Dickinson College Dickinson Scholar Student Honors Theses By Year Student Honors Theses 5-23-2010 Pressure and Duration of Constriction in Boa Constrictor is Influenced by a Simulated Prey Heartbeat Allison Elizabeth Hall Dickinson College Follow this and additional works at: http://scholar.dickinson.edu/student_honors Part of the Biology Commons Recommended Citation Hall, Allison Elizabeth, "Pressure and Duration of Constriction in Boa Constrictor is Influenced by a Simulated Prey Heartbeat" (2010). Dickinson College Honors Theses. Paper 86. This Honors Thesis is brought to you for free and open access by Dickinson Scholar. It has been accepted for inclusion by an authorized administrator. For more information, please contact [email protected]. Pressure and Duration of Constriction in Boa constrictor is Influenced by a Simulated Prey Heartbeat By Allison E. Hall With the collaboration of Amanda Hayes and Katelyn McCann Submitted in partial fulfillment of Honors Requirements for the Department of Biology Dr. Scott Boback, Supervisor Dr. Charles Zwemer, Supervisor Dr. David Kushner, Reader May 18, 2010 Abstract Constricting prey is energetically costly for snakes and therefore it would be beneficial to minimize this cost. However, the consequences of arresting a constriction event too soon could be deadly. Thus, the duration of constriction is bounded by competing demands to kill prey and conserve energy. Snakes possess mechanoreceptors within their ventral and dorsal skin that are used for detecting approaching predators and prey. This experiment sought to determine whether Boas (Boa constrictor) can sense a simulated heartbeat in their prey. It was predicted that if snakes possess this ability, those constricting rats with a simulated heart would constrict with greater pressure and increased duration than snakes constricting rats without a simulated heartbeat. We recorded constriction pressure from snakes constricting rats with and without a simulated heartbeat. Using a two-way unbalanced analysis of variance (ANOV A) we found that boas constricting rats with a simulated heart did so for longer and with greater total pressure relative to those constricting rats without a simulated heart. These data suggest that snakes may be capable of sensing the simulated heartbeat and will adjust constriction pressure and duration accordingly. Introduction Boa constrictor is a species of snake within the family Boidae with a geographic range from Central Mexico to Argentina (Stafford and Meyer, 2000). Boas are non• venomous and use constriction to subdue and kill their prey prior to consumption (Cundall et al., 2000). In general, constricting snakes use their axial musculature to constrict their prey; specifically, the three main epaxial muscles (spinalis-sernispinalis, longissimus dorsi and iliocostalis) produce the force needed to constrict a prey item (Lourdais et al., 2005; Figure 1). The epaxial muscles are thought to play a major role in flexing the vertebral column 2 laterally, which is an important part of both constriction and locomotion (Moon, 2000). Hence, the performance of these muscles may vary among and within species of snakes according to the type of locomotion used and the type of prey consumed. Snakes use a variety of sensory reception including infrared (IR), vibration, and olfactory mechanisms to locate prey (Cundall et al., 2000). Boas possess temperature sensitive neurons located beneath scales of their upper jaws (labial scales), giving them the ability to detect IR radiation (Ebert et al., 2007; Von During, 1974). This provides them with the ability to detect endothermic prey without the use of visual cues. The IR receptors provide input to the tectal region of the brain of the snake, where visual and heat information can be integrated (Buning, 1983). Like other snakes, boas also have the ability to detect vibrations (Proske, 1969). Proske ( 1969) demonstrated that Pseudechis porthyriacus has vibration sensitive nerves in the ventral and dorsal skin. These receptors also were found in branches of the trigeminal nerve feeding the mandible and maxilla. Other work has corroborated these findings, indicating that the vibration sensitivity is detected by two distinct sensory systems. The first is composed of receptors in the skin of the snake (as indicated by Proske) whose signals are processed by the midbrain. The second set of signals can be detected by the eighth cranial nerve and are thus termed the 'VIII nerve system' (Hartline, 1971 ). These systems were found within three snake families: Boidae, Colubridae and Crotalidae (Hartline, 1971 ). The third recognized method of prey detection is olfactory reception. These sensory receptors are located in the vomeronasal organ and detect specific chemical cues from the snake's prey (Cundall et al., 2000). Once a potential prey item has been detected, the snakes initially strike, throw a coil around the prey, and proceed to constrict. In most strikes boas contact their prey with the 3 mandible alone initially, which then is followed by the maxilla, although this pattern can vary slightly between individual strikes (Cundall and Deufel, 1999). Although the initial strike pattern may vary amongst snakes, prey restraint behaviors vary little within boas and pythons (Cundall and Deufel, 1999). Boas strike the anterior portion of their prey and use a horizontal coil; the prey item is thus maintained horizontal to the substrate. The coil is a ventral-lateral coil, such that the first loop contacts the prey ventrally while the second loop contacts the prey laterally (Mehta and Burghardt, 2008; Heinrich and Klaasen, 1985). Additionally, boas constrict prey by placing their anatomical right against the prey significantly more often than their left (Heinrich and Klaasen, 1985). Constriction of prey is energetically costly for the snakes. For example, Canjani et al. (2003) measured the aerobic metabolism of Boa constrictor amarali before, during, and after constriction and found constriction times of up to an average of sixteen minutes and average oxygen consumption of up to 0.276 ml 02 g-1 h-1, a six-fold increase from rest. Comparatively, Python molurus averaged 0.20 ml 02 g-1 h-1 during digestion, nearly a six• fold increase above standard metabolic rate (SMR). However, because digestion spanned an eight-day period this cost is likely greater than constriction (Secor and Diamond, 1997). Moon (2000) suggested that snakes use only as much force, and thus energy, as is necessary to subdue a prey item. Regardless, because constriction is costly, it is plausible that a mechanism exists for a snake to determine precisely when a prey item has expired and constriction is no longer necessary. This study aims to determine if that signal is the prey's heartbeat. There are many different cues that could indicate to a snake when the prey has expired. Movement, struggling, and breathing in addition to a beating heart would indicate a 4 living prey item (Moon, 2000). Snakes possess the ability to detect vibration, and movement in general, so they potentially have the ability to sense a beating heart in a prey item when it is being constricted (Hartline, 1971; Young and Morain, 2002). The cessation of the heartbeat would indicate that the prey item is dead and constriction is no longer necessary. Further, the mechanism by which prey are killed via constriction may be circulatory arrest (Hardy, 1994). If this is true, detecting the cessation of cardiac activity could be important in determining when it is safe to release. A similar study, conducted by Moon (2000) examined the constriction pressure produced by gopher snakes (Pituophis melanof eucus) and king snakes (Lampropeltis getula) in response to prey movement, simulated heartbeat, and respiratory movement. In preliminary trials, it was found that the snakes responded to the prey movement by increasing their constriction pressure, but the simulated ventilation and heartbeat did not prolong constriction times or increase constriction effort and were thus eliminated from the rest of the study (Moon, 2000). We hypothesized that boas have the ability to detect the heartbeat of their prey during a constriction event and alter their constriction pressure based on the presence or absence of a heartbeat. If the snakes have the ability to detect prey heartbeat, and utilize this to assess when prey have expired, we predict that snakes constricting prey with a sustained heartbeat would maintain a higher pressure for a longer duration relative to snakes constricting prey without a heartbeat. Methods Subjects The fourteen snakes of the species Boa constrictor used in this study were collected between 2002 and 2003 from the mainland and islands off of the coast of Belize and several 5 were born in captivity (Boback, 2006). Six males and eight females were used, nine were wild caught snakes and five were captive born snakes. Selection of prey In free-ranging boas, a typical prey item is between 20 and 40% of the snake body mass (Moon, 2000; Loop and Bailey, 1972). Therefore we used rats that were 20% (± 1 %) of the snake body mass for all constriction tests. The selected rats (Rodentpro.com, Inglefield, IN) were thawed from a frozen state to the room temperature of a snake holding room (28° C) overnight, prior to dry heating with a heating blanket. Instrumentation In order to monitor the pressure produced by the constricting snake, two pressure systems were used, one in the abdomen and one in the thoracic region of the rat. The thoracic system included both the simulated heart and a pressure-sensitive probe. Both were outfitted with a pressure transducer (Gould P.T.J. 4771, Holliston, MA, Figure 3). A 4.0 mm endotracheal (ET) tube (Sheridan, Loveland, CO) was used for the simulated heart. The 4.0 ET tube was cut along its length to create a pocket in which a 3.0 mm ET tube could reside. This allowed the thoracic system (simulated heart and pressure-sensitive probe) to be as compact as possible so that insertion into the rat was minimally destructive.