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Darren Naish said: "...eggs are too rare a resource for any
endotherm to utilise them as a full-time resource...there are not
seek-and- destroy mammalian egg-predators, nor have there ever
been...(and so eggs are unlikely to have been significantly
implicated in dinosaur extinction)."
Apart from the obvious blunder here (hedgehogs, badgers, and
squirrels, for example, do seek eggs and when they find them they
destroy them), this statement, as a response to my views, does
not accurately reflect them! Nor does: "John doesn't seem to be
aware...that terrestrial crocodiles were eaters of dinosaur
eggs..." since I have specifically invoked them, at least twice.
But perhaps the fault is mine. In trying to defend mammals as
agents of egg predation, I may have appeared to be suggesting it
was they alone which were responsible. I was not suggesting
This is my position: Perhaps there were no specialist egg
predators (but there may have been). However, there may well
have been mammal, bird, dinosaur, crocodile, lizard, and snake
omnivores whose diet consisted, in a time when there _were_ more
eggs, OF MORE EGGS! Surely this is a parsimonious view. I have
also invoked predation on juveniles by new aerial agents (see
post called "Bird competency"). While I have argued for mammal
competency, I have never said they did it by themselves. Rather,
I have suggested a summation--on top of existing predation by new
agents towards the end of the Cretaceous. Still, the challenge
of Darren's (and others) posts, since they readily grant the
business-as-usual egg predation of other agents throughout the
Mesozoic, is to find in the mammals and birds (i.e., new agents)
at the K/T, potential for increasing the predation load on the
dinosaurs. This does not have to be a dramatic bolide-style
instant mammalian victory. Instead, it should be thought of as a
gradual winding down of dinosaurs caused by the tipping of
dinosaur reproduction from, say, level population growth, to
slightly negative "growth". And this could happen, not in a year
or so (as predicted by the bolide idea), but in 50,000 or more
years (a purely arbitrary number). In this post I'll argue for
Why mammals? Tell me if this is right: Diversity data over
the Cretaceous shows a gradual decline of dinosaurs along with an
increase of mammals. This doesn't prove causality, but it might
be a hint.
How well were they suited to preying on eggs? Many people,
most notably, I should say _vocally_, Gould, have trouble
imagining little mammals doing any damage to dinos. But a friend
of mine who works at keeping up Soldier's Field (Baltimore) says
they hardly ever find deer antlers because they are eaten by tiny
voles!! When they do find antlers, they find tiny gnaw marks on
them. So size _per se_ is not a problem.
Perhaps dentition is. I am trying to acquire an emu or an
ostrich egg and an analogue K/T mammal tooth. Assuming K/T
mammals had something like the famous tenacity for gnawing that
we see in rats, how long do you think it would take to "gnaw" it
through? One hour, two hours, a day? Considering the bounty
that is mine when I break through, how long is too long? In this
regard, can anyone suggest an analogue to a 4mm thick sauropod
If mammals are competent in these respects, what else is
needed? I believe it was the mammalian ability to plan and
learn, their flexibility of response, that contributed most to
their competency. This ability was due to two facts of mammal
1. The stealthy existence of a mammal, fine tuned by millions of
years of dinosaur predation, paid a selective premium to
cognitive abilities. As John McLaughlin says in his wonderful
book, _Synapsida_: "Inside (mammal) heads evolved analogues of
their frighteningly unpredictable world in the form of "inner
space" in which odors and sounds replaced vision. Here...
distances in time and space were bridged by _planning_.
Sequences of discrete behaviors were strung together like beads
to bridge the gap between odor-of-prey and the prey itself, and
these sequences varied to suit the vagaries of the invisible
world without. So appeared the first purposeful use of a sense
of time and a new awareness of consequences...The making of a
(sniffed) molecule into a meal or a monster, and then bridging
the time and space distance to that meal or monster in
appropriate response, required all sorts of cerebral additions."
2. The enforced stealth of mammals also caused the development of
reproductive security. This, along with the enhanced
survivorship of adults and babies as they hid out in burrows,
amplified their cognitive powers. Firstly, since babies had a
lot more time to develop (secure either in womb or pouch, and
then later, in the burrow), they could prolong the installation
of ever more complex neuronal connections. And then (according
to McLaughlin) "the advent of suckling further tightened the bond
between mother and offspring, eventually permitting the young to
test their growing inner space against the real world while still
enjoying the protection of an experienced parent."
Given these abilities, we can think of mammals, towards the
K/T, as being preadapted to take advantage of the dinosaurian
egg. These abilities evolved, not to exploit, but to avoid
exploitation. Nevertheless, for the dinosaurs it was a black day
when the defences they had created were turned on themselves!
I challenge anyone to put up a reasonable pre K/T scenario
involving mammal egg-predation on dinosaurs. I shall volunteer a
mammals-win response by invoking only the most parsimonious
mechanisms. Here is one: The hadrosaur herd settles down for the
evening. Up from a burrow near the egg beds, a group of small
mammals creep, unseen, unheard, and unsmelled, amongst the egg
beds. Lapping from fluids released the previous night, or
gnawing fresh holes, the group feeds quietly all night long.
Before morning they return to their burrows, their stomachs full
of rich lipids and proteins.
Does this sound too easy to be true? What response could be
made by the dinosaurs to these novel tormentors? Again, the
fundamental problem was not the fluke evolution of freak
mammalian powers, but the non-stealthy nature of the dinosaurs'
nests. As McLaughlin says: "Eggs are highly fragile objects, the
most vulnerable interval in an egg-layers life cycle. An egg
cannot flee danger, nor can it strike out in its own defence.
Also, eggs are nutritious and delicious..." (I should qualify
that McLaughlin was not talking about dinosaur eggs here, and nor
does he feel that the non-stealthy nest theory is likely true).
Whether or not they ate eggs, mammals must have been
competent egg exploiters. Perhaps they added to the
egg-predation ambience of the Cretaceous, enough at least to send
the responseless dinosaurs into a negative population slide.
But many have difficulty with (I hesitate to say bias
against) the concept of biological causation _of any kind_ as a
cause of extinction:
DEREK SMITH: "I seriously doubt that (egg-eaters) would drive
dinos into extinction...simply because it doesn't make good
ecological sense to do so. If you eat all of your primary food
source, you starve."
This view seems to deny the possibility of predation being
responsible for _any_ extinction. It is also group-selectionist
and wrong. When animals are hungry--they eat!
DALMIRO: "(Many dinosaur eggs have been found intact, therefore
dinosaur eggs must not have been preyed on significantly.)"
This is the same as claiming: "If dinosaurs existed,
significant dinosaur egg predation could not have." Someone ask
the Nene if this logic follows.
I cannot win if this nonsensical line of argument is
supported. Yet the _non sequitur_ is praised by...
STEVEN S. LAZARUS: "Right on!"
But he then makes a more standard denial: "...normal
(biological) processes (cannot account for dinosaur extinction)."
JONATHAN WAGNER: "...I have grown more than a bit weary, of
theories that involve one species out competing another..."
And yet a large body of evidence supports this: Gause's
competitive exclusion principle and his predator/prey
experiments, natural selection, Wilson's island biogeographical
studies, island extinctions caused by novel predators,
MacArthur's niche partitioning. A powerful
message of predator/prey studies is this: if the prey species
cannot avoid being eaten by the predator (i.e., by evolving new
tactics, morphologies etc.,), they will become extinct. The
vaunted hare/lynx oscillation is just a snap shot in time. What
we are seeing are the elaborate adaptations of two species which
have been associated for a long time. An arms race between them-
-teeth and speed vs. stealth and speed--has been in progress for
millions (probably) of years. But the fossil record is full of
species no longer with us, with species whose tactics and
morphologies were no longer sufficient. Did they become extinct
for no good reason--as one post put it: "Because their time had
come"--or has there been replacement of species by competition
and predation? Or must all species wait for a bolide to come
along before they can go extinct?
Finally, my arguments have been based on biological causes
for extinction. I have not seen any other biological arguments
put forward--disease has been (rightly) rejected, temperature of
eggs causing too many of one sex has been (rightly) rejected.
Crop species overturn (gymnosperm -> angiosperm) has merit but
does not, by itself, explain the selectivity of the extinction
pattern (but see _Precocial Dinosaurs_ post!) Ditto climate
change. What else?
Mind you, I am not saying we should KNOW what killed them,
only that we should be able to put forward, in the absence of
satisfying physical explanations, and because many on this net
have spent alot of time studying biological phenomena, reasonable
biological hypotheses for a biological phenomenon: the taxon-wide
extinction of non-avian dinosaurs. The non-stealthy nest idea is
a reasonable biological hypothesis.