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

Re: my working hypothesis for body size




On Wed, 29 Apr 1998, Colette H. Adams wrote:
> I see
> nothing in population or community ecology that would indicate that an
> insectivorous mammal is inherently more or less likely to survive than an
> insectivorous dinosaur of similar size and habitat preferences. 

Two barnacles compete in the intertidal zone.  One has greater
physiological tolerance of dessication and so has an advantage at the
upper limits of the high tide.  Similarly, I can think of many reasons for
different degrees of success in a dino vs. mammal of similar size and
niche requirements.  1. If mammals had more efficient endothermy they
could better exploit nocturnal niche. 2. Better sensory
equipment helps in nocturnal niche. 3.Notwithstanding rails and
company, tetrapods may be better able to exploit arboreal or any niche
with complex structure better than a bipedal creature. 4. Mammalian
reproductive strategies may give them an edge over nesting dinosaurs
(especially if parental attendance is practiced).  5. Mammalian dentition
might process insects more completely thus enabling them to get by on less
(and so survive times of scarcity).

But I agree with you about the fuzziness of the the idea of competition.
For example, in some of the examples above its hard to know when
competition ceases and predation begins.  They seem to me to be a part of
the same thing.

> So competition is a difficult problem.  Even where it is obvious that two
> species are feeding on the same things, it is often very difficult to
> demonstrate that one affects the population of the other. 

I don't know if you've read it.  Forgive me for suggesting Wiens, J.A.
1977 On competition and variable environments. American Scientist 65:
590-597.  He notes that populations are often affected by weather more
than direct competition.  He also questions the idea of populations held
at some "equilibrium" and that they may be held way below carrying
capacity by predation and other things. 

> I believe this was the
> beginning of an evolutionary "arms race", with dinosaur herbivores trying
> to achieve an escape from their predators. 

In what sense do you mean "escape"? Large size beyond say, horse size,
would slow one down!  If you mean escaping predation by scaring a predator
off or using bigness to defeat it, then I would agree.

> Anyways, that is my happy little story to explain the difference in body
> size between dinosaurs and mammals during the Mesozoic.  Predator-prey
> interactions, not competition.

One thing your analysis doesn't address is why so few huge post-dino
mammals?  Why aren't they driven so dramatically by an evolutionary arms
race.

Last thing from me.  There were no grasslands in the Cretaceous.  I'm not
being sarcastic when I ask what you mean by "grassland style" niches?