[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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 second big question going into the paper: how does
> McNab support the implicit assumption that the lineages in
> question actually reached their maximal body size?
> Cheers,
> --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 this.

McNab operates under the assumption (laid out in the paper) that the largest 
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.