Determining Breeding Status in Green Anacondas (Eunectes murinus): A Condition Index Assuming Isometry

Abstract

Abstract. Capital breeders are animals that breed with the stored energy they have gathered prior to the reproductive event. The amount of energy stored by a given individual can be quantified using morphological measurements and estimating a condition index that can help study the reproductive biology of wild-caught animals. Several condition indices have been used to study the biology and natural history of snakes, with the residual of the log-transformed data being one of the most popular. Here, I develop a simple condition index that assumes that the animals do not change shape throughout life except for changes in condition. This condition with isometry assumption (CIA) is unbiased for all size classes and simple to calculate. With data of animals whose breeding condition was known, I determine a cutoff of 0.875. Animals with a higher condition had a high chance of breeding, while those with lower condition did not. This method correctly predicts the breeding status of 94% of the observations. Using the same data set, I tested the residuals of the log-transformed regression and found far worse results. The CIA was also able to predict breeding status in other snakes opportunistically found in the study. The fact that it is possible to reliably predict the breeding status of anacondas based on their condition confirms that anacondas are capital breeders.