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Re: Big = Old = Advanced?
On Thu, 28 Aug 1997, Ronald Orenstein wrote:
> John Bois writes:
> I was arguing that relative to
> >_birds_, non-avian dinosaurs were at a relative disadvantage because they
> >probably couldn't put as much distance between their nesting grounds anfd
> >their "home" predators. The comparison is useful because of their
> >relatedness and similarity of reproductive styles, i.e., they both lay
> >shelled eggs. As such their reproductive effort is relatively expensive.
> I think that there is no way of knowing if this is true. First of all,
> even though birds can fly their nests cannot, and the majority of birds do
> NOT nest in areas separate from their "home predators".
This is true. And I know it. All I meant above was that _regarding that
particular strategy_ non-avian dinosaurs were at a relative disadvantage.
As you know, I also believe that regarding the strategy of being cryptic,
non-avian dinosaurs (generally) were at a relative disadvantage.
> Secondly - there is no reason to assume that dinosaurs did NOT employ this
> strategy. Large dinosaurs could have nested on islets (as seabirds do)
> that they could reach by wading or swimming but which may have been beyond
> the reach of smaller predators. We know Maiasaura nested in colonies, and
> in birds colonial nesting usually occurs in areas out of reach of (or
> easily guarded against) nest predators for obvious reasons.
Maiasaura, though, didn't appear to have a way of distancing themselves
from their worst predators. The nasties could, for all we know, just
follow the herds up and down the continent--at least their pred./prey
system is not analagous the kind of territoriality shown in the
lion/wildebeest system and the consequent predator territoriality.
> have no idea how pairs of dinosaurs split brooding duties. Perhaps one
> stayed with the nest for long periods while the other headed for distant
> feeding grounds, just as many seabirds do today. Like Emperor Penguins,
> they could have undergone lengthy fasts during nesting season in order to
> nest in areas remote from both food and predators.
> As I have said before in discussion of Mr Bois' views, I believe he greatly
> underestimates the range of adaptations for nesting dinosaurs may - I would
> say must - have employed to bring off successful clutches - after all, they
> did it for 250 million years.
On the contrary, I believe that such difficulties forced a _greater_ range
of adaptations than we currently recognize. Perhaps, for example,
maiasaurs mounted colonial defence. I have, however, consistently argued
that there is a danger in assuming they shared the strategies of today's
dinosaurs. In my view, and excuse the legalese, non-avian dinosaurs were
relatively less able to employ strategies of concealment and remote
laying than avian dinosaurs.
> As Mr Bois points
> out the egg-laying habits of sea turtles are forced on them by their
> amniotic eggs - thus they need not be regarded as having evolved to avoid
> nest predation (in fact they almost certainly did not, beyond the extent to
> which they did so for all turtles, many of which nest at no great distance
> from their "home predators", whatever these are).
Okay. But turtles can take advantage of things that dinosaurs could not.
A fascinating question is whether non-avian dinosaurs attend the nest.
While this has obvious benefits in protection and other forms of parental
investment, it has a cost (unless parent goes in to torpor) in alerting
preds to the presence of a nest. Turtles avoid this by laying and leaving
(i.e., some degree of concealment operates until hatching). I'm not sure
what the issues are for nest attendance in non-avian dinosaurs. Apart
from guarding, is there a constraint--need to turn eggs, for example--on
avian dinosaurs? Why do the vast majority of birds stay with there nests?
I think megapodes are the only exception to this.
Turtles can also hatch at a relatively immature stage--presumably because
they need less complex wiring and they have protective shells that give
them a margin of protection. This means that they can lay more, smaller
eggs, i.e., their eggs are less expensive. These are not claims by me
only interesting hypotheses. But I come from the idea that eggs are
highly desired and need strategic assistance. Different creatures solve
the problems in different ways. Other creatures try to unsolve the
problem in different ways. And there are winners and losers in this game.