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RE: Croc growth (was RE: More about Erickson et al. 2004 and RE: Delayed "growth spurt" in T. rex)



Some of Erickson & Brochu's conclusions in their 1999 paper didn't add up,
particularly where a key point rested on croc longevities not exceeding 50
years of age. Unfortunately I'll have to pull this off the top of my head,
because my copy was lost several months ago, but much was based on bone
growth data that we know is highly unreliable in tropical crocs (and
typically produces an underestimate of the actual age). Unreliable because
a) growth rings can be indistinguishable where growth rates vary little
seasonally (ie. in the warm tropics) and b) earlier rings are absorbed later
in life, particularly in females. The differences between counted rings in a
known-age croc can be in the order of 25 to 50% depending on the person
doing it (unpubl. data!). Ecological studies on croc longevity naturally
take a long time to produce any results, but there is now mounting evidence
that several wild croc species reach ages well in excess of 50 years. A
population of freshwater crocodiles (C. johnstoni) that we're working on has
yielded individuals that, by conservative estimates, are in the 60 to 70
year old range, whereas this species was previously thought to live around
30 to 40 years at most. There are also reliable data indicating that larger
species such as C. porosus often exceed 60 to 70 years. Despite this, crocs
that achieve large size are characterised by rapid growth in their early
years, usually starting to tail off by the age of 20. You can influence this
directly even during incubation, where eggs incubated at higher temperatures
result in more rapidly growing animals that select warmer temperatures - ie.
early environment has a significant influence on adult size, though clearly
there is a genetic component also. One particular croc farmer in Thailand
has been going to great lengths to produce faster growing crocs in less
time, his greatest success to date was a hybrid that reached 19.6 feet
(roughly 6 metres) in 20 years. In the past 7 years, however, that croc has
grown less than a foot, though it has increased to a greater degree in mass.
While captive manipulation is an extreme case, such variables affect crocs
across their geographical range.

Perhaps Greg had considered these points in the paper that I cannot fully
recall, but the oft-quoted conclusions that I keep reading about often don't
gel with wild croc data.

Best wishes,

Adam


--
Dr Adam Britton, abritton@wmi.com.au
Senior Researcher, Wildlife Management International
Tel. 61 8 8922 4500,  Fax. 61 8 8947 0678
http://wmi.com.au, http://crocodilian.com
-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu] On Behalf Of
Thomas R. Holtz, Jr.
Sent: Friday, 13 August 2004 4:40 AM
To: bigelowp@juno.com; dinosaur@usc.edu
Subject: Croc growth (was RE: More about Erickson et al. 2004 and RE:
Delayed "growth spurt" in T. rex)

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Phil Bigelow
>
> I still haven't had a chance to see the paper, but I've got a question.
> Did the authors compare the _T. rex_ growth curve with the growth curve
> of the salt water croc or the Nile croc?
>
> Considering that these are the only extant large archosaurian predators,
> and considering that we have good ecological data on these animals, it
> would have been the logical comparison to make.  And this is a case where
> a negative correlation with crocs would be just as informative as a
> positive correlation!
>
Although they didn't compare those per se in this paper, such comparisons
were made earlier in the context of Erickson & Brochu's 1999 (Nature
398:205-206) Deinosuchus growth paper. Modern and fossil crocs have long
extended period of growth, and non-determinate size. It would be nice to see
their Deinosuchus data replotted as mass vs. age rather than length vs. age,
though, to make it more comprable to the dinosaur plots.

So it has already been established that dinosaurian growth curves and rates
are NOT like those of living crocodilians or other non-avian reptiles.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796