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Re: Resources, energetics and dinosaur maximal size

If they were given a mammalian metabolism, and concomitant FMR (as
detailed in the paper) then a 56 tonne _Brachiosaurus altithorax_ would have to digest some 561,000 calories a day. This is asking a lot for plant productivity of any time period.<<<

Ignoring the issue of specific mass estimates for the moment (after all, there are titanosaurs estimated to get to at least that size)...how is 561,000 calories a lot to ask of plant production in absentia of population details? After all, you would only need lower adult population density to have the same impact on the local flora. That's relevant since indricotheres are presumed (like other rhinos and like elephants) to have had a small number of large offspring and therefore a more adult-heavy population, while sauropods appear to have large numbers of offspring each year and to have full-sized adults comprise a much smaller portion of the population.

I'd go back to the rate of consumption arguments before I'd be concerned about what the plant biomass could support.

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333


-----Original Message-----
From: Jura <pristichampsus@yahoo.com>
To: Dinosaur Mailing List <dinosaur@usc.edu>
Sent: Mon, Jul 27, 2009 10:33 am
Subject: Re: Resources, energetics and dinosaur maximal size

--- On Mon, 7/27/09, Mike Habib <habib@jhmi.edu> wrote:

From: Mike Habib <habib@jhmi.edu>
Subject: Re: Resources, energetics and dinosaur maximal size
To: xrciseguy@q.com
Cc: "Dinosaur Mailing List" <dinosaur@usc.edu>
Date: Monday, July 27, 2009, 11:25 AM
I look forward to reading this paper,
and it would appear to have some very interesting ideas
contained within. What I am most interested to see,
though, is how McNab supports the second statement in the
abstract: " The factors most responsible for setting the
maximal body size of vertebrates are resource quality and
quantity, as modified by the mobility of the consumer, and
the vertebrate's rate of energy expenditure".

That sounds very reasonable at first, but upon a second
look, it's actually a remarkably bold statement. I
would, for example, expect maximal body size to be set (at
minimum) by an interaction of resource usage and morphology.
For example, basic structural limits come into play at large
sizes. The consumer mobility aspect mentioned by the
author does include morphology, of course, but I'm not sure
it includes the full range of relevant variance attributable
to shape. There is also a certain matter of dumb luck,
it would seem: there is no guarantee that an evolving
lineage will actually "find" the morphospace that includes
shapes capable of hitting the maximal body size, which leads
to my second20big question going into the paper: how does
McNab support the implicit assumption that the lineages in
question actually reached their maximal body size?


--Mike H.


McNab details the morphology bit in the paper by saying ( [] mine ):

"That is, given a restraint on total energy expenditure, an individual with a lower mass-independent expenditure (represented by a) can attain a larger mass than one with a higher expenditure, at least as long as the low-expenditure
individual has sufficient mobility to find resources ade
res. However, at 1 extreme along a continuum, the most sluggish of species would not be able to sustain the largest masses potentially permitted by resources because they could not find a sufficient resource base in a limited area to support a large mass, which therefore would reduce K [maximal daily expenditure]
and maximal m [mass]. "

McNab goes further with this, by showing that a hypothetical sauropod with an "average lizard" metabolism (i.e. the metabolism of a sit and wait iguanian), would "need" to grow to 330 tonnes in order to consume the same ammount of calories as a large African elephant (_Loxodonta africanus_). That no sauropod comes close to this estimated size, suggests that sauropods had higher FMRs than

McNab operates under the assumption (laid out in the paper) that the
terrestrial mammal known to have existed (_Paraceratherium_ sp.), was probably operating at, or near the maximum FMR for terrestrial animals in a terrestrial ecology. In this case, it would be about 170,0000 calories a day. The largest dinosaurs were about 8 times the size of _Paraceratherium_. If they were given a mammalian metabolism, and concomitant FMR (as detailed in the paper) then a 56 tonne _Brachiosaurus altithorax_ would have to digest some 561,000 calories a
day. This is asking a lot for plant productivity of any time period.

McNab assumes Mesozoic plant communities were about as productive as that of those in the East African plains (obviously a simplification on his part, but a necessary one. Probably best to look at it as the "average" Mesozoic plant community), and does consider Hummel et al's recent work on fern nutrition. Using Hummel et al's data, Mesozoic plant communities would come off as less than, or equal, nutritionally, to those of extant plant communities. It is under this assumption that McNab posits that sauropods would have been unlikely to
have sustained a mammalian level FMR.