Re: [Part 2: Terramegathermy (very long, too)]

["Adam Britton" <abritton@wmi.com.au>]

I'd love to join this discussion because it's fascinating, and there's much
scope for further clarification, but alas the best discussions always
coincide with major work deadlines that always take priority.

However, it's Saturday morning, and I can't resist just one, tiny post...

Jura wrote:

> > how can we really know how fast any dinosaur grew? Is there
> > anything else dino growth rates are being based on, other than bone
> > histology?

David replied:

> Not much. Mainly the argument from generational turnover: *Deinosuchus*
> needed 50 years to reach 10 m.

Reptile growth rates are highly dependent upon a large number of factors,
primarily temperature and food intake. This also affects age to maturity.
Hawksbill turtles in cool water are estimated to take 30 to 50 years to
become mature, yet those in tropical waters can do it in 5 to 10 years.
Crocs can reach maturity in half the time in captivity (6 years vs 12 to 14
yrs) if provided with optimal temps and food intake, and they grow up to 4
times as fast. Similar variation occurs in the wild, but it's rare for wild
growth rates to even approach those achievable in captivity. However, the
biggest crocs in the wild coming from tropical areas where food is available
nearly year-round. Also, there are some smaller crocs which live in tropical
areas with lots of food, but their ecology selects for smaller size.

Incidentally, Deinosuchus must have grown much faster than any modern croc
(even in captivity with optimal conditions) to reach 10 m in 50 years. Even
known-age crocs of 70 to 80 years old top out at 5 to 6 metres, although the
formative years of their life were spent in the wild which dictates future
growth rates and maximum size. The biggest captive-born croc I know of is
just shy of 6 metres after 30 years.


Jura wrote, and David replied:

> > As for reptiles, how big do you want.
>
> Over about 1 tonne and terrestrial.
>
> > We have huge snakes, turtles, crocs and
> > lizards.
>
> All stopping at or near 1 tonne, AFAIK.

The largest captive crocs that have been weighed have approached 1200 kg.
Extrapolations from known-sized crocs would put the largest measured crocs
(6.3 metres) closer to 1500 kg. As you can imagine, it's not easy to measure
animals of that weight very often, especially not in wild animals - captive
animals would normally be slightly heavier (fatter).


Regarding Deinosuchus, David wrote:

> Semiaquatic, and only 10 m long (for decades it had been estimated at 15
m).

I've always wondered: what reason is there to believe that fossil
Deinosuchus found to date represented animals of maximum size for that
species, or that they were male (which exceed females in size by up to 40%
in extant crocs)? On a normal distribution curve, mean size for adult male
C. porosus for example is around 4 to 5 metres, yet rare individuals
exceeding 6 metres have been found. Mean female size is around 3 metres,
with rare individuals reaching 3.5 metres.


Jura wrote:

> > Crocs are no to migrate long distances over land to find new waterholes,
> > though it isn't as regular as the other examples.

David replied:

> How far?

There's a lot of literature on croc movements, which I don't have time to
look up, so I'll just throw this into the mix. As you can imagine, most croc
migrations are aquatic not terrestrial, taking place along river systems and
coastlines. Radio-tracking and marking studies, usually following relocated
animals, have found they will home between 100 and 150 km if necessary.
Short-term movements are typically in the region of 10 to 60km within 6 to
12 months, although I've tracked one animal that moved at least 90 km in
less than 6 weeks. The longest known aquatic "migration" for C. porosus was
1390 km between New Guinea and the Eastern Caroline Islands (time taken
unknown). As for terrestrial migration, this normally occurs during the dry
season or drought where water holes start to dry up. Distances between
adjacent water holes along a river course are rarely more than a few km (and
typically less than 1 km), but the longest distance I know of was around 50
km over several weeks (I don't remember the exact time): an animal which was
relocated and which homed back to its point of capture overland, eventually
getting caught in a bushfire. It died about 5 km from its destination.
There's not enough known to say whether such a distance is usual.

Observations of croc movement overland suggest that they use the high walk
form of locomotion (the animals lifts its entire body trunk off the ground),
and that speeds are relatively slow (less than 5 km / hr) but sustained.
However, this is strongly dependent upon croc size - bigger animals have
greater anaerobic capacity, but they have much greater bulk to move around.
I would doubt that most adult crocs could keep this up for more than a few
minutes at a time, and galloping (in some species) typically lasts for no
more than 5 to 10 seconds before exhaustion. Crocs have enough latitude to
apply terrestrial movement to common situations, but being terrestrial for
long periods just isn't their strong suit. I don't hold that against them,
though, just as I don't think less of dolphins for not being particularly
crash hot in an arboreal environment.


Someone (sorry, I got lost in the quotes!) said:

> > Um, what captive leatherbacks? Leatherbacks can't be kept in captivity
> [...]

Juvenile leatherbacks have been kept in captivity - there's no special trick
to that. Don't know about adults.


Jura wrote re: captive alligators:

> > I disagree; while it might not be viable to use captive growth rates to
> normal
> > wild ones, it does show that growth is highly dependent on available
food
> in
> > many reptiles (tortoises might be exceptions).

David replied:

> The implication is that such lots of food aren't available in the wild,
and
> never were, so bradymetabolic dinos couldn't have grown big enough.

I agree with Jura. High levels of food can be available in the wild,
depending on location, season etc, and this can have significant effects on
the growth of reptiles dependent upon that food. Best barramundi fishing in
Australia, some say, is Shady Camp on the Mary River, and it has the highest
density of C. porosus anywhere in the wild, and the fastest-growing.
Contrast that with dwarf populations of eg: Nile crocodiles in Mali - right
on the edge of their range, cooler yearly temperatures and less reliable
food source. These crocs are half the normal maximum size of the same
species found at warmer latitudes in high density food areas.


David wrorte:

> > But what of their prehistoric ancestors? Well there's a question eh,
croc
> > biology preadapts them (one could say) to a high energy existence. They
> have
> > an efficient gizzard digestive system coupled with xenomorph like
stomach
> acid
> > that can even handle bone. They also have a four chambered (though
> aquatically
> > modified) heart and a diaphragm. None of these would be expected in a
> > semi-aquatic ambusher, yet there they are.

I think they make good sense given the croc's ecology, hunting method and
diet (ie. hard-shelled organisms - the biggest dietary item in a crocs diet
is crustacean and mollusc, with high-energy food coming from less frequent
large prey captures from the water's edge where the ability to submerge for
extended periods is highly advantageous for both concealment and handling,
not to mention predator evasion)

Jura wrote:

> > Of course these are more leftovers than they are preadaptations, but
> > regardless it would seem that immediate crocodylian ancestors were more
> active
> > than extant ones. This probably means that they grew much faster too.

Please elaborate.


Jura wrote, regarding weights over 1 t:

> > _Deinosuchus_

David replied:

> Semiaquatic. And probably not much heavier either.

I disagree. Taking extant crocs as a baseline, most species reach 1000 kg
around 5.5 to 6.0 metres in length. A 10 metre Deinosuchus could be
extrapolated to weigh between 4000 and 5000 kg (ref: Webb and Messel 1979 -
ask me for full ref).

Going back to an earlier post regarding acidosis and activity of crocs,
David said:

 > hyperanaerobiosis
> is an inefficient process (that consumes ten times > as much food as
> aerobiosis) that works only for a few minutes, and is > followed by toxic
> effects (Bennett, 1991). For example, anaerobic    > power falls off so
> quickly that big crocs may be unable to drag      > smaller ungulates into
> deep water to drown them if they do not       > succeed with the first
lunge
> (Deeble & Stone, 1993; contrary to the  > assertion of Bennett et al.
(1985)
> that big reptiles can produce     > hyperanaerobic power for long
periods).

I'd go with Bennett's findings here. Exercise time to exhaustion has been
confirmed by several studies to scale with body mass - so very large crocs
have much greater anaerobic capacity. However, it's not so simple.
Crocodilians exert massive amounts of power over short periods of time (a
large croc [>5 m] can separate the front half of an average sized ungulate
from its posterior portion with a flick of its head), then they have a
period of rest with little activity, and then they exert massive power
again, and so on until exhaustion. Capturing crocodilians reveals this
activity pattern. In other words, they can exert hyperanaerobic power for
long periods, but not continuously (and decreasingly over time).


Jura said:

> Interesting proposal, but quite different from what I've read. Crocs have
a
> phenomenal degree of anaerobic endurance. They can withstand a lactic acid
> content of near 80%, which is higher than any known animal, and it doesn't
> take them that long to get rid of it either (I believe it is about 3 hours
or
> so, but I'll have to get the ref to be sure).

Agree on the low pH tolerance (most animals cannot tolerate blood pH below
7.0, but crocs can go down to 6.4 without ill effects). However, time to
recovery is dependent upon body mass. Adults typically take 4+ hours to
recover, but recovery time is logarithmic with body mass, and animals over 5
m can take close to 30 hours before they approach normal blood pH levels
again. It is not uncommon for very large crocs to die from effects of blood
acidosis when captured, normally because they drown (eg. head submerges
following an extended period of struggling). Big crocs are ambush
predators - they can only repeat short-term bouts of extreme aggression
after a suitable pause, and they can't keep that up for long. As most
attacks are doomed to failure if the predator gets a chance to run (on
land), this aspect of their physiology suits them just fine.

Refs for this:

Baldwin, J, Seymour, RS, Webb, GJW (1994). Scaling anaerobic metabolism
during exercise in the estuarine crocodile (_Crocodylus porosus_). Comp.
Biochem. Physiol. 112A (2): 285-293

Bennett, AF, Seymour, RS, Bradford, DF and Webb, GJW (1986). Mass-dependence
of anaerobic metabolism and acid-base disturbance during activity in the
salt-water crocodile, _Crocodylus porosus_. J. exp. Biol. 118: 161-171

Seymour, RS, Webb, GJW, Bennett, AF and Bradford, DF (1987). Effects of
capture on the physiology of _Crocodylus porosus_. Pp. 253-257 in Wildlife
Mangement: Crocodiles and Alligators, ed by GJW Webb, SC Manolis and PJ
Whitehead. Surrey Beatty & Sons: Sydney.

Cheers,

Adam Britton