RE: gigantism as liability

[John Scanlon <riversleigh@outbackatisa.com.au>]

A factor not apparently mentioned so far is likely synchronization of
breeding within regional populations and among species, as with sea turtles
(where 3 or 4 spp. may hit the same nesting beaches at the same time). This
increases the odds of survival for any individual young by swamping the
predator population, and particularly (if nesting events are far enough
apart, most likely annual but potentially even longer) preventing a large
population of specialist (hence efficient) nest- and juvenile-predators from
arising.
There do exist nest-predation specialists among ectotherms (e.g. snakes
restricted to eating bird eggs or lizard eggs available for a short period
each year), but such a seasonal crop is probably not available, as a major
or sole component of lifetime food intake, for either dinosaurs or mammals.

Could there have been 13-year and 17-year sauropods? 

-----------------------------------------------
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
riversleigh@outbackatisa.com.au
http://tinyurl.com/f2rby
 
"Get this $%#@* python off me!", said Tom laocoonically.
-----Original Message-----
From: john bois [mailto:jbois@verizon.net] 
Sent: 17 November, 2008 7:36 AM
To: dinosaur@usc.edu
Subject: Re: gigantism as liability

----- Original Message ----- 
From: "Graydon" <oak@uniserve.com>
To: <dinosaur@usc.edu>
Sent: Saturday, November 08, 2008 9:42 PM
Subject: Re: gigantism as liability
> Newly hatched sea turtles are effectively defenceless.  (Swallowed whole
> by gulls, etc. = effectively defenceless.)
>
> By analogy, just getting bigger involves incremental predator immunity;
> remember that there are many fewer large predators than small predators,
> just on trophic web/food pyramid grounds; get too big for the 10 kg
> predators and your risk drops by an order of magnitude, and this keeps
> happening until you're eventually not worth the bother for the 2,000 kg
> predators.

The initial argument here is that I claimed that simply being an r 
strategist was not enough for sauropods, that they needed defence of young 
in addition to producing lots of them. I don't believe turtles are a good 
comparison because 1) they have a body plan specialized for predator 
defence; the shell effectively limits predation soon (relative to 
non-shelled creatures) after hatching, and 2) they live in a vast 3 
dimensional medium that affords protection not available to terrestrial 
animals...if sauropods relied strictly on numbers it is hard (for me) to 
come up with a scenario whereby babies who are defenseless at chicken size, 
who would have had predators at least all the way up to elephant size, could

avoid total reproductive failure. The argument on "web/food pyramid grounds"

doesn't work so well when the whole ecology is scaled up relative to the 
size of today's animals. Predators were larger, yet babies were 
smaller...meaning that mesozoic babies had to pass through more predatory 
classes than babies of today's mammals. Turtle hatchlings had a mad scramble

to the ocean, sauropod hatchlings had a mad scramble to predator immunity, a

scramble to get big. In the meantime, I believe, they needed considerable 
protection from their massive parents.

> Whales, rhinos, horses, elephants, pretty much all ungulates, it's
> single births with rare twins.  This is probably -- but I don't know
> that it has been definitively demonstrated -- due to a cost/benefit case
> with investment in offspring.  (Your best odds of success are dependent
> on the ability of the offspring to keep up with the herd, so the
> larger/fitter it is at birth, the better the odds are.)

Yes...a case of one full glass rather than two half empties.

 > The putative 20 ton parent is defending 2 kg eggs.  How -- what possible
> mechanism -- does it do that, against the 5kg mammal digging away on top
> of the nest?  Or the ~100 kg juvenile allosaur standing among the
> hatchlings?

While I deeply relish the image you have presented, I am reminded of the 
exceptional tenderness of croc parents in bringing hatchlings to the water. 
And I imagine that even the 20 ton parent could deliver a nasty bite to 
either of these nest predators. But I agree that large size is not helpful 
here. It is helpful against predation of adults at the nest however.