The Effect of Altitude on the Metabolic Rate in Sceloporus Occidentalis

The Effect of Altitude on the Metabolic Rate in Sceloporus occidentalis.

Jennifer Doncost and Andrew Naillon

Department of Biological Sciences

Saddleback College

Mission Viejo, California 92692

The resting metabolic rates of ten Sceloporus occidentalis normally living at an altitude of 186 meters were calculated from CO2 production data collected at an elevation of 2194 meters (ALT) and at 3 meters (SL). The oxygen content at altitude (2194 m) was 77% of the 100% available sea level (3 m). The mean resting metabolic rate (RMR) for S. occidentalis at ALT wasfound to be 0.1994 ± 0.0201 ml O2 g-1 hr-1 (MEAN ± SE, n = 10). At theSL,testing site the mean RMR was 0.1922 ± 0.0215 ml O2 g-1 hr-1 (MEAN ± SE, n = 10). The difference in the mean resting metabolic rates was .0072 ml O2 g-1 hr-1. A one-tailed, pairedt-test (p = 0.3791) with a 95% confidence interval indicated that there was not a significant difference between the data collected at the different altitudes.

Introduction

Most of the energy that animals expend is through their metabolic processes (Angilletta 2001). The process can only occur through specific sets of chemical reactions that produce energy for the organism to survive. This reaction as a whole, of converting different chemicals to energy, heavily depends on the oxygen availability (Wilson, Erecińska 1986). Adenosine triphosphate (ATP) fuels the body during its homeostatic processes and the body’s primary provider of this fuel is produced through oxidative phosphorylation (Wilson, Rumsey 1988). ATP can be made through anaerobic glycolysis, but it is substantially inefficient for many life forms to be able to survive solely off this alternative fermentation process. A severe lack of ATP production can result in the slowing or even halting of other bodily processes and can lead to a lower chance of survival because of inadequate survival performance, organ damage, or even death (Longmuir 1957).

Sceloporus occidentalis’dependence on oxygen enables the reptile to maintain bodily functions that include, but are not limited to, energy production and body temperature regulation with the ambient temperatures. S. occidentalis species appear in habitats that range from western Idaho and western Utah to the Pacific Ocean (Davis, Verbeek 1972). Their preferred habitats range anywhere from sea level to 4600 meters in altitude (Andrews 1998). Other species observed inhabiting higher altitudes acquire adaptive behavioral changes that can lead to physiological adjustments to the lower concentration of O2 available (Angilletta 2000) in order to avoid the body remaining in a distressed state of hypoxia (Rogowitz 1996) (Bennett, Nagy 1977). In a study of various highland native Tibetians, the adaptations induced an increase in the supply of oxygen to tissue to overcome the hypoxic stress (Frisancho 1975). Some adaptations are more long term. “Muscularized” pulmonary arteries with an extra layer of elastic lamina can result from more constant stares of pulmonary vasoconstriction (Heath, Williams 1989). Effects on the body due to the sudden increase in altitude also include pulmonary vasoconstriction. This is a common immediate response to less oxygen flowing into the body and results in elevated arterial pressure in the lungs due to the narrowing of the blood vessels. As a result, the constriction increases blood flow to more-aerated regions of the lungs to better use the area involved in gaseous exchange. With the increase of arterial pressure, an increased heart rate will occur and produce a heightened metabolic rate (MR). Oxygen saturation also decreases with a sudden decrease in oxygen availability (Beall 2001) but will increase with time as adaptation occurs. A decrease in oxygen can translate to an increase in the rate of oxidation in the citric acid cycle (Raugi, Liang, Blum 1975). Ultimately, if the body must work harder to keep up the homeostatic processes, especially due to a sudden change and is not allowed sufficient time for the body to adjust, an increase in the metabolic rate will follow.

This study is introducing S. occidentalis to a target altitude about 2000 meters higher than the elevation of the habitat from which they were collected and then also at sea level, whichwould expose them to a 31% decrease and 2% increase in oxygen availability, respectively. It was hypothesized that with these specific altitudinal changes, there would be an increase in the resting metabolic rate (RMR) of S. occidentalis at the 2194 meter testing site.The amount of O2 inhaled can be obtained from the CO2 collected at the testing sites and later analyzedto determine the impact of altitude on the lizards’ resting metabolic rates.

Materials and Methods

Sceloporus occidentalis (the western fence lizard) were first collected from the Orange county area during a two week period spanning from the months of September to October 2009. A total of ten lizards, five males and five females, were collected at an altitude of about 186 meters and were housed in a glass aquarium and fed a steady diet of crickets. The lizards were fasted for a period of 24 hours after which they were weighed. The males weighed 11.27 ± 3.09 g (MEAN ± SE) and the females were 11.39 ± 1.52 g (MEAN ± SE). After a seven day period the lizards were again fasted for 24 hours and then taken to Fawnskin, California, at an altitude (ALT) of 2194 meters. The pressure was measured at 591 mmHg using a barometer set up with the Pasco GLX unit, and the altitude was recorded using a GPS. Temperature was recorded at 21.3°C using a temperature probe also for the GLX unit. One specimen at a time was placed into a 500 milliliter container with a CO2 probe linking the container to a GLX unit. The lizard was allowed a five-minute adjustment period for anxiety to subside. During this time and also during the actual testing period, noise and disruptions were kept to a minimum to help ensure the specimen stayed calm. After five minutes, the containers were sealed with only the probe connecting the container to the GLX unit and data was collected for a period of 300 seconds. The lizards were faced away from the probe to prevent CO2 from being exhaled directly into the device. This procedure was repeated for all ten lizards and data was recorded to a USB flash drive to be later converted to ml O2 g-1 hr-1. The lizards where allowed a two-day rest period, again fasted for 24 hours, and were then brought down to Aliso Creek Beach in Laguna Beach, California, at an altitude of 3 m where the procedure that was performed at altitude was repeated at sea level (SL). The temperature at sea level was recorded at 20.9°C and barometric pressure was at 760 mmHg. The data collected from both events were then downloaded to Data Studios and calculations were made. In order to use the CO2 produced to determine their resting metabolic rate (RMR), the following equation was used:

RMR =

The CO2 produced by each lizard was taken in units of ppm/sec and was converted to ml/L and was then used as O2 ml/L. The amount of CO2 exhaled by each lizard was equal to the O2 consumed. The weight of the lizards was taken in grams, and the volume of the container the lizards were housed in while testing was 0.500 liters. The lizards RMRs were found and the mean for the SL RMRs was calculated as well as the mean for the ALT RMR. A paired, one-tailed t-test was run on the data.

Results

During exposure to 31% lower oxygen levels than that of what was normal in the lizards’ everyday environment, the mean RMR obtained was close to that at sea level.

The mean RMR for S. occidentalis at sea level was 0.1922 ± 0.02152 ml O2 g-1 hr-1 (± SE, n = 10) (Figure 1). The lizards mean RMR at altitude was 0.1994 ± 0.02007 ml O2 g-1 hr -1 (± SE, n = 10).

Figure 1.The average resting metabolic rate for S. occidentalis at 3 meters (SL) was 0.1922 ± 0.0215 ml O2 g-1 hr-1 (± SE, n = 10). At 2194 meters (ALT), the average resting metabolic rate for the lizards was 0.1994 ± 0.0201 ml O2 g-1 hr-1 (± SE, n = 10) with 95% confidence.

The overall resting metabolic rates of the sample group of S. occidentalis were not significantly higher during the ALT testing exercise than during SL testing. There were a few cases in which altitude RMRs were lower than the sea level RMRs. The RMRs for both the SL and ALT testing sites mixed within a similar output range of ml O2 g-1hr-1(Figure 2).

Figure 2. The distribution of the RMRs of the lizards (n = 10) is represented. The lowest RMR was at sea level (light-colored bars), and the highest at altitude (dark-colored bars). The mean for this figure was 0.213 ± 0.0187 (± SE) at altitude and 0.207 ± 0.0217 (± SE) at sea level. There was a range of 0.150 at ALT and 0.180 at SL.

When broken down into genders, the distributions of values from each testing location are still in close proximity to one another (Figure 3). The females generally had higher RMRs at both testing locations than the males. The maximum and minimum RMRs calculated were obtained from the males.

Figure 3. The above is the distribution broken down at the level of the sexes (male = blue squares, females = red diamonds) and their RMRs at the ALT and SL testing sites. The males (blue squares) had the highest (max = 0.2953 ml O2 /g/hr) as well as the lowest (min = 0.1469 ml O2 /g/hr) RMR recorded for the group (n=10) The best fit linear lines are visible (red for females, blue for males, and then solid for altitude, dotted for sea level).

The conclusion after running the one-tailed, paired t-test was that p = 0.3791 with a 95% confidence interval. Since p > 0.05, our hypothesis could not be supported by the data found. The p-value indicated that there was no significant difference. The RMR was not significantly affected by altitudes of 3 meters and at 2194 meters.

Discussion

Though there were slightly higher resting metabolic rates found in S. occidentalis(n = 10) at ALT compared to at SL, the difference was not significant (p > 0.05). The data suggest that the heightened altitude had no effect upon the RMRs of S. occidentalis. It is possible to reflect upon the outcome of the study with facts kept in mind.

Oxygen Availability

It is known that with a decrease in oxygen supply, the mitochondria inevitably receive less O2, a key component constantly required for mitochondrial metabolism (Beall 2001). As an altitude increases, there is a decrease in pressure and a decrease in the partial pressure of O2 that can be collected from the air to sustain the homeostatic processes. The oxygen available at 2194 meters was 31% less than that available at sea level.

The 31% decrease in oxygen did not affect S. occidentalis in this study. Other past experiments have had results with MRs having a positive correlation with altitude, all tested at elevations exceeding 3000 m with humans (Beall 2001), Tribolium confusum and Camponotus (Kennington 1957), andPeromyscus maniculatus(Hayes 1989). This study supports the fact that at about 2200 m, there is no metabolic variation than from at sea level with S. occidentalis collected from 186 m. Future studies could determine where RMR does begin to increase at altitude (between 2200 m and 3000 m if the specimens were used to a habitat of about 186 m.)

Temperature

Temperature can be a key factor in accounting for higher metabolic rates at altitude than at sea level. Although the temperature difference at 2194 m was minimal, increasing the altitude further will continue to lower the temperature, resulting in even greater SMRs. This change is due to the fact that the standard metabolic rate of an ectotherm is affected greatly by temperature (Angilletta). These lizards behaviorally select a body temperature of 35ºC (Bennette, Gleeson 1976), so an increase or decrease in this temperature can have a significant effect on the metabolic rate of s. occidentalis. Also ectotherms from temperate locations with seasonally varying temperatures appear to have a greater metabolic sensitivity to temperature variation (Rogowitz, 1996). This means that s. occidentalis will be more sensitive to a change in temperature. Since the mean temperature at altitude (21.3ºC) was higher than the mean temperature at sea level (20.9ºC), the difference in temperature did not have a significant effect on the metabolic rate of the lizards. Related studies have shown that at lower temperatures, significant lags occur in oxygen uptake, which appear to result from restricted ventilation (Bennett, Gleeson, 1975). With this taken into account, if lizards where taken to a higher altitude where a significant temperature decrease occurs, metabolic rate would increase until the temperature threshold for neuromuscular activity is reached, which would then cause the lizards to enter a state of torpor (Rogowitz, 1996).

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