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Eunectes murinus (Green Anaconda). Subduing behavior.

 Constricting snakes coil around their prey preventing the prey from breathing. Additionally, they may cause circulatory arrest in their prey by applying pressure to the thoracic cavity that prevents the prey’s heart from beating (Hardy 1994. Herpetol. Rev. 25:45-47). Here, I present evidence that when a constrictor handles potentially dangerous prey, the violence of the attack, and method of constricting may produce structural damage to the prey that reduces its ability to defend itself or escape. The following observations were taken in the Venezuelan llanos, Distrito Muñoz, Apure State (7°30’N, 69°18’W).

On 26 April 1992, a female anaconda (455 cm total length, 46 kg mass), during the process of killing a young capybara (2.5 kg mass) dislocated the capybara’s spine at the cervical level. The snake did not eat her prey because apparently other capybaras attacked her. The capybara was found floating in the river the next day and examination of the body showed that the capybara had a dislocated spine and evidence of anaconda teeth marks on its skin, matching the size of the snake’s head.

On 24 March 1992, I found a female anaconda (413.5 cm TL; 40 kg mass) that regurgitated a female white-tailed deer (Odocoileus virginianus) weighing 10 kg. Upon examination of the regurgitated deer, I found that it had two broken ribs. I assume that the constriction process caused the deer’s ribs to break.

On 27 January 2001, a female anaconda (460 cm TL) regurgitated a full-grown male white-tailed deer (O. virginianus) that had a disjointed spine at the cervical level.

In May 1999, a large snake (ca. 450 cm TL) was observed constricting a large (ca. 180 cm TL) spectacled caiman (Caiman crocodilus). During the process of constriction, it was apparent because of the angle between the caiman’s tail and body, that the caiman’s spine was broken (Fig.1).

In a recent account, an anaconda constricted a white collared peccary (Tayassu tajacu) (Valderrama and Thorbjarnarson 2001. Herpetol. Rev. 32: 46-47) where the authors report that: “At some point, a muffled crackling sound was heard, resembling that of many bones breaking all at once.”  It is uncertain if the bones (e.g. ribs) of the peccary were actually breaking or if the sound was that of vertebrae being dislocated. The following statement by the authors: “ ...the snake coiled itself round the peccary’s torso and squeezed, visibly stretching the peccary length-wise...” suggests the latter rather than the former.

The evidence presented here demonstrates that constriction by anacondas can produce structural damage to prey in the form of broken bones and dislocated vertebrae.  Hardy (op. cit.) argues that the violence and pressure exerted on the prey is higher than what is needed to cause suffocation and contends that the violence and excessive pressure serves the purpose of producing circulatory arrest. While I do not disagree with Hardy’s interpretation, I believe the extra pressure and violence of the strike may also serve the purpose of disjointing the spine or breaking ribs to reduce a prey’s ability to escape or defend itself and to expedite death.

I thank the Wildlife Conservation Society, National Geographic Society, and Asociaccion para la Conservacion y Recate Ecologico ACRE, Zoo de Doue la Fontaine-France for financial and logistic support. I also thank COVEGAN, Estación Biologica Hato El Frío, for permits to work on their properties. I thank Tony Crocceta for providing photographic material. I am also in debt to M. Quero, P Azuaje, Mirna Quero, J. Thorbjarnason, M, Muñoz for their cooperation in the development of this research.


Submitted by Jesús A. Rivas, Department of Ecology and Evolutionary Biology, University of Tennessee.  Knoxville, Tennessee 37996, USA.




FIG. 1.  Female anaconda (ca. 450 cm TL) found constricting a large spectacled caiman (ca 180 cm TL) in the Venezuela Llanos.  It is apparent by the spine of the caiman has been dislocated.