Week 4 Quiz

Back in 1980, Dr. George Somero, a world-renowned animal physiologist, published a paper in Physiological Zoology showing that, among teleost fishes, the activity of lactate dehydrogenase (LDH), an enzyme involved in anaerobic metabolism, exhibited significantly higher activity per gram body weight in larger species. What is the significance of this finding, with regards to our desire to understand the basis of the negative allometric relationship between body mass and metabolic rate in animals?

There are two ways by which animal cells can meet their demand for ATP: aerobic metabolism and anaerobic metabolism. If we presume that the negative allometric relationship between body mass and metabolic rate observed in class applies only to aerobic metabolism (which it does), then Dr. Somero's findings that anaerobic metabolism shows a positive allometric relationship instead seems to refute any of the "demand" hypotheses, e.g., ion-motive ATPases are less active in larger individuals, because clearly the demand for ATP is high in large animals, but is being powered more anaerobically. Perhaps this observation can be seen as supporting the "supply" hypotheses, particularly with regards to the capacity to supply oxygen to the cells of larger animals. If oxygen supply were to decrease with larger size, yet demands for ATP were to remain the same, then a greater anaerobic capacity would be required to make up the difference, consistent with the results of Dr. Somero's study.

Broadly speaking, birds exhibit two different flight behaviours: flapping flight and bounding flight. A 1996 paper in The Auk examined the relative utilization of these two flight behaviours in woodpeckers (and related birds), which ranged in size from 27g (Downy Woodpecker) to 262g (Pileated Woodpecker). As you can see, larger woodpeckers were more likely to utilize flapping flight than smaller woodpeckers. Given that bounding flight is less costly than flapping flight--at least according to a 2013 study published in Journal of Theoretical Biology--what is the significance of this finding with regards to our understanding of the biological basis of the negative allometric relationship between body mass and metabolic rate?

One hypothesis with regards to the negative allometric relationship between body mass and metabolic rate is that that large animals have relatively slower metabolic rates because the cost of their locomotion is cheaper, which reduces their energetic demands associated with movement. However, in this study, larger woodpeckers were more likely to utilize a more costly mode of flight (i.e., flapping), which is contrary to this hypothesis. The significance of this finding, then, is that, at least for birds, the negative allometric relationship between body mass and metabolic rate is not underlied by reduced locomotory energy demands in larger individuals.

A 1958 study published in American Journal of Physiology (and conducted by Knut Schmidt-Nielsen, the father of comparative animal physiology) showed that hemoglobin affinity for oxygen was significantly higher in larger mammals than smaller mammals. (P₅₀ is the O₂ concentration at which hemoglobin O₂ saturation is 50%.) Are you surprised by this finding? Explain.

No, I am not surprised by this finding because it is consistent with our observation that there is a negative allometric relationship between body mass and metabolic rate in mammals, and offers some insight into the possible etiology of this relationship. A higher hemoglobin O₂ affinity can have two implications: i) it can mean more O₂ can be loaded into the blood from the lungs, which would, in turn, make more O₂ available to the body; or ii) it can mean that hemoglobin is less likely to release the O₂ that it carries to the tissues, which would, in turn, constrain cellular metabolism. It is possible that the negative allometric relationship between body mass and metabolic rate in mammals results from the reduced ability of hemoglobin to deliver O₂ to the tissues in larger mammals, which forces them to reduce their mass-specific metabolic rate. Of course, it's also possible that a higher hemoglobin O₂ affinity evolved in larger mammals in order to prevent over-delivery of O₂ to tissues whose metabolic demands are relatively lower, as a mechanism to prevent ROS production for occuring.

In a 2020 study published in Journal of Experimental Zoology, researchers determined the scaling exponent for the relationship between resting metabolic rate (RMR) and body mass for goldfish both before and after the removal of 50% of their gills. The results are shown below. As you can see, neither RMR nor the scaling exponent were affected in any significant way by the gill excision.

a) What insight does this finding provide for researchers trying to understand the origins of the negative allometric relationship between RMR and body mass in animals?

This finding suggests that, at least in fishes, regardless of their body size, the resting metabolic rate is not constrained by their capacity to acquire oxygen from their environment via their gills. Gill excision only reduces a fish's ability to supply oxygen to their tissues and not their demand for oxygen or nutrients; therefore, the lack of change in RMR or scaling exponent suggests that there is no constraint in fishes with regards to their capacity to acquire O2 from the water via their gills. This would suggest that fish maintain a higher gill number and/or larger gill size than is necessary to meet their body's O2 demands.

b) Do you think the results of this study would have been different had the researchers removed 75% of the fish's gills? Justify your answer.

If a larger number of gills had been removed, the surface area of the remaining gills would likely become too low to supply the body with adequate oxygen to meet its metabolic needs. This should lead to a noticeable reduction in RMR. Moreover, since smaller fish have higher mass-specific RMR (due to negative allometry), one should presume that this additional gill removal would constrain the RMR of smaller fish to a greater degree than larger fish, which might lead to the scaling exponent becoming larger. That is, as smaller fish see their RMR reduced by this greater gill excision, but larger fish do not, the slope of the relationship between RMR and body mass would become steeper. And, as we noted in class, the scaling exponent can be determined by the slope of a log-log plot of body mass vs. metabolic rate.

The life cycle of Manducta sexta, the tobacco hornworm, is shown below. In a 2012 study published in Physiological and Biochemical Zoology, researchers determined the scaling exponent for the relationship between body mass and metabolic rate in each of the five larval instar stages, separately. Their findings are shown below.

a) Are their findings consistent with Dr. Craig White's "life history optimization" hypothesis? Explain.

I think so. In the 2012 study, the researchers observed that the value of the scaling exponent for the relationship between body mass and metabolic rate decreased from the 1st to the 5th instar. Dr. White's hypothesis is that animals adjust the scaling exponent for the relationship between body mass and metabolic rate in order to optimize their lifetime reproductive output. During all the larval instar stages in Manducta, the insects should be putting most of their energy into growth (as they cannot reproduce at this stage) in order to achieve a large adult body size, which will minimize their predation risk and, thus, maximize their chances of surviving to reproduce. As Dr. White showed in his paper, there is an inverse relationship between growth rate and the scaling exponent. On this basis, as the growth rate increased from the 1st to 5th instar, which is highlighted by the figure showing larger increases in body size between later instars, the scaling exponent should have decreased to facilitate these faster growth rates, which is what was observed. 

b) In light of your answer to part a) above, if the researchers had determined the scaling exponent for the relationship between body mass and metabolic rate in adult moths, what would you predict the value to have been? Justify your answer.

Given that the value of the scaling exponent seems to align with Dr. White's hypothesis, and given that adult moths should invest their energy into reproduction rather than growth, I would presume that the scaling exponent would have a high value (i.e., higher than observed in any larval instar) because it is known that their is positive relationship between reproduction rate and the scaling exponent. Thus, adults moths could optimize their reproduction rate by adopting a higher scaling exponent value.  

Classically, fishes have been divided into four main groups based on their swimming mode: anguilliform, subcarangiform, carangiform, and thunniform. Anguilliform swimming is characterized by undulations along the entire body length, whereas thunniform swimming is characterized by undulations in the tail region only. In a 2010 study published in Ecology Letters, researchers used previously published data to determine the scaling exponent (b) for the relationship between body mass and standard metabolic rate (SMR) for fishes exhibiting each swimming mode. Can the results of this study explain why tuna are such large fish? Explain.

The scaling exponent for thunniform swimmers, which includes tuna, was found be significantly lower compared to other swimming modes. A lower value for scaling exponent (in this case 0.6) means that, as body sizes increases, mass-specific metabolic rate decreases to an even greater degree. On this basis, larger tuna clearly have much lower mass-specific metabolic rates compared to smaller tuna (and compared to larger fish exhibiting other swimming modes). This would suggest that tunas may benefit more than other fish by growing to a large size; that is to say, when tuna grow to a large size, they experience the benefits of large size (e.g., reduced predation risk) without experiencing as much cost (i.e., their metabolic demands do not increase as much). This would encourage tuna to grow to a large size over evolutionary time. 

One could also interpret these data through the lens of Craig White's Life History Optimization idea, as he found that lower scaling exponents were correlated with higher growth rates. Thus, perhaps tuna, seeking to grow to a larger size than other fish, have adapted their metabolic scaling to permit faster growth rates. At the same time, given how lower scaling exponents are correlated with lower reproduction rates, the larger body size of tuna may come with a lower reproductive capacity. 

In a 1987 study published in Journal of Comparative Physiology, researchers examined the relationship between body mass and resting metabolic rate in 14 different boid snake species at three different environmental temperatures. Their results are shown below. At this time, few, if any, researchers were thinking about the negative allometric relationship between body mass and metabolic rate through a Darwinian lens. If you were to consider the findings of this study through such a lens, what insight does it provide, especially in the face of warming global temperatures due to climate change?

Because the scaling exponent for the relationship between body mass and resting metabolic rate did not change as environmental temperature changed in this study, it suggests that boid snakes may not be able to adapt their metabolic scaling in the face of increasing ambient temperatures, as are occurring due to climate change. This could mean that boid snakes may face difficulty adapting to climate change. For example, as temperature increase, leading to increased body temperature in snakes, this might allow them to grow faster and achieve adult body sizes more quickly, allowing them to reproducing earlier. But, as Craig White's research demonstrate, faster growth rates are correlated with lower scaling exponents. From the data presented in this study, it does not appear that boid snakes can decrease their scaling exponent in the face of higher temperatures in order to take advantage of how a warm body could speed up chemical reactions and lead to faster biosynthesis rates for faster growth.

The figure below has been reproduced from a 2012 study published in Journal of Experimental Biology. It shows the relationship between body mass and various parameters in migratory locusts, which are known to exhibit discontinuous gas exchange at rest. Given the relationships shown between body mass and CO2 production/mass-specific CO2 production, for which other relationship (body mass vs. F-phase duration or body mass vs. C-phase duration) do you think that the researchers expressed their surprise? Explain.

The relationship between body mass and CO2 production rate was nearly isometric (leading to a nearly independent relationship between body mass and mass-specific CO2 production rate). On this basis, if one locust is twice as big as another locust, then its CO2 production rate is also nearly twice as fast. When we discussed discontinuous gas exchange, we noted that, during the closed and fluttering phases, CO2 is unable to escape the insect's body and so accumulates within the trachea (thereby increasing tracheal pressure) and/or within the hemolymph (thereby decreasing hemolymph pH). Assuming that both tracheal and hemolymph volumes are isometric with respect to body mass (as all these parameters are proportional to volume), then we would expect that the duration of the flutter and closed phases would both be constant with respect to body mass. That is, if one locust is twice as big as another locust and producing CO2 twice as fast, but only has twice as much tracheal and hemolymph volume, then it should only be able to store that CO2 for the same duration as the smaller locust. For this reason, both the closed and fluttering phase should have the same duration in locusts regardless of their body mass, yet, according to the data, the closed phase duration increased with body mass in locusts. This finding suggests that i) tracheal and/or hemolymph volume increase at a faster rate than body mass in locusts, such that larger locusts can accommodate more CO2 than smaller locusts, or ii) hemolymph buffering capacity must be higher in larger locusts such that they can handle a larger volume of CO2 before pH alterations force the spiracles to open.

A stock photograph of a woman with (right) and without (left) makeup is shown below. Do you think students should have been instructed not to wear makeup when coming to this week's laboratory session, in which we used RGB analysis of photographs to assess changes in hemoglobin oxygen saturation during exercise and recovery with and without a mask? Justify your answer.

Yes, maybe. Because students were encouraged to used photographs of their forehead for color image analysis in this week's laboratory session, any makeup (e.g., foundation) applied to the forehead may have interfered with this procedure by masking color changes in the hemoglobin flowing through the blood vessels in the forehead. Of course, makeup applied elsewhere to the face (e.g., eyelids, lips) would have no effect on the results of this experiment, so maybe students could be directed to avoid foundation only, rather than all makeup.

Based on the premise of this week's laboratory experiment, in which we employed color image analysis of facial photographs in order to assess an individual's hemoglobin oxygen saturation, it seems reasonable that a family physician could visually assess whether a patient has adequate hemoglobin oxygenation or not simply by looking at their forehead (or other body regions) Of course, given how much time family physicians spent on their computer each day, completing patient paperwork, it also seems reasonable that a bespectacled family physician may have been advised by their optometrist to add a blue light filter coating to their glasses. If so, how would impact their perception of their patient's hemoglobin oxygen saturation? 

The equation that we used to calculate hemoglobin oxygen saturation (SpO2) had the blue colour of the face in the numerator and the red color of the face in the denominator. On this basis, if a physician is less able to perceive blue light because their glasses are outfitted with a blue light filter coating, then they are likely to underestimate the hemoglobin oxygen saturation of their patients, which would lead them to conducting unnecessary tests, either to confirm such low hemoglobin saturation (via pulse oximetry) or to assess potential underlying causes.