How much did a T-Rex eat in one day?

[theropod]

Barrick & Showers estimated food requirements for a 6t T. rex at 17kg. BUT:
They used a metabolic rate based on an avian regression, whose metabolic rates start out higher than similar-sized mammals, but decrease more with larger size, resulting in a much lower estimated metabolic rate for large dinosaurs than for similar-sized mammals (a hypothetical 6t bird has only 22% the metabolic rate of a hypothetical 6t mammal based on these regressions). Then they calculated the mass of a mammal with an equivalent metabolic rate and calculated feeding requirements for it using a second regression. I don’t really understand why they did that, when there was a feeding-regression for birds in the same paper they cited, but Nagy’s bird equation suggests essentially the same value, 16.8kg of feed per day.

Intermediate metabolic rates ("mesothermy") have been proposed elsewhere for dinosaurs, and I’m not saying they are generally implausible, but the method and result are problematic because it is extrapolated allometrically very far beyond the range of the original data (no extant birds in the sample were above 10kg, whereas mammals ranged up to 84). This estimated metabolism should be verified using quantitative histological methods, but sadly I’m not aware of any study that did this for T. rex. Does anyone know is something like this has been done?

If it’s metabolism was that of a 6t eutherian mammal, it would require 88kg of food based on Nagy 1987.

So it’s a question of which we trust more, that large theropods fell on the same curve as birds, their closest relatives, but alltogether far smaller in body size, or that they might plot closer to more similar-sized mammals.
To me the latter seems more likely, I don’t see a T. rex needing just a fifth of the caloric intake of a same-sized mammal, I think that’s more likely an artifact of using a regression never suited to estimate something at those body sizes.

Another problem is that the feeding amount were based on the mean metabolizable energy content of various foods mixed together for a mean of 14kJ/g, as in an omnivore, but T. rex definitely wasn’t an omnivore. Nagy estimated the metabolizable energy of fish (the only kind of meat he provided data on) at 18.7 and 16.2 kJ/g for mammals and birds respectively. Estimated metabolic rate for a hypothetical 6t bird and hypothetical 6t mammal is 237,380kJ/d and 1,085,630kJ/d respectively, so a fish-eating T.rex would probably require between 15kg and 58kg of fish a day. If someone knows the energy content of some other types of meat, that might be more informative.

This is the closest I came to finding scientific information on this, but as you can see it should all be taken with a grain of salt.

So if you want my best guess, I’d say around 60kg of meat, perhaps a little less, but with a massive room for error in either direction.

As for how often they would need to eat, I’ll wager there would be room for far more than 60kg in a T. rex’ digestive tract, which is good considering it would have periodic availability of large prey alternating with periods of scarcity, and it would presumably have a lot of fat for storage as well for that purpose.

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Barrick, R.E. and Showers, W.J. 1999. Thermophysiology and biology of Giganotosaurus: comparison with Tyrannosaurus. Palaeontologia Electronica 2 (2): 22pp.
Nagy, K.A. 1987. Field metabolic rate and food requirement scaling in mammals and birds. Ecological monographs 57 (2): 111–128.

[Infinity Blade]

Intermediate metabolic rates ("mesothermy") have been proposed elsewhere for dinosaurs, and I’m not saying they are generally implausible, but the method and result are problematic because it is extrapolated allometrically very far beyond the range of the original data (no extant birds in the sample were above 10kg, whereas mammals ranged up to 84). This estimated metabolism should be verified using quantitative histological methods, but sadly I’m not aware of any study that did this for T. rex. Does anyone know is something like this has been done?

Well, I don't know about any histological studies for T. rex, but if it's where T. rex lay in the ectothermy-mesothermy-endothermy spectrum you're looking for, there is Pontzer et al. (2009). The title of their paper says it all, really: "Biomechanics of Running Indicates Endothermy In Bipedal Dinosaurs". Especially large taxa.

[theropod]

I would tend to agree with this, but my point was an independent quantification of metabolic rate, because that’s what’s relevant for.

If you ask me, I think the strong negative allometry in birds’ metabolism is probably not informative with regard to the metabolism of larger dinosaurs (there are other factors apart from the huge size discrepancy, for example small birds have much higher wing-beat frequencies and are much more active fliers, while the largest bird in the sample is a soarer, the wandering albatross, and even larger birds not even in the sample would be flightless alltogether, which means they have entirely different metabolic needs that would cause a regression to drop off very steeply), but that leaves us with no solid way to estimate their metabolic rate in absolute terms…except for bone histology.

Biomechanics is strongly indicative of a high metabolic rate in dinosaurs, as are integument and palaeogeography. I wouldn’t exclude the possibility that some dinosaurs are more "mesothermic" in having such a lower metabolic rate as suggested by the bird regression (sauropods for example are far larger than other terrestrial animals, they are biomechanically very efficient, and mass-specific metabolic rates always decreases with larger size, so they would probably have lower metabolisms than modern endotherms simply by virtue of being so large), but T. rex definitely isn’t a good candidate due to its ecology. Even then, "mesothermic" isn’t that widely used a term (it requires us to draw another arbitrary line where we think animals are mesotherms and where they are endotherms). Most people would subsume all variations of metabolic rates elevated above that of typical modern non-avian reptiles as "endothermic", as do Pontzer et al., who simply conclude that large dinosaurs would have "exceeded the maximum aerobic capabilities of modern ectotherms".

[theropod]

It’s unlikely any predator in search for food would pass up an opportunity like that, and predators that exclusively feed on large animals are more of a myth than reality with very few exceptions. A T. rex would eat anything it could catch if it was hungry.

There is no evidence for or against social hunting in T. rex. The majority of predators seem to do fine as solitary hunters, and gregarious and solitary predators can be closely related (such as within Pantherinae or Caninae), so phylogenetic bracketing cannot help us. Really, how you envision it is entirely speculative.

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This has more to do with metabolism that directly answering the thread title's question. But while this isn't a peer-reviewed paper or anything (though it definitely cites and refers to some), I think it's a good summary of the question of dinosaur metabolism.

www.geol.umd.edu/~tholtz/G104/lectures/104endo.html

[Infinity Blade]

Just wanted to put these here for future reference. The first study finds an ancestrally high metabolic rate for archosaurs (interestingly, Maiasaura was found to have a more intermediate metabolism).

academic.oup.com/sysbio/article/65/6/989/2281633


This later study (2020) finds endothermy for all dinosaurs (including, interestingly enough, Maiasaura), but also suggests heterothermy for at least some (e.g. Troodon).

advances.sciencemag.org/content/6/7/eaax9361