In this post, we travel back to 1947 to explore a fascinating review by Dr. Max Kleiber from the University of California – Davis. Published in Physiological Reviews, this review examined the relationship between body size and metabolism.

Simply asking the question of whether horses produce more heat per day than humans or rats will lead one to conclude that indeed they do because of their larger body size. However, if we look at metabolic rate relative to their body mass in kilograms, we would come to a much different conclusion as metabolism does not scale uniformly with body mass. Instead, smaller animals generate more heat per kilogram of their body weight than larger animals. Researchers have determined that metabolic rate can be calculated for animals of different body sizes by dividing their metabolic rate by their body mass in kilograms raised to ¾ power (Kleiber, 1947).

To illustrate this, consider a whale. While a 70-ton whale produces more metabolic heat overall than a tiny mouse, when we consider metabolic rate relative to body weight, it is only about one-tenth that of a mouse (Kleiber, 1947). This has important implications in field like veterinary medicine, where the dose of medications for animals of different sizes cannot be calculated simply by dividing the dose by their weight. Instead, the dose should factor in differences in metabolic rate, not simply body mass.   

A well-known case exemplifying this error was published in 1962 when researchers mistakenly administered entirely too much LSD to an elephant named Tusko who lived at the Oklahoma City Zoo (see review: Lindstedt and Nishikawa, 2015, and YouTube video below). It goes without saying that no amount of LSD is appropriate for an elephant. In their study, psychiatrists Dr. Louis West and Dr. Chester Pierce worked with Warren Thomas, the zoo’s director, to see whether LSD could induce “musth” in elephants, a period of aggression that males naturally experience. It was important for them to understand this behavior as captive elephants experiencing musth are at risk of harming and even killing other animals as well as zookeepers. Since preliminary experiments had been done in cats, the researchers adjusted the dose based on Tusko’s weight, then halved the dose – just to be safe. But, they failed to scale the dose based on differences in the metabolic rate of cats vs. elephants. Tragically, Tusko was given a whopping dose of 297 mg LSD, instead of a properly calculated dose of ~30 mg. He died within 2 hours. The authors incorrectly concluded his death was the result of higher sensitivity to the drug in this species.  

These reviews underscore the importance of understanding how metabolic rates differ across species, especially when conducting experiments or administering medications. They also highlight the importance of animal ethics oversight committees who are responsible for overseeing research to prevent or regulate such experiments in the first place.

Sources:

M Kleiber. Body size and metabolic rate. Physiological Reviews. 27(4): 511-541, 1947. Doi: 10.1152/physrev.1947.27.4.511

SL Linstedt, KC Nishikawa. From Tusko to titin: the role for comparative physiology in an era of molecular discovery. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology.

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Categories: Comparative Physiology, Most Popular, Time Machine

Tags: American Journal of Physiology, American Physiological Society, animals, health, LSD, metabolism, Musth, Physiological Reviews, Tusko, veterinary, zoo