Comparative Analysis between Mass-Specific Metabolic Rates of Endotherms and Exotherms
BIOL-241L-02 Comparative Animal Biology Lab
Cedar Xiao
March 13th, 2026
Abstract
Metabolism dictates how animals convert food into usable energy, a process heavily influenced by their evolutionary thermoregulatory strategies and body mass. In this study, we tested the combined effects of thermoregulation and body size on mass-specific metabolic rates (MSMR). We hypothesized that endotherms would exhibit higher MSMRs than ectotherms of similar size, and that within both thermal categories, smaller animals would possess higher MSMRs due to negative allometric scaling. Using flow-through respirometry, we measured the resting gas exchange rates of a human (Homo sapiens), a mouse (Mus musculus), a toad (Rhinella marina), and three Madagascar hissing cockroaches (Gromphadorhina portentosa). Our results supported both hypotheses: the endothermic mouse exhibited an MSMR (\(1.1713 \text{ kcal/gday}\)) roughly 144 times greater than the human (\(0.0268 \text{ kcal/gday}\)), and significantly higher than both ectotherms. Furthermore, MSMR scaled negatively with body mass across both thermal strategies. While uncontrolled variables such as locomotion and Specific Dynamic Action (SDA) likely introduced minor overestimations in our ectotherm data, our findings strongly demonstrate the high energetic costs of endothermy and the impact of body size on metabolic demands.
Introduction
Oxygen is essential for the survival of almost all macroscopic life on Earth. It drives aerobic respiration, the shared metabolic pathway that first evolved 3.2 to 2.8 billion years ago in the Mesoarchean age (Husain et al., 2026). From our early ancestors, different evolutionary lineages split off over hundreds of millions of years, leading to the massive physiological differences we see today across arthropods and vertebrates like amphibians and mammals. Even though all these diverse groups still use the exact same ancestral cellular machinery in their mitochondria for aerobic respiration, their different evolutionary histories mean they get and spend their energy in different ways. For example, the early evolutionary split between vertebrates and invertebrates, and the much later divergence between ectothermic amphibians and endothermic mammals, created distinct respiratory structures and different energy demands.
Metabolism is how we turn energy in the food we eat into the energy our body can use for biosynthesis, maintenance, and external work. (R. W. Hill et al., 2016 p. 169) Animals’ metabolic rates dictate how much energy they require to survive, and it heavily influences their ecology and behavior. One of the biggest drivers of metabolic variation among animals is their thermoregulatory strategy. Endotherms such as birds and mammals constantly maintain a high body temperature by producing their own metabolic heat. This strategy is very energetically expensive, so endotherms usually have basal metabolic rates that are significantly higher than ectotherms, who rely on the environment to regulate their body heat instead of producing heat themselves.
Along with thermoregulation, body size is another factor influencing metabolic rates. While absolute metabolic rate goes up as an animal gets bigger, mass-specific metabolic rate (MSMR) scales negatively with size. (R. W. Hill et al., 2016 p. 178) This is partly due how, given the same body shape, small-sized animals will always have a higher surface area to volume ratio than their larger-sized versions. A sphere surface area-to-volume equation demonstrates this: \[\text{Ratio} = \frac{\text{Surface Area}}{\text{Volume}} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}\]
As the radius (dimensions) of the sphere (animal body) increases, the surface-area-to-volume ratio decreases. Heat dissipation relies on surface area, which means that smaller animals lose heat to their environment much faster relative to their total mass, requiring them to maintain a proportionally higher mass-specific metabolic rate to compensate. This is a partial reason why the metabolic rate doesn’t increase proportionally with body mass within animals of the same general body layout and thermoregulation strategies, such as within carnivorous mammals and species of crabs. (R. W. Hill et al., 2016 p. 180) We see similar negative allometries in other energy-related relationships in the animal kingdom, such as the cost of transport in cockroaches (Herreid et al., 1980) and the metabolic rates of crocodiles as they grow from hatchlings to adults. (Gienger et al., 2017). This relationship is not universal though: for example, in Chassin et al.’s study (1976) on lion locomotion, they found that as their lions grew from ~30 \(kg\) to ~60 \(kg\), their MSMRs remained constant.
This leads to our research question: what happens to these messy and mysterious relationships when comparing between vastly different species? In this lab, we aim to test the combined effects of thermoregulatory strategy and body mass on metabolic rates. We measured the resting metabolic rates of four different common lab animals representing ectothermic amphibians, ectothermic insects, and endothermic mammals big and small. We hypothesized that the endotherms of roughly the same body size would have higher mass-specific metabolic rates than the ectotherms. We also hypothesized that within both groups, the smaller animal would have a higher mass-specific metabolic rate than the larger animal following the general rule of negative allometric scaling.
Methods
Subjects
For mammalian subjects, we used one human student and a laboratory
mouse. The human subject was a 20-year-old female student (Homo
sapiens) enrolled in Mount Holyoke College (MHC)’s Comparative
Animal Physiology class (course code: BIOL-241-01) in the
spring semester of 2026. The mouse (Mus musculus) was
an adult from the MHC Psychology colony, housed in small social groups
of three mice (up to four) per cage in ventilated solid-bottom cages
bedded with Teklad corncob. They were fed Teklad Global Rodent Diet and
water ad libitum.
For the ectotherms, we used a toad (amphibian) and three cockroaches (insects). The toad (Rhinella marina) was sourced from Josh’s Frogs and fed a diet of mealworms. The Madagascar hissing cockroaches (Gromphadorhina portentosa) came from an MHC colony and were fed Premium Roach Diet from Josh’s Frogs with water ad libitum.
All of our studies were approved by the MHC Animal Care and Use
Committee, protocol #BT-44-0226.
Equipment and Experimental Procedures
We determined metabolic rates via indirect calorimetry using flow-through respirometry. Oxygen and carbon dioxide, the gaseous reactant and product of aerobic respiration, were measured as a proxy for aerobic respiration rates. To measure our subjects’ oxygen consumptions and carbon dioxide productions, we used a Q-teach 300 oxygen meter for endotherms and a Q-teach 101 \({CO}_2\) meter for ectotherms (Qubit Systems, Kingston, Ontario). We pumped air through the system using a Q-teach 201 pump/flow monitor at 0.45\(L/min\). We collected all our data using the LoggerPro 3 software from Vernier Science Education.
The first step of all four gas measurements was to record ambient oxygen/carbon dioxide by running the pump and the oxygen/carbon dioxide meters without the animals. With the ambient \(O_2\) and \({CO}_2\) levels recorded, we then measured the gas exchange rates of animal subjects.
For the smaller lab animals (the mouse, toad, and cockroaches), we put the subject(s) into a transparent plastic tube to create a closed system with measurable changes in gas concentrations. We connected an airway lines connected to the suction pump through one end of the tube and another line to air in the lab. We wired the pump’s output to the gas detectors (oxygen and carbon dioxide) to measure the gas concentrations post-exhalation. When using the oxygen meter, the exhaled gases were put through a dehumidifying capsule before its \(O_2\) concentration were measured.
After letting the animals acclimate for a bit, we recorded their steady-state gas exchange. For the mouse, we measured oxygen consumption; for the toad and the cockroaches, we measured their carbon dioxide production (\(mL\) \({CO}_2/min\)) instead. We did this assuming ectotherms consume less oxygen, so the higher-precision \({CO}_2\) meter is more suitable for detecting their gas exchange rates.
Because the cockroaches are so small and have very low individual metabolic rates, we measured three cockroaches in the transparent plastic tube simultaneously. We weighed three cockroaches altogether and divided the total weight by three, and similarly measured their total recorded gas exchange rate and divided by three for the individual gas exchange rates.
Because the human was too large to fit in a chamber and very compliant to the experimental procedures, we set up the gas exchange measurement differently for the human subject compared to the lab animals. Instead, they breathed directly into an exhaler connected to a bag of air, which was then connected to the pump, the dehumidifier and the oxygen meter.
Data Collection and Analysis
We recorded body masses of every subject except the human with a common scale precise to three decimal places in the biology lab. A standard mass of 60 \(kg\), or 60,000 \(g\), was assumed for the human. We calculated the absolute metabolic rates (\(kcal/day\)) from the gas exchange rates. To convert the gas exchange values to energy consumption, we used a standard metabolic conversion factor of about 4.8 \(kcal/L\) \(O_2\).
We took averages of \({CO}_2\) production/\(O_2\) consumption rates over approximately 45-90 seconds after the subjects’ gas exchange rates were observed to have stabilized.
For the ectotherms, we used their average measured carbon dioxide production as a proxy for oxygen consumption. Aerobic respiration consumes the same number of molecules of carbon dioxide as oxygen, which according to Avogadro’s law would also take up the same volume, so we could also apply the 4.8 \(kcal/L\) oxygen conversion factor equivalently to the carbon dioxide production. To compare species so vastly different in size, we calculated the mass-specific metabolic rate (MSMR) for each subject by dividing their daily metabolic rate by their body mass (\(kcal/gday\)).
Results
The gas exchange rates and the metabolic rates we calculated varied a lot across the four species (Table 1).
| Species | Strategy | Mass (g) | Gas Exchange (mL/min) | Rate (kcal/day) | MSMR (kcal/gday) |
|---|---|---|---|---|---|
| Toad\(^1\) | Ectotherm | 45.750 | 0.1130 | 0.781 | 0.0171 |
| Cockroach\(^2\) | Ectotherm | 7.977 | 0.0451 | 0.312 | 0.0391 |
| Mouse | Endotherm | 22.830 | 3.8400 | 26.740 | 1.1713 |
| Human | Endotherm | 60000.000 | 232.9000 | 1609.800 | 0.0268 |
| Note: | |||||
| 1: We measured \({CO}_2\) production for the toad and cockroach because the \({CO}_2\) meter is more precise for low metabolic rates. We measured \(O_2\) consumption for the mouse and human. |
Looking at absolute daily metabolic rates, the human subject expended the most energy by a large margin (~1609.8 \(kcal/day\)), while the cockroach expended the least. However, when we divided the metabolic rates by body masses, we saw an inverse relationship with size. Of all 4 lab common lab animals, the mouse had the highest MSMR at 1.172 \(kcal/gday\). The human’s MSMR was only 0.02683 \(kcal/gday\), which means the mouse’s MSMR was roughly 143.7 times greater than the human’s, even though they are both endotherms. (Figure 1)
Among ectotherms, the smaller cockroach (7.977 \(g\)) had a higher MSMR (0.0979 \(kcal/gday\)) than the larger toad (45.75 \(g\)), which had an MSMR of 0.01707 \(kcal/gday\). The cockroach’s MSMR was also slightly higher than the human’s. The toad was the only lab animal we tested that had a MSMR lower than the human subject.
Discussion
Interpretation
The results of this study show how much thermoregulatory strategy (ectothermic versus endothermic) and body mass simultaneously impact an organism’s energy requirements, even across species and families. The negative allometry and the significantly higher MSMR of the endotherm compared to the ectotherm within the same order of magnitude are both in support of our initial hypotheses.
Firstly, our data highlights the high energetic costs of endothermia. The endothermic mouse (22.83 \(g\)) had an MSMR significantly higher than both the ectothermic toad and cockroach (See Table 1). This makes sense because endotherms constantly generate metabolic heat, making their baseline cellular respiration rates significantly higher. This aligns with our first hypothesis that among endotherms and ectotherms of similar body sizes, the endotherm will have a significantly higher MSMR.
Secondly, our data aligns with our second hypothesis that within of negative allometric scaling within endo/ectotherm categories (See Figure 1). Within endotherms, the mouse’s MSMR was over 140 times greater than the human’s. We saw this same relationship in ectotherms of different families, where the cockroach (insect) exhibited a higher MSMR than the toad (amphibian).
Nuances
Despite finding evidence agreeing with our initial hypothesis, it is important to note several factors that may have resulted in errors in our results.
Firstly, throughout the measurement for Gromphadorhina portentosa, one of the three cockroaches in the tube was relentlessly crawling on top of the other two cockroaches, who were at rest. We observed the cockroaches’ \({CO}_2\) production visibly increase when the cockroach was in motion compared to when it rested. When we took the average \({CO}_2\) production over time, it was impossible to exclude the portions where the cockroach was active. As ectotherms, the cockroaches have lower basal metabolic rates, and additional activity will greatly impact its metabolic rates and gas exchange rates. According to a cockroach locomotion study by Herried et al. (1981), a cockroach’s metabolic rates could increase by a factor of 2.5-5 when moving, depending its speed. Although having two other resting cockroaches in the chamber helped average out the impact of the restless cockroach, we still likely overestimated our cockroach MSMR. This would put in question whether the MSMR of a single cockroach is truly higher than that of humans: suppose the cockroach was moving at a slow speed of 0.03 \(km/h\), its metabolic rate will still have increased by a factor of approximately 2.5, which is equivalent to having 4.5 resting cockroaches in the chamber. This would decrease the MSMR of each resting cockroach to 0.0261 \(kcal/gday\), which is lower than the MSMR of humans. Table 2 shows the recalculated MSMR per cockroach under different speed assumptions of the restless cockroach.
| Species | Speed (\(km/h\)) | Strategy | Individual Locomotion MR factor | Mass or Mass Equivalent (\(g\)) | Rate (\(kcal/day\)) | MSMR (\(kcal/gday\)) |
|---|---|---|---|---|---|---|
| Cockroach\(^1\) | At Rest | Ectotherm | 1.0 | 7.977 | 0.312 | 0.0391 |
| Cockroach | 0.03 | Ectotherm | 2.5 | 11.970 | 0.312 | 0.0261 |
| Cockroach | 0.07 | Ectotherm | 3.5 | 14.630 | 0.312 | 0.0213 |
| Cockroach | 0.12 | Ectotherm | 5.0 | 18.610 | 0.312 | 0.0168 |
| Human | At Rest | Endotherm | 1.0 | 60000.000 | 1609.800 | 0.0268 |
| Note: | ||||||
| 1: The mass equivalents are calculated using cockroach locomotion metabolic rate measurements from Herreid et al.’s 1980 paper. |
Next, let’s not forget that despite the subjects were fed ad libitum, we did not monitor their feeding pre-experiment. Therefore, we failed to take into account the animals’ specific dynamic action (SDA) in their measured metabolic rates. The size of animals’ SDAs relative to their daily metabolisms depend on their diets, fasting duration, meal size, body mass, thermal strategies, and ambient temperatures. Specifically, endotherms typically experience a lesser increase in metabolic rates post-feeding, very rarely exceeding a ratio of 2 between peak postprandial and basal metabolic rates. (Secor, 2008) Meanwhile, ectothermic animals can experience a significantly greater post-prandial metabolic rate hike, up to 44 times for the Burmese Python (Python molurus), because they have lower metabolic rates and consume more per meal to fast for longer periods. (Secor & Diamond, 1997) We can estimate SDAs for the human, the cockroach, and the mouse with data of animals with similar survival strategies and body size in Secor’s 2008 paper. Because Secor’s 2008 paper did not contain similar animals as our toad (Rhinella marina), used data from another paper by Secor and Faulkner (2002) that specifically looked at SDAs of toads. Table 3 lists the dietary habits of our subjects and estimates of their SDAs .
| Species | Mass (\(g\)) | Thermal Strategy | Feeding | Eating Habit | Kilocalories per Meal | \(\frac{\text{Peak Postprandial Metabolic Rate}}{\text{Basal Metabolic Rate}}\) |
|---|---|---|---|---|---|---|
| Toad\(^1\) | 45.750 | Ectotherm | \(ad\) \(libitum\) | Opportunistic eater, once every 2~3 days | 2.25 ~ 9 | 2.9 ~ 6.4 |
| Cockroach\(^2\) | 7.977 | Ectotherm | \(ad\) \(libitum\) | Detritovore, Constant Nibbler | 0.05 ~ 0.1 | 1.78 |
| Mouse\(^3\) | 22.830 | Endotherm | \(ad\) \(libitum\) | Constant Nibbler | 1 ~ 2 | 1.27 ~ 1.73 |
| Human\(^4\) | 60000.000 | Endotherm | \(ad\) \(libitum\)* | 2-4 Meals/Day | 500~ 800 | 1.31 |
| Note: | ||||||
|
1: For the Toad (Rhinella marina), we
used SDA Scopes from Secor and Faulkner (2002), with meal sizes ranging
from 5% to 20% of the toad’s body mass. 2: For the Cockroach (Gromphadorhina portentosa), we used the Sand Field Cricket (Gryllus firmus, another) numbers from Secor. (2008) 3: For the lab mouse mus musculus, we used the Brown Rat (rattus norvegicus) numbers from Secor. (2008) 4: For the human subject, we used numbers from Secor (2008) describing the SDA of a female human (Homo sapiens) consuming 600 kilocalories of mixed (solid and liquid) food. |
As seen from Table 3, the cane toad (Rhinella marina) is subjected to the most error from unaccounted SDA. This is due to a combination of the following factors: 1) it has a relatively small body size; 2) it is ectothermic; 3) it is naturally an opportunistic eater, normally consuming up to 1/5 its body mass per meal that lasts for 2-3 days (Secor 2008; Zayas, 2025). In comparison, the Madagascar hissing cockroach is a detritovore and an intermittent feeder, and the house mouse is also not only an intermittent feeder but also an endotherm. Therefore, there is a possibility of overestimating the MSMR of the toad, depending on when it was last fed.
So, all in all, our study has a lot of room for improvement. Ideally, further experiments should have all subjects at basal/standard metabolic rates by eliminating excess movement during measurements and keeping them fasted for a suitable period of time. This is especially important for ectothermic subjects, because they have low standard metabolic rates and are proportionally under more influence from environmental conditions.