Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Life Sciences

Major Professor

Gordon M. Burghardt

Committee Members

John Echternacht, Susan Richert, Gary McCracken


Many species of a secretive nature that inhabit remote areas are largely unknown to science and have the potential to provide the diversity of life styles and factual information that is needed to unravel important questions regarding behavioral ecology. In this contribution I present some of the relevant information regarding the reproductive biology and general natural history of the green anaconda (Eunectes murinus, Family Boidae) gathered during 7 years of data collection in the Venezuelan llanos. Due to the novelty of the study animal I had to design methods to collect much of my data. I developed new methods for subduing and measuring the animals. I also document the efficacy of force feeding transmitters as a way to radio-tag the animals. Although the emphasis is on reproductive biology, I also present information on many other aspects of the anaconda's life history collected both systematically and opportunistically. Anacondas use relatively small home ranges in wet and dry seasons but they perform relatively long migrations from one season to the other. Anacondas seem to be generalist ambush predators that feed on wading birds at early ages, but switch to larger prey as they grow older. They risk being injured or even killed by their prey when attacking large, dangerous prey items. Anacondas experience relatively high mortality in early ages that decreases as they grow larger. Adult males always face relatively high risk of predation by caimans, which seems to be specially dense in the breeding season. Other causes of mortality are overheating, parasites and diseases. The determinants of breeding output were analyzed in detail using data collected from wild animals. Larger females produce large clutches of large individuals, but breed less often than smaller females, incurring a smaller reproductive investment in every breeding event, as well as on an annual basis, the maximum size of females seems to be optimized to maximize their breeding output. The maximum expected size for anacondas, as well as the maximum recorded in this study, are well below the maximum reported in the literature. I discuss this contradiction in light of my findings and possible environmental differences. The mating system of the species was analyzed using data collected from the field and from captive observations. Anacondas show a striking female biased Sexual Size Dimorphism (SSD), larger than the SSD reported for any other terrestrial vertebrate. This is especially surprising because males mate in multiple-male breeding aggregations, where larger males seem to benefit from their large size. Anacondas breed in large breeding aggregations composed of one female and 1 to 13 males. These aggregations last up to four weeks and are scattered in the landscape fairly unpredictably. Larger males seem to be selective in their mating, selecting larger females, and larger females are courted by a higher number of males. Males spend a considerable amount of time and energy in courtship and the mating season is relatively short. Hence, factual polyandry is proposed as the main mating system in the species. Multiple mating increases the breeding success of the females. Large variance in the female's breeding success related to male preference sets the scenario for the action of sexual selection on female size. The possibility of a runaway process acting on female size is proposed. I also review the mating system of other species of snakes as well as the evolutionary environment of the group and conclude that polyandry might be more widespread among snakes than formerly believed. Finally, I use my findings to review the possibilities of sustained management of the species. Due to their secretive nature, low commercial value of the skin, female biased sexual size dimorphism, reproductive biology, and slow growth rate, I conclude that harvesting wild populations is not a likely possibility. Ecotourism is a recommended way to incorporate the anacondas into the local economic activities.

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