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Re: Big = Old = Advanced?
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". In fact nest
predation can be very high in birds, and most bird adaptations for coping
with this are not involved in nesting in inaccessible or distant areas -
seabirds are the one really obvious exception. Birds avoid nest
predation, most often, by crypticity - disguising the nest or the eggs so
that they are difficult to spot - or making it difficult to get at (though
this rarely stops snakes, for example). This can reach extreme lengths -
African Penduline Tits actually build a false entrance into the nest to
In fact I would say that among the passerine birds (which include more than
half of living species) I cannot think of one offhand that nests out of
reach of at least some predators, particularly snakes (not to mention other
birds - Blue Jays and Toucans, for example, are regular nest-robbers that
can fly from nest to nest).
There are birds that do locate nests far from regular feeding grounds - the
extreme example may be the Gray Gull of western South America, which flies
long distances to nest in the almost lifeless Atacama Desert of Chile - but
they are in the minority as birds go. In other words, putting distance
between nesting and feeding areas is a useful strategy but not a necessary
one, and in fact most birds do not employ it.
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. Further, we
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.
>And, they are both fixed-site modes, i.e., with some exceptions possible,
>most dinosaurs didn't regualrly move their eggs.
I would say that though this may well be true I know of know evidence to
support this claim. After all, we only have definite evidence that
dinosaurs even laid eggs for a few species, and more information on nest
type is available for still fewer. I cannot even imagine what sort of
fossil evidence you would be able to find that could show whether a nesting
dinosaur could or did move its eggs or newly hatched young (as some
crocodilians and a very few birds are known to do today).
>Sea turtles may be constrained to lay eggs on land.
>So, they make the best of a bad lot by laying on an island in the middle
Many sea turtle beaches are on continents, not islands.
It is in fact utilizing a skill differential that its
>once upon a time "home" predators didn't have--it could swim thousands of
Thereby exposing itself to a different set of predators. 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).
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:firstname.lastname@example.org