This essay examines a research paper, Allometric scaling of maximal metabolic rate in mammals, written by Weibela and Hoppelera. The paper analyzes the existing facts on maximum metabolic rate (MMR) in several mammals (Weibel, Bacigalupe, Schmitt & Hoppeler, 2004). As illustrated in the paper, MMR depicts the rate at which oxygen is consumed by a homoeothermic animal’s body, when at maximum aerobic output, during maximum exercises. On the other hand, basal metabolic rate (BMR) refers to the lowest energy needed to fulfill an animal’s daily living needs, calculated when an animal is at rest, fasting and under its normal temperature (Weibel et al, 2004). All through their investigations and findings, the researchers attempt to uncover the paper’s hypothesis that stated, “To determine the factors that influence MMR in mammals.”
By examining the existing literature, the researchers noted how aerobic scope varied from one mammal to another. From their research findings, Weibela and Hoppelera confirmed that aerobic scope in large-sized mammals is much higher than in small-sized mammals (Weibel et al, 2004). Similarly, the report illustrated that athletic mammals posses a higher aerobic scope as compared to non- athletic mammals. Weibela and Hoppelera’s work indicated that a lot of BMR related researches have been done as compared to MMR related researches. For instance, the report indicates that more than 600 mammal species have been used in the analysis of BMR, while less than 50 mammals’ species have contributed to the recent knowledge in MMR (Weibel et al, 2004). Through this, Weibela and Hoppelera were able to develop a comprehensive report on the topic.
These research findings did not only rely on other researchers’ works, but also on their own investigations (Weibel et al, 2004). During the research, many animals were used. To select the best data, the researchers employed the standardized approach used in the estimation of the VO2max. Under this approach, several animals were placed on a treadmill running on varying velocity. Thereafter, VO2max estimation, morph-metric examinations, and psychoanalysis were carried out on the animals (Weibel et al, 2004). Afterwards, the animals were killed, and their lungs replaced with intratracheal installation of glutaraldehyde. It was not long before some of their organs and muscles tissues were removed for further analysis. Using the collected information, the researchers correlated their data with the existing MMR and BMR information (Weibel et al, 2004).
After data correlation, morph-metric analysis was initiated. The animals’ half bodies were subdivided into several strata. With each stratum, the length, depth and circumference were measured and recorded to represent a three dimensional specimen (Weibel et al, 2004). From then on, the remaining animals’ half bodies were dissected and weighed. The weights were labeled Mm to represent approximations of the weighed carcasses. Subsequently, approximations of mitochondrial volume densities were evaluated and recorded as biased approximations. Thereafter, the samples were analyzed to obtain their capillary length densities. Finally, the volume densities were evaluated by multiplying capillary circumferences with length densities (Weibel et al, 2004).
From the collected data, the researchers compared their findings with the existing literatures. Out of all the samples, 57 approximations of Vo2max matched with the literature findings (Weibel et al, 2004). The estimates represented the 35 mammalian species selected. The findings were then represented using tables and graphs. With the help of these tables and graphs, scaling of VO2max, muscle capacity and capillary blood supply were deduced. From the findings, researchers confirmed that in mammals MMR scales largely varied with body mass than BMR scales. From the earlier findings, BMR exponents were recorded as 0.75; however, the researchers’ findings were recorded as 0.872 (Weibel et al, 2004). Similarly, it was observed that the factorial the ratio of MMR and BMR was higher in small mammals than in huge mammals. Reduced factorial ratio in large animals was attributed to their agility.
During the experiment, comprehensive data on Vo2max, functional and structural characteristics of loco-motor muscles were collected (Weibel et al, 2004). The mammals sampled ranged from 20g to 450 kg. The data collected represented the whole muscle mass of the carcasses, which in quadrupeds are active when animals work in MMR. In this range, the scale exponent was found to be 0.96 rather than the usual 0.87 for the entire samples (Weibel et al, 2004). Similarly, the researchers noted that the volume of the mitochondria obtained from the loco-motor musculature, scaled with the same exponent of 0.96. This implies that the total volume of the mitochondria is strictly proportional to the Vo2max for all animals. After evaluation, the researchers evaluated the ratio of VO2max and muscle capillary erythrocyte volume as 4.9 (Weibel et al, 2004). However, it should be noted that the value obtained varied from the standard metabolic capacity of mitochondria. As such, higher values have been obtained in similar experiments by increasing the degree of oxygen supply in the mitochondria. This limitation is attributed to the oxidative phosphorylation process in the mitochondria.
In animals, mitochondria, and capillary blood determine aerobic capacity of muscles. It should be noted that their proportions vary directly with the maximal metabolic rate an animal can achieve (Weibel et al, 2004). Weibela and Hoppelera generalized their findings on a subset of species with MMR scale of 0.87. Generalization was implemented for several reasons. First, in their findings the scaling components were not statistically different from the entire population. Similarly, the selected subset was evenly distributed over the entire range (Weibel et al, 2004). Equally, in the subset both athletic and non-athletic species were represented. With these, the authors confirmed that scaling of aerobic capacity of loco-motor muscle is directly proportional to Vo2max. Similarly, Weibela and Hoppelera illustrated the relationship between mechanistic explanations and scaling components(Weibel et al, 2004). In the article, the authors’ findings confirm that in mammals, MMR scales differently with BMR. In the article, the researchers emphasized that there is no prior reason why MMR should scale with BMR (Weibel et al, 2004). However, differences in their performance were attributed to the BMR oxygen and BMR blood flow. In general, the authors’ explanations clearly relate with the paper’s hypothesis.
In the paper’s conclusion, Weibela and Hoppelera acknowledged that their findings failed to indicate how the process relates with vasculature (Weibel et al, 2004). Using Darveau’s model, the authors asserted that the approximations of partial scaling on the sequential functions depend on scaling exponents of metabolic rate. In their attempt to expound on this process, the authors examined several body organs analyzed during the examination of MMR and BMR (Weibel et al, 2004). Through this analysis, mechanistic explanations of the scaling of MMR are clearly outlined.
Reference
Weibela, E. R., & Hoppelera, H. (2004). Allometric scaling of maximal metabolic rate in mammals: muscle aerobic capacity as determinant factor. Respiratory Physiology & Neurobiology, 1(1), 18.