RE: crocs eating hadrosaurs (was Gr. 9 sci proj)

[Nathan Myhrvold <nathanm@MICROSOFT.com>]

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From:  John Bois [SMTP:jbois@umd5.umd.edu]
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> Am I right in saying that an animal bearing one baby which survives has
> higher fecundity than another which has 20 but they all die?  We cannot
> census dino populations, but surely the teeming Serengeti plains are
> evidence of healthy mammalian fecundity.  I claim that large clutches in
> dinos are evidence for high predation rates on eggs 'n offspring, not of
> teeming Laurasian plains.

The terminology generally used is a bit different.  Fecundity is the term 
used for simply how many young you have per unit time.  Surviorship is the 
tendency for them to stay alive (often expressed by the opposite term 
mortality counting those that die) .   Biologists have broadly speaking 
classified animals as being either r-strategists (high fecundity, balanced 
by high mortality) and K-strategists (low fecundity, but lower mortality).

Large mammals are K-strategists, rabbits are r-strategists.

Usually r-strategists also have short lives.   Crocodillians are an odd mix 
- very high fecundity (large clutches of eggs every year), and thus high 
juvenile mortality, however they also have very long lives (up to 90 years 
in captivity) and very low adult mortality.

Dinosaurs appear to have been like crocs in this respect.   They must have 
had fairly long lives to be able to grow that big - probably 100 to 200 
years for the big ones.

> The hypothesis is that this scale dependent fecundity effectively
> limited gigantism in terrestrial mammals.  Dinosaurs appeared to have
> high fecundity, irrespective of their size.  Even the biggest dinosaurs
> seem to have had the fecundity of a rabbit (loosely speaking).

Rabbits need lots of babies because lots of them get eaten.  This was
probably true for dinos as well.  And it still doesn't argue for
superior fecundity.

Steady state in a stable population deaths balance births.   The only 
difference between the r-strategy and K-strategy is average age at death. 
  It is hard to see any measurable difference or "superiority" of one 
versus another.

However when there is a displacement in the environment and favorable 
conditions emerge, the dinosaur strategy should be superior - they can 
radiate MUCH faster than slow breeding K-strategists.  This may be what 
kept them so successful for so long.

Also, it underscores the weirdness of the K/T event - even a very small 
remaining population should have been able to breed like crazy and 
re-radiate.   One pair of hadrosaurs living in one valley someplace would 
be enough to grow millions in 20 years, especially if their predators were 
gone (like introducing rabbits to Australia).

Nathan