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Re: Response to Orenstein's "explanation" of mammal success.

John Bois' response to my message strikes a tone that is a bit more
personal and emotional than I want to get in this discussion.  I am
not trying to put anyone down.  However, as readers of this list
should know by now, I have very firm views on the degree to which we
should allow speculation to run away with us with respect to
prehistoric organisms.  This can become particularly problematical
when we see an unsolved mystery and find it very hard to accept that
we simply may not have - and may never have - the pieces to solve it.
It is very tempting in such cases to fall back on broad
generalizations about animal behaviour and interrelationshps - because
if the real truth is in the details, the little individual adaptations
to situations taken species by species, then it becomes all the more
unlikely that we shall ever get at it for the dinosaurs and their

>Then perhaps we agree on this.  My difficulty is with what I see
>as the standard Gouldian claim that mammals were "almost
>surely...small and insignificant...restricted, and unendowed with
>consciousness" (in _Dinosaur in a Haystack_)......Again, this is pure

Of course, ANYTHING we say about the ecological role of mammals in the
Cretaceous is pure speculation.  I'm not sure what Gould means by the
"consciousness" bit from the quote, though.  There is a considerable
debate among students of living animals about the presence of
consciousness, so I can hardly see us drawing useful conclusions about
extinct ones.

>     Anyway, Archibald's find says more about the increased
>_quality_ not the quantity of diversity, i.e., innovations in the
>mammals stock were making the clades of today.  And the viability
>which these innovations subsequently demonstrated creates a
>_greater_ probability of their increased significance in the pre-
>K/T ecology.

I confess I don't see why this follows at all.  The selective value of
adaptations relates only to present circumstances.  I can imagine no
selective pressure that would preserve adaptations because of their
future evolutionary viability.  Thus the only valid measure of Pre-K/T
"significance" is the situation in the Cretaceous.  What happened
afterwards may be likened to the "ecological release" of organisms on
islands.  The fact (for example) that tarweeds evolved into
silverswords on Hawaii says nothing whatever about their ecological
significance as tarweeds in California.

>1. You use extinct creatures to show the viability of non-
>mammalian open-niche life-styles as in your response to my claim
>that we see (today) no big ground-laying birds, lizards, turtles,
>and snakes: "there WERE big open-country lizards...[terrestrial]
>crocs lasted until fairly recently...As for birds, don't forget
>Diatrymids, Phorusracids, and Dromornithids..."

This is not speculation.  Certainly these creatures existed, and we
know something about the habitat in which they lived from fossil
evidence.  I mention them only to show that such things were not only
possible, they happened.  What this says about the relative survival
value of, say, mammals vs pristichampsid crocodiles I would hesitate
to guess, but one can hardly say that such creatures were

>     Ron, there is _one_ big ground-laying bird in all of Africa. 

Well. to be picky, this depends on what you mean by "big".  The
heaviest flying bird in the world, the Kori Bustard, is an African
ground-nester (as are other African bustards almost as large).  More
to the point, though - size aside, many species of birds, large and
small, nest on the ground - including some, like many American
warblers, that forage in trees.  If ground-nesting were an unviable
strategy because of predator pressure, one would not expect to see

As I said, the truth is in the details.  You cannot lump
"ground-nesting" into one category and judge its ecological
significance in toto.  Ground-nesting birds include colonial seabirds
nesting on islands, megapodes that bury their eggs in huge heaps of
earth and rotting vegetation, burrowers like bee-eaters and some
swallows, shorebirds that lay camouflaged eggs on bare sand and
songbirds that build complex, covered nests.  They include island
species that succumbed rapidly to introduced egg predators (an
excellent example is the ground-nesting Socorro Dove of Socorro Is.,
Mexico, which has been pretty much replaced by the newly-immigrant
tree-nesting Mourning Dove, its probable ancestor, since feral cats
got loose on the island) and mainland birds that raise young quite
successfully on the ground surrounded by an even greater mix of
potential predators.

Given the range of variation in living forms, and the range of
ecological success these variations have shown, I assume (here is my
speculation) that there must have been quite a range of variation in
the same way (though surely with different techniques) in
ground-nesting dinosaurs.  As I said earlier, egg predators were
probably always around throughout dinosaur evolution; surely it seems
reasonable to assume that dinosaurs had evolved ways of dealing with

>The general rule, then, applies: mammals do better in the
>big-animal, open-field niche than egg-layers.

I would say that mammals do better TODAY (and have in many areas since
the K/T) in such niches, if by "better" you mean that they are more
diverse and numerous as a whole, than other vertebrate groups, most of
which happen to be egg-layers - but egg-laying is not necessarily the
critical factor.  Why, for example, have viviparous lizards not
evolved into such niches if a switch to live birth makes all the

>First of all "any combination of these or other factors" is
>legalese.  It is an answer that works for just about _any_
>complex ecological question.  But it is of not much use in answer
>to the question: What specific advantages did mammals have over
>other creatures which enable (and enabled) them to dominate the
>open-field niche of today?

If by "dominate" you mean only with respect to large size (I am not at
all convinced that small mammals "dominate" birds, for example, in
such habitats - and in deserts there is a good argument for saying
reptiles dominate), the question is of course valid; this does not
mean it is definitely answerable.  I am, however, not convinced that
reproductive strategy is the whole answer, for reasons I have already
set out.  For example, one of the major adaptations that led to
large-mammalian ability to exploit open country after the radiation of
grasses in the Miocene was almost certainly the development of a
dentition (and presumably a digestive system) capable of handling
grasses.  Birds, for example, have evolved very few grazers (geese
come to mind) and these certainly do not handle the kind of grasses
one finds in open prairies.  Mammal patterns of tooth-formation and
replacement probably gave them a hand-up in developing specialized
high-crowned dentition.  Remember that before the Miocene there may
not have been much in the way of open-country niches.  Anyway, none of
this applies to grazing dinosaurs, which were eating completely
different plant groups.

>really don't see how a ferocious Phorusracid, for example, with
>its bone-crushing jaws could have been bested by _any_ small
>mammal, no matter how finely its teeth were put together (unless
>of course egg-gnawing ability were involved!).  But they went the
>way of all other open-field pretenders. 

Being big and fierce is no guarantee of evolutionary success, of
course.  And the phorusracid example may make part of my case for me:
despite their long evolutionary history, and considerable radiation,
they never developed grazing adaptations.  They were "bested" at this
by South American mammals of several lines.  And any open-country
mammal community is dominated not by predators but by herbivores.
There were bird versions, and croc versions, of open-country predators
- and they persisted for millenia, even if they succumbed in the end.
But there were, and are, no bird equivalents of bison, wildebeest or
zebra.  Even on islands the moas and elephant-birds seem to have been
predominantly browsers or tuber-eaters.  Why not?  It could be the
dentition.  I will admit that it might be more difficult for
egg-layers to evolve into nomadic herd animals like bison, but it
seems that ceratopsian and hadrosaurian dinosaurs did just that - was
the difference the nature of the vegetation?  And the mechanisms
required to eat it?

>the only thing that is unique to mammals is teeth. 
>Archibald's 2mm tooth looms large indeed!

To repeat - in terms of future evolutionary potential, yes indeed -
though even Palaeocene and Eocene ungulates did not have the kind of
high-crowned dentition we see in modern cattle or horses, for
instance.  In terms of ecological significance in the Cretaceous -
possibly a very different matter.  As I noted, multituberculates
seemed marked for success, too.

>Ron says that mammals were generalists and had not gone "down
>roads of specialization that it would be extremely difficult to
>retrace."  As such they were more able to step into the niches
>made available by the dinos exit.  But birds were generalists,
>too and did, in fact, step into those niches before mammals.

Birds are not as generalized as cretaceous mammals, because (early
forms aside) they had lost their teeth altogether and had greatly
minimized the ability of their front limbs to diversify (one possible
result of this, for example, is that though there are at least half a
dozen independent living lines of fossorial mammals there are no truly
fossorial birds, though as I mentioned there are some that can
excavate nesting tunnels).

> Ron uses snakes as an example and suggests that they could not be
>herbivores because of "early specialization" (for carnivory, I
>presume).  But this is silly.  Any snake worth the title has a long,
>thin body.  Why?  So it can hide down holes.  Why?  Because a
>selective pressure existed to make this a viable strategy to resist
>predation.  Having adapted for laying low, carnivory was the only
>option since herbivores must forage longer.

It is generally accepted, true, that snakes evolved as burrowing
forms, but there is a lot more to burrowing than "laying low" (as any
root-eating gopher could tell you).  Many lines of reptiles have
evolved this habit, and all as far as I know are active predators that
seek their food underground, rather than hunting on the surface and
retreating to holes for protection; the most "primitive" snake
families today, such as Typhlopidae and Leptotyphlopidae, are entirely
burrowers.  Snakes are, in fact, extremely diverse within a certain
narrow confine; they have certainly evolved extremely sophisticated
jaw mechanisms (especially in venomous forms).  They have also
developed considerable dietary specializations - there are slug-eating
snakes, egg-eating snakes, fish-eating snakes, even snake-eating
snakes like the king cobra.  But not one herbivore - nothing eating
the fallen fruits on the rainforest floor (as many other animals -
even Amazonian fishes - do), no burrowing snakes eating roots like
many burrowing rodents.  No arboreal fruit-eating or leaf-eating
snakes, though arboreal snakes abound in the tropics.  Why not?
Because, I think, they are not built for it.  Let me quote from
Richard Shine's excellent "Australian Snakes: A Natural History" (Reed

"The last major problem faced by elongate animals is feeding.  A snake
is basically a long tube... An elongate animal has really only got two
options.  It can either eat many small prey items, or a few large prey
items.  Lizards have generally used the former option, but this is
rare in snakes.... Snakes don't eat plant material (apart from
occasional "mistakes" by over-enthusiastic individuals) although this
is common in lizards.  Most snakes have evolved to use the second
option: to eat a few, large prey items."

In other words, snakes "boxed themselves in" evolutionarily at an
early stage (unlike lizards) into a body form that simply was not cut
out for herbivory.

>Look also at the marine iguana of the Galapogos.  It is a vegetarian
>among predominantly carnivores, lizards I mean.  Yet it has adapted
>herbivorous structures.

Actually many iguanas are herbivores, including large mainland species
like the common iguana.  It is likely that the marine iguana evolved
from a herbivorous, not a carnivorous, ancestor that developed
herbivory in a non-isolated, mainland situation (as have many other
lizards).  If lizards could do that, why not snakes?

>Ron wonders what "the advantages of altriciality" are that are
>allowed by mammalian milk.  He believes the additional time spent in
>the helpless state would have a "certain cost."  I believe the
>security of a well hidden and less frequently visited nest, den,
>burrow, or marsupium (remember the window of insecurity is smaller
>in live birth than egg laying)

Why?  Many birds nest in well-hidden nests or cavities, and many have
young that are fully precocial on hatching (some, like mergansers and
related ducks, do both).  Why are these less secure than (say) a
mammalian den?  Or the nest of a harvest mouse?

>I believe species will "opt" for altriciality if possible simply
>because their is a selective advantage to having more time for
>development.  Why else would most ground-laying birds be precocial
>and tree-laying birds be altricial (To keep the babies from falling
>out of the nest?  I don't think so.  The pattern is the same for
>mammals if you compare burrowers to grazers.)

There are other potential reasons.  For example: a bird that forages
in trees may need to be able to fly, and fly well, to find its own
food (including hovering and other behaviours).  This involves a
degree of development found, among hatchling birds, only in the
ground-nesting megapodes (which couldn't hover if they tried).  A
ground-foraging species needs only to be able to walk and pick things
up (eg domestic fowl).  To allow the time to develop such abilities
altriciality may be necessary (the same argument fits hawks, for
example - and, yes, it may literally be to keep them from falling out
of the nest.  The only precocial tree-nesters I can think of are ducks
which nest in deep cavities.)  Many seabirds also require long periods
of physical development - an extreme example is among albatrosses
which may stay in the nest for months.  In this case altriciality has
nothing to do with learning as the young do not learn foraging
techniques from their parents - in fact in many shearwaters the young
are extremely fat (hence the "muttonbirds" of former commerce) and are
abandoned by their parents well before they are able to fly and fend
for themselves.  This has necessitated remote, usually island,
nest-sites for these birds.

The fact is that there is a great range in living birds from fully
precocial to "semi-precocial" and altricial.  It is impossible to say
that one type is in general advantageous over the other - it depends
on the lifestyles involved.  For some birds altriciality is
advantageous - for others precociality is. There is no general trend
towards altriciality.  In fact in ducks the trend has probably been
away from altriciality - thus the most aberrant and possibly
"primitive" duck, the Australian Magpie Goose, is the only one to feed
its young - all other ducklings feed themselves.  This may be evidence
of a trend away from, not towards, altriciality in thse birds.
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court                  Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net
Office: 130 Adelaide Street W., Suite 1940    
Toronto, Ontario Canada M5H 3P5