length among snakes in the different meal size groups.

In general terms, females initially grew more rapidly than conspecific males in all three species, and showed a more gradual decline in the growth rate with time. This pattern resulted in the females attaining a larger adult size than the males. The relationships of the patterns of growth for males and females could be modified by changes in the feeding regimen.

Although N. fasciata attains the smallest adult size of the three species, it initially grew more rapidly than N. cyclopion and N. rhombifera. The latter two species, however, showed a more gradual decline in the growth rate with time than the former species. As a result, N. cyclopion and N. rhombifera eventually were growing at a more rapid rate per gram of food consumed than N. fasciata. Significant individual variation in the rate of increase in length and weight per gram of food consumed was evident in all the litters for every species.

The results of this investigation are discussed in relation to natural selection for certain growth patterns under varying conditions in natural populations. Questions are presented that could lead to further research into the forces shaping the pattern of growth in natural populations of snakes.