Metabolism, and metabolic rate tend to feature pretty highly in literature related to dinosaurs and other reptiles. For instance it is often stated that reptiles have metabolic rates around 1/10th those of similar sized mammals and birds, but what exactly does that mean? Talks of thermoregulation focus heavily on the role of metabolism, while allometric studies focus on how metabolism is affected by size. Given the prevalence of metabolic terminology in dinosaur and reptile papers/books, I thought it might be best to quickly give a review of metabolism, metabolic studies, and what all of that means for real animals.
Metabolism is everything
Metabolism is defined as the sum total energy expenditure of an organism. That is to say metabolism is the total energy an organism uses during its life. It is often broken up into the chemical reactions that build up resources (anabolism) and the reactions that break those resources down (catabolism). The amount of metabolism, or energy expenditure during a specific interval of time (seconds to days) is referred to as metabolic rate. From bacteria to blue whales, metabolism is the measure of all the energy that lets these critters go, and metabolic rates determine how much energy that is going to take. It can be measured in a variety of ways from respirometry to doubly labeled water and heart rate telemetry. The diversity of metabolic rate measurements is reflected in the units used to measure metabolism; which can range from watts/hour to milliliters of oxygen per minute, and even to joules per second.
Specificity is important
A key thing about metabolic rates is that they are plastic. They change depending on the situation presented. For instance one could measure the metabolic rate of a sleeping cat, and then compare it to measurements from that same cat while playing, or after eating a big meal. Metabolic rates ramp up when energy demand increases, and then ramp down when that energy demand decreases, or when the environment demands drastic energy cuts (e.g. starvation). Thus when measuring the metabolic rate of an animal it is important to decide exactly what kind of metabolic rate you are trying to measure.
And boy, oh boy are there a lot of different flavours to choose from.
One can measure: BMR, SMR, RMR, MMR, AMR, and FMR just for starters.
Those are a lot of initialisms, and they are just the most common ones. The choice of metabolic rate that one decides to measure is also going to dictate the technique that will be employed. So what do all these things stand for, and what technique is best for what? Let’s find out.