Alligator, American

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Pelvic aspiration in the American alligator (Alligator mississippiensis)

Pelvic aspiration in the American alligator (Alligator mississippiensis)

the ribs caudally and medially to decrease the volume of the thorax, as is the case in most amniotes; (ii) by activity of the transversus abdominis muscle, and possibly other body wall muscles, which decreases the volume of the abdomen and shifts the liver cranially, reducing the volume of the thoracic cavity; and (iii) by activity of the rectus abdominis muscle, and possibly the transversus abdominis muscle, which rotates the pubic bones in the cranial–dorsal direction, decreasing the volume of the abdominal cavity. This also aids in shifting the liver cranially. Inspiration is produced (i) by cranial rotation of the ribs caused by activity of the intercostal muscles; (ii) by caudal translation of the abdominal viscera caused by contraction of the diaphragmaticus muscle; and (iii) by posterior abdominal expansion, caused by contraction of the ischiopubis and ischiotruncus muscles. We believe that these two muscles function to expand the posterior abdomen to make space for the caudal displacement of the viscera as a result of the action of the diaphragmaticus muscle. Although we have no direct recordings of movements of the pubic bones, videofluoroscopy recordings made recently by Dr Elizabeth Brainerd and Tomasz Owerkowicz confirm that the pubic bones of young American alligators rotate ventrally during inspiration (E. Brainerd and T. Owerkowicz, personal communication). Fig. 6. Illustration of the mechanism of ventilation in the American alligator. (A) Expiration is produced by caudal rotation of the ribs and constriction of the abdominal cavity, which produces a cranial translation of the viscera. Constriction of the abdomen results from activity of the transversus abdominis muscle and the rectus abdominis muscle, which rotate the pubes dorsally. (B) Inspiration is produced by cranial rotation of the ribs and caudal translation of the viscera. The viscera are pulled posteriorly by contraction of the diaphragmaticus muscle. The ischiopubis muscle increases the volume of the posterior abdomen by rotating the pubes ventrally.
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Population Status of the American Alligator (Alligator mississippiensis) in North Carolina.

Population Status of the American Alligator (Alligator mississippiensis) in North Carolina.

We conducted a wide-range two-phase adaptive sampling approach to provide insight into American alligator relative abundance and distribution at the northern edge of their range. Site characteristics coupled with our analyses indicated that several covariates affect alligator occupancy in North Carolina. Northern and western survey sites were less likely to be occupied by alligators. Alligators in North Carolina are most likely limited by temperature at the northern extent of their distribution. Alligators were less abundant or not observed in several lakes located in northern latitudes, despite the presence and availability of seemingly suitable conditions. Although American alligators are the northernmost occurring species of crocodilian and have behavioral adaptations to freezes (Hagan et al 1983), their distribution limits are most likely driven by cold temperatures (Brisbin et al. 1982). When ambient temperatures remain below 38˚ F for extended periods of time, alligators may die of
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Cardiovascular development in embryos of the American alligator
Alligator mississippiensis: effects of chronic and acute
hypoxia

Cardiovascular development in embryos of the American alligator Alligator mississippiensis: effects of chronic and acute hypoxia

The American alligator demonstrated a substantial tolerance to hypoxia during embryonic development relative to other developing terrestrial vertebrates. Chronic hypoxic incubation induced significant changes in embryo and organ mass, as expected, but it also changed normoxic control cardiovascular variables (heart rate and Pa) and blunted the cardiovascular response to acute hypoxic challenge. It is of interest that the earliest differences between normoxic and hypoxic embryos were the physiological responses to acute hypoxic challenges (70%). Not until 90% of incubation did gross morphological differences appear in the form of impaired growth and cardiac enlargement.
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Recruitment of the diaphragmaticus, ischiopubis and other respiratory
muscles to control pitch and roll in the American alligator (Alligator
mississippiensis)

Recruitment of the diaphragmaticus, ischiopubis and other respiratory muscles to control pitch and roll in the American alligator (Alligator mississippiensis)

Five captive raised American alligators (Alligator mississippiensis Daudin 1801) were used in this study (0.77–2.2·kg). Surgery was performed on each animal to implant four patch electrodes over different muscles associated with respiration. After implantation of electrodes, the animals were given at least 2·weeks to recover from the surgery in a dry, clean cage before data collection. Following the recovery period, the electrode leads were connected to an AC amplifier (Grass P5, Quincy, MA, USA), and a small inclinometer (2.5·cm ⫻ 2.5·cm; SQ-SI-360DA SignalQuest, Lebanon, NH, USA) was secured to the back of the alligator just caudal to the scapula. The animal was then placed in a 284-l aquarium (122·cm ⫻ 45·cm ⫻ 61·cm; L ⫻ W ⫻ H) filled three quarters full with 30°C water. The alligator was then given 1·h to adjust to the new environment before data collection commenced. During the observation period, the animals were monitored for muscle activity during three different conditions; with no weight added to the animal, with weight equaling 2.5% of the animal’s body mass taped under the jaw, and with weight equaling 2.5% of the animal’s body mass on the base of the tail. The order in which the different conditions were monitored was randomly chosen. The period for monitoring muscle activity for each condition lasted for a minimum of 1·h and at least five dives were recorded for each condition. Only dives that had a change in pitch greater than 45° and little change in roll were used for this analysis.
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Rib kinematics during lung ventilation in the American alligator (Alligator mississippiensis): an XROMM analysis

Rib kinematics during lung ventilation in the American alligator (Alligator mississippiensis): an XROMM analysis

In the present study, the skeletal kinematics of ventilation in the American alligator [Alligator mississippiensis (Daudin 1802)] are analysed using marker-based XROMM. These observations will allow us to investigate whether the axis of rotation of the vertebral ribs in crocodylians in vivo is the same as that predicted based on joint anatomy, as well as whether the migration of the parapophysis and the change in the orientation of the costovertebral joint are reflected in differences in the rotations of the vertebral ribs as we look at the cranio-caudal sequence of rib movements. It will also allow us to see whether rotations at the dorsal intracostal joint occur at all, and how these compare with the motion across the other costal joints. Additionally, this will serve as a valuable exploratory study in terms of simply describing how the rib segments move relative to one another, the sternum and the vertebrae compared with other sauropsids (e.g. Iguana, Salvator), which will also help inform reconstructions of ventilation kinematics in fossil taxa.
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Ventilation and gas exchange during treadmill locomotion in the American alligator (Alligator mississippiensis)

Ventilation and gas exchange during treadmill locomotion in the American alligator (Alligator mississippiensis)

American alligator illustrating the exercise/recovery transition. The top trace shows the signal from the pneumotachometer; exhalation occurs when the values are positive. The high-frequency ventilatory signals that sometimes occur during apnea are due to movement of air into and out of the buccal area. This air-flow did not contribute to lung ventilation and was not included in the analysis of ventilation. The periods between the vertical dashed lines represent the last minute of exercise and the first minute of recovery. A dramatic decrease in ventilatory frequency is apparent upon cessation of exercise. The bottom trace is the signal from the strain gauge. Each wave cycle indicates one locomotor cycle. The constant frequency and amplitude of the oscillations in this trace are indicative of steady locomotion. Large breaths occur during walking. The small-amplitude oscillations during recovery are due to motion of the thorax and abdomen during ventilation, not to locomotion.
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Terrestrial locomotion does not constrain venous return in the American
alligator, Alligator mississippiensis

Terrestrial locomotion does not constrain venous return in the American alligator, Alligator mississippiensis

The effects of treadmill exercise on components of the cardiovascular (heart rate, mean arterial blood pressure, central venous pressure, venous return) and respiratory (minute ventilation, tidal volume, breathing frequency, rate of oxygen consumption, rate of carbon dioxide production) systems and on intra-abdominal pressure were measured in the American alligator, Alligator mississippiensis, at 30°C. Alligators show speed-dependent increases in tidal volume and minute ventilation, demonstrating that the inhibition of ventilation during locomotion that is present in some varanid and iguanid lizards was not present in alligators. Exercise significantly increases intra-abdominal pressure; however, concomitant elevations in central venous pressure acted to increase the transmural pressure of the post caval vein and thus increased venous return. Therefore, despite elevated intra-
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Ontogeny of bite force in a validated biomechanical model of the American alligator

Ontogeny of bite force in a validated biomechanical model of the American alligator

Three-dimensional computational modeling offers tools with which to investigate forces experienced by the skull during feeding and other behaviors. American alligators (Alligator mississippiensis) generate some of the highest measured bite forces among extant tetrapods. A concomitant increase in bite force accompanies ontogenetic increases in body mass, which has been linked with dietary changes as animals increase in size. Because the flattened skull of crocodylians has substantial mediolaterally oriented muscles, crocodylians are an excellent model taxon in which to explore the role of mediolateral force components experienced by the feeding apparatus. Many previous modeling studies of archosaur cranial function focused on planar analysis, ignoring the mediolateral aspects of cranial forces. Here, we used three-dimensionally accurate anatomical data to resolve 3D muscle forces. Using dissection, imaging and computational techniques, we developed lever and finite element models of an ontogenetic series of alligators to test the effects of size and shape on cranial loading and compared estimated bite forces with those previously measured in vivo in A. mississippiensis. We found that modeled forces matched in vivo data well for intermediately sized individuals, and somewhat overestimated force in smaller specimens and underestimated force in larger specimens, suggesting that ontogenetically static muscular parameters and bony attachment sites alone cannot account for all the variation in bite force. Adding aponeurotic muscle attachments would likely improve force predictions, but such data are challenging to model and integrate into analyses of extant taxa and are generally unpreserved in fossils. We conclude that anatomically accurate modeling of muscles can be coupled with finite element and lever analyses to produce reliable, reasonably accurate estimate bite forces and thus both skeletal and joint loading, with known sources of error, which can be applied to extinct taxa.
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Home range and movements of American alligators (Alligator mississippiensis) in an estuary habitat

Home range and movements of American alligators (Alligator mississippiensis) in an estuary habitat

study of American alligator home ranges in marsh habitat, researchers used radio transmitters to conduct intensive daily and weekly tracking [8]. However, such tracking is lo- gistically challenging for monitoring movements of highly mobile species for more extended periods in remote areas. Satellite telemetry has been recommended as a better tool to gather location data on crocodilians over extended spatial and temporal scales [9]. Recently GPS - another contender in remote telemetry techniques - was used with acoustic telemetry to track two alligators in the Everglades estuary [10] and with these tools, researchers deciphered different movement patterns between the tracked animals. Whereas GPS telemetry is a viable alternative for remotely monitoring animal movements, the lower cost and in- creased battery longevity is an advantage of satellite tele- metry. The effectiveness of satellite telemetry to track crocodilian movements was shown for Crocodylus porosus in an estuary habitat in Cape York Peninsula, Australia [11]. However, the tracked crocodiles exhibited large scale movements around the coastline. Therefore, the effective- ness of satellite telemetry to track species movements over a smaller ecosystem-scale remained uncertain.
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Motor control of locomotor hindlimb posture in the American alligator
(Alligator mississippiensis)

Motor control of locomotor hindlimb posture in the American alligator (Alligator mississippiensis)

Although force platform data suggest that the activity patterns of several hindlimb muscles might be modulated between sprawling steps and high walk steps in crocodilians, force platforms provide only an indirect indication of muscle action. In the present study, we use electromyographic (EMG) recordings from the hindlimb muscles of American alligators synchronized with video of locomotor kinematics to test for modulations of hindlimb motor patterns correlated with the use of different limb postures. EMG patterns have been recorded for alligator hindlimb muscles during the high walk (Gatesy, 1994, 1997), but explicit analyses of postural effects on muscle motor patterns have not been performed previously. Our analyses will, therefore, provide insight into the basis for the ability to use both sprawling and more upright limb postures, a trait that has made crocodilians feature prominently in many analyses of the evolution of tetrapod locomotion (e.g. Bakker, 1971; Charig, 1972; Kemp, 1978; Brinkman, 1980; Parrish, 1987; Gatesy, 1991; Reilly and Elias, 1998; Blob and Biewener, 1999, 2001). In addition, our analyses of motor pattern modulation across limb postures in alligators will provide a new data set for comparison with other studies of behavioral modulation of motor patterns (e.g. Gruner and Altman, 1980; Nilsson et al., 1985; Buchanan et al., 1986; Macpherson, 1991; Johnston and Bekoff, 1996; Gillis and Blob, 2001), allowing us to explore potential general patterns in how muscle activity and patterns of recruitment change to allow the same morphological structure to perform a variety of tasks.
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Effects of chronic exposure to 12‰ saltwater on the endocrine physiology of juvenile American alligator (Alligator mississippiensis)

Effects of chronic exposure to 12‰ saltwater on the endocrine physiology of juvenile American alligator (Alligator mississippiensis)

American alligator (Alligator mississippiensis) habitats are prone to saltwater intrusion following major storms, hurricanes or droughts. Anthropogenic impacts affecting hydrology of freshwater systems may exacerbate saltwater intrusion into freshwater habitats. The endocrine system of alligators is susceptible to changes in the environment but it is currently not known how the crocodilian physiological system responds to environmental stressors such as salinity. Juvenile alligators were exposed to 12 ‰ saltwater for 5 weeks to determine the effects of chronic exposure to saline environments. Following 5 weeks, plasma levels of hormones [e.g. progesterone, testosterone, estradiol, corticosterone, aldosterone (ALDO), angiotensin II (ANG II)] were quantified using liquid chromatography and tandem mass spectrometry. Compared with freshwater-kept subjects, saltwater- exposed alligators had significantly elevated plasma levels of corticosterone, 11-deoxycortisol, 17 α -hydroxyprogesterone, testosterone, 17 β -estradiol, estrone and estriol whereas pregnenolone and ANG II were significantly depressed and ALDO levels were unchanged (slightly depressed). On the one hand, saltwater exposure did not affect gene expression of renal mineralocorticoid and glucorticoid and angiotensin type 1 (AT-1) receptors or morphology of lingual glands. On the other hand, saltwater exposure significantly reduced plasma glucose concentrations whereas parameters diagnostic of perturbed liver function (aspartate aminotransferase and alanine aminotransferase) and kidney function (creatinine and creatine kinase) were significantly elevated. Except for plasma potassium levels (K + ), plasma ions Na + and Cl − were significantly elevated in saltwater
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Surgical removal of right to left cardiac shunt in the American alligator
(Alligator mississippiensis) causes ventricular enlargement but does
not alter apnoea or metabolism during diving

Surgical removal of right to left cardiac shunt in the American alligator (Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

Crocodilians have complete anatomical separation between the ventricles, similar to birds and mammals, but retain the dual aortic arch system found in all non-avian reptiles. This cardiac anatomy allows surgical modification that prevents right-to-left (R–L) cardiac shunt. A R–L shunt is a bypass of the pulmonary circulation and recirculation of oxygen-poor blood back to the systemic circulation and has often been observed during the frequent apnoeic periods of non-avian reptiles, particularly during diving in aquatic species. We eliminated R–L shunt in American alligators ( Alligator mississippiensis ) by surgically occluding the left aorta (LAo; arising from right ventricle) upstream and downstream of the foramen of Panizza (FoP), and we tested the hypotheses that this removal of R–L shunt would cause afterload-induced cardiac remodelling and adversely affect diving performance. Occlusion of the LAo both upstream and downstream of the FoP for ~21  months caused a doubling of RV pressure and significant ventricular enlargement (average ~65%) compared with age-matched, sham-operated animals. In a separate group of recovered, surgically altered alligators allowed to dive freely in a dive chamber at 23°C, occlusion of the LAo did not alter oxygen consumption or voluntary apnoeic periods relative to sham animals. While surgical removal of R–L shunt causes considerable changes in cardiac morphology similar to aortic banding in mammals, its removal does not affect the respiratory pattern or metabolism of alligators. It appears probable that the low metabolic rate of reptiles, rather than pulmonary circulatory bypass, allows for normal aerobic dives.
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Characterization of Chitotriosidase Enzyme Activity in the Serum of the American Alligator (Alligator mississippiensis)

Characterization of Chitotriosidase Enzyme Activity in the Serum of the American Alligator (Alligator mississippiensis)

Chitotriosidase (ChT) is an endoglucosaminidase enzyme that cleaves chi- tinous substrates and has been strongly associated with innate immune activ- ity and the ability to identify non-selftissues. This enzyme activity was de- tected and characterized the serum from the American alligator ( Alligator mississippiensis ) using a fluorometric probe. Alligator serum exhibited vo- lume-dependent activity, with activity (2.1 ± 0.3 μmol/min) observed at dilu- tions as low as a 1:150, and maximum activity (5.2 ± 0.6 μmol/min) measured at a dilution of 1:30. Alligator serum ChT showed linear activity for approx- imately 20 min, at which time activity decreased exponentially, presumably due to the depletion of substrate. In addition, the ChT activity in alligator se- rum was temperature-dependent with low activity at 5˚C, a sharp increase from 10˚C - 30˚C, and maximal activity from 30˚C - 40˚C. The activity was inhibited in the presence of water-soluble chitin, but not mannan, indicating the specificity of the enzyme. The presence of ChT in alligator serum is likely to be partially responsible for the potent innate immune system of these cro- codylians, and particularly antifungal activities.
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Contribution of the diaphragmaticus muscle to vital capacity in fasting
and post prandial American alligators (Alligator
mississippiensis)

Contribution of the diaphragmaticus muscle to vital capacity in fasting and post prandial American alligators (Alligator mississippiensis)

Fig.·2. Sample recordings of ventilation during 5% hypercapnic anoxic treatment from two post-prandial American alligators. (A) Data from an alligator with a transected diaphragmaticus (body mass 93·g). (B) Data from a control alligator (body mass 94·g). A breath cycle normally begins with an exhalation, followed by an inhalation and subsequent apnea. Expiration and inspiration occur when the trace is above and below the solid line (zero marker for airflow), respectively. The beginning of inspiration has been aligned to better illustrate the difference between the animals in both time to inspire and breath size. Calibration bar, 1·s.
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Structure and function of the esophagus of the American alligator
(Alligator mississippiensis)

Structure and function of the esophagus of the American alligator (Alligator mississippiensis)

Esophageal structure and function were studied in juvenile American alligators (Alligator mississippiensis). The anatomy of alligators differs from humans in several important aspects: the crocodilian esophagus is more muscular and is composed entirely of smooth muscle. Functionally, the crocodilian esophagus is similar to that of mammals, but alligators have peak esophageal peristaltic pressures that are 2–3-fold greater than pressures in the human esophagus. As is found in humans, the incidence of esophageal reflux increased in postprandial animals compared with the fasting state. We
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Contribution of active atrial contraction to cardiac output in anesthetized American alligators (Alligator mississippiensis)

Contribution of active atrial contraction to cardiac output in anesthetized American alligators (Alligator mississippiensis)

Ventricular filling may occur directly from the venous circulation during early diastole or via atrial contraction in late diastole. The contribution of atrial contraction to ventricular filling is typically small in mammals (10 – 40%), but has been suggested to predominate in reptiles. We investigated the importance of atrial contraction in filling of the ventricle in American alligators (Alligator mississippiensis) by bypassing both atria (with the use of ligatures to prevent atrial filling) and measuring the resultant effects on cardiac output in anesthetized animals. Atrial ligation had no significant effects on total systemic blood flow before or after adrenaline injection. Unexpectedly, pulmonary flow was increased following atrial ligation prior to adrenaline treatment, but was unaffected after it. These findings suggest that the atria are non-essential (i.e. redundant) for ventricular filling in alligators, at least under anesthesia, but may serve as important volume reservoirs.
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Ontogeny of baroreflex control in the American alligator Alligator
mississippiensis

Ontogeny of baroreflex control in the American alligator Alligator mississippiensis

Baroreflex regulation appears in different species at different points in embryonic development. This study was designed to understand the development of the baroreflex in embryos of the American alligator at four different points of embryonic development (60%, 70%, 80% and 90% of a total incubation period of 72·days) and in 1- week-old hatchlings. Data from a separate study on 1- year-old alligators were included for comparison. The gain of the cardiac limb of the baroreflex was calculated from heart rate changes triggered by pharmacological manipulation of arterial pressure with sodium nitroprusside and phenylephrine. The results demonstrated that a vagally mediated hypertensive baroreflex was present during the final 30% of alligator development. A hypotensive baroreflex was not present in embryos but appeared in hatchlings, mediated by a combined effect of vagal and sympathetic efferents. Absolute baroreflex gain was maximal at 80% of
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Using the American alligator and a repeated measures design to place constraints on in vivo shoulder joint range of motion in dinosaurs and other fossil archosaurs

Using the American alligator and a repeated measures design to place constraints on in vivo shoulder joint range of motion in dinosaurs and other fossil archosaurs

Using the extant phylogenetic bracket of dinosaurs (crocodylians and birds), recent work has reported that elbow joint range of motion (ROM) studies of fossil dinosaur forearms may be providing conservative underestimates of fully fleshed in vivo ROM. As humeral ROM occupies a more central role in forelimb movements, the placement of quantitative constraints on shoulder joint ROM could improve fossil reconstructions. Here, we investigated whether soft tissues affect the more mobile shoulder joint in the same manner in which they affect elbow joint ROM in an extant archosaur. This test involved separately and repeatedly measuring humeral ROM in Alligator mississippiensis as soft tissues were dissected away in stages to bare bone. Our data show that the ROMs of humeral flexion and extension, as well as abduction and adduction, both show a statistically significant increase as flesh is removed, but then decrease when the bones must be physically articulated and moved until they separate from one another and/or visible joint surfaces. A similar ROM pattern is inferred for humeral pronation and supination. All final skeletonized ROMs were less than initial fully fleshed ROMs. These results are consistent with previously reported elbow joint ROM patterns from the extant phylogenetic bracket of dinosaurs. Thus, studies that avoid separation of complementary articular surfaces may be providing fossil shoulder joint ROMs that underestimate in vivo ROM in dinosaurs, as well as other fossil archosaurs.
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Atmospheric oxygen level affects growth trajectory, cardiopulmonary
allometry and metabolic rate in the American alligator (Alligator
mississippiensis)

Atmospheric oxygen level affects growth trajectory, cardiopulmonary allometry and metabolic rate in the American alligator (Alligator mississippiensis)

Incubation of alligator eggs under hypoxic conditions resulted in diminutive hatchlings (Fig. 1). This finding is in agreement with previous studies, which have consistently shown chronic hypoxia to retard growth in embryos from all major clades of extant vertebrates: fish (Sundt-Hansen et al., 2007), amphibians (Bradford and Seymour, 1988; Mills and Barnhardt, 1999), mammals (Mortola et al., 2000; Zamudio et al., 2005; Julian et al., 2007); as well as reptiles: squamates (Herman and Ingermann, 1996; Andrews, 2001), turtles (Kam, 1993), crocodilians (Warburton et al., 1995; Crossley and Altimiras, 2005) and birds (Wangensteen et al., 1974; Black and Snyder, 1980; Xu and Mortola, 1988; Dzialowski et al., 2002; Crossley et al., 2003b) (reviewed by Chan and Burggren, 2005). Most recently, domestic chicken eggs have become a popular model to study the effects of chronic hypoxia on embryonic growth at altitude (Giussani et al., 2007). Some of the earlier studies measured significantly lower blood oxygen saturation and depressed metabolic rates in embryos exposed to chronic hypoxia, and used these findings to explain why hypoxic embryos show reduced utilisation of egg yolk and reduced somatic growth (Kam, 1993; Crossley and
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Mechanics of limb bone loading during terrestrial locomotion in the green iguana (Iguana iguana) and American alligator (Alligator mississippiensis)

Mechanics of limb bone loading during terrestrial locomotion in the green iguana (Iguana iguana) and American alligator (Alligator mississippiensis)

sampled through an A/D converter at 500 Hz and stored on computer. The plate was designed to accommodate a 50 N vertical load; amplifier gains for each channel were adjusted appropriately for the weight of the animal to allow more sensitive resolution of forces. Calibrations verified that the response of the platform to loads was linear over the range of forces recorded. The natural unloaded frequencies of the platform were 210 Hz (vertical), 150 Hz (anteroposterior) and 75 Hz (mediolateral), all sufficiently greater than the stride frequencies of the animals studied (less than 4 Hz in both species) so as not to confound the signal produced by the ground reaction force. Cross-talk between channels (<3 %) was corrected in the data-analysis software. Force data were filtered digitally with a zero-lag, second-order, low-pass Butterworth filter (cut-off frequency ⭓ 10.5 times stride frequency) prior to analysis. The point of application of the ground reaction force was calculated initially as half the distance between the toe and ankle. As the heel lifted off the substratum, the point of application was recalculated for each frame as half the distance between the toe and the most posterior part of the foot contacting the ground. Thus, the ground reaction force shifted anteriorly through the step so that it was applied at the end of the toes by the end of the support phase (Carrier et al., 1994). Steps in which the right hindfoot contacted the force platform in isolation were selected for analysis. However, the behavior of the animals was difficult to control, and such steps were uncommon for the alligator. Therefore, additional steps were analyzed in which the right hindfoot was in isolated contact with the platform prior to peak force (i.e. overlapping contact by the right forefoot did not influence measurements of peak forces). All steps with left limb contact or substantial tail contact with the plate were excluded. The Pascal program used for kinematic analysis also synchronized force and kinematic data, with the beginning of the force traces indicating the beginning of foot contact with the platform (i.e. the first video frame digitized). Calculations of force components in particular directions and joint moments due to the ground reaction force then were performed. Inertial and gravitational moments about the hindlimb joints were assumed to be negligible during stance because they are typically small relative to the moments produced by the ground reaction force during stance (Alexander, 1974; Biewener and Full, 1992).
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