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Re: Ouranosaurus' sail



I hadn't really thought about the possibility for some sort of mimetic
relationship between the African sailbearers (or humpsters, whatever the
case may be), and my intial reaction to Silvio's post was pretty sceptical.
However, further reflection has led me to think that it's not that far
fetched.

I stand to be corrected on this, but I am assuming that _Ouranosaurus_ and
_Spinsosaurus_ are known from the same, or at least, equivalent units.  If
they are not, then Silvio's suggestion fails the first test of mimcry - that
the species concerned be sympatric for at least some part of their life
cycle.  However, if they were sympatric, then the development of mimicry is
a possibility.

Where you have a number of sympatric species - two being the minimum number,
but I don't think that there is a theoretical maximum - which display some
sort of phenotypic similarity, it is actually quite reasonable to think that
mimicry could be involved. Classical Batesian mimicry involves three
parties - a model, a mimic, and a dupe.  The dupe (I might have just made
that term up, but I think it has been used before in this context) is the
party who is being deceived by the mimcry - usually a predator of the mimic.
The model is the species who is both unappealing to predators and
conspicuous in some aspect of its appearance (aposematic).  The mimic is the
(as far as the dupe is concerned) perfectly edible prey species
which is 'hiding' from its predators through some degree of similarity to
the model.

There are other sorts of mimcry, where there is more than one species of
model (e.g. classical Mullerian mimicry), but I think the situtation that
Silvio and Jean-Michel were discussing is more in line with Batesian mimcry.

Silvio's original suggestion, that _Ouranosurus_ was mimicing _Spinosaurus_,
would appear to be a straightforward example of Batesian mimicry.  Any
predator of _Ouranosaurus_ (except, perhaps, _Spinosaurus_) might be
confused by
the superficial similarity between the two species.  The fact that both
share (what it for dinosaurs) an unusual body shape, potentially noticable
from a distance, is consistent with the theoretical requirements for
Batesian mimicry.  The
similarity in body shape might have been augmented by _Ouranosaurus_
adopting a
similar colouration to that of its model, whatever that may have been -
there is no obvious reason why _Spinosaurus_ would have needed to adopt
aposematic (warning) colouration, and thus is may have had a cryptic colour
pattern that may have assisted it in hunting.  Even if you had been able to
have been in Early Cretaceous Africa to observe these animals, it would then
have been difficult to test a theory of mimicry between these two species
against a (null) hypothesis that they were both simply cryptic.  Unless
_Ouranosaurus_ was mimicing any intraspecfic display colouration used by
_Spinosaurus_ as well....

Note that I am using 'cryptic colouration' in the sense used in entemology,
i.e. any colouration / pattern that makes the animal harder to see against a
natural background).  Applied in a straightforward sense to larger animals,
this would include the colouration seen in large mammals such as
countershading, outlining disrupting stripes, spots, etc.  There is a
difference in how well certain 'cryptic' colour schemes work between small
and large animals - colour schemes that replicates very precisely a certain
part of the environment (leaves and flowers in mantids, stony ground in
quail) don't appear to work as well in large animals, where more general
pattern disruption is used.

Returning to a hypothetical mimicry relationship between _Ouransaurus_ and
_Spinosaurus_, one interesting difference between it and the more 'typical'
examples of Batesian mimicry found in insects would be that the model would
probably not suffer any ill effects from the presence of the mimic.  In
insects, where the mimicry often involves deception of birds which are
potential predators of both the model and mimic, the model is actually
disadvantaged by the presence of the mimic - the
effectiveness of its aposematic colouration is compromised by the presence
of animals with a similar colouration that are quite edible.  (Incidentally,
this is believed to be one of the processes by which very striking warning
colours can evolve - the model is under selection pressure to evolve
colouration that is different to the mimic, whilst the mimic is under
selection pressure to copy the model.  In any situation where the mimic
starts off by being more cryptic than the model, this can lead - in theory -
to a 'colour race' between the model and mimic, the model evolving to even
more spectacular and uncryptic  warning patterns, the mimic evolving to keep
up.)  However, it is hard to see how the presence of _Ouranosaurus_ as a
mimic
is going to have any harmful effects on _Spinosaurus_ - unless we think that
predators of _Ouransaurus_ may also have been important predators of
_Spinosaurus_ as well.

We may therefore have to acknowledge a situation where _Ouranosaurus_ could
have developed quite accurate mimicry of _Spinosaurus_ without _Spinosaurus_
coming under any significant selection pressure to 'evade' the mimicry.
However, of _Spinosaurus_ was a significant predator of _Ouranosaurus_, it
would
have been under signifcant selection pressure to be able to distinguish its
prey from conspecifics.  The question would then become - would the
selection pressure acting upon _Ouranosaurus_ to perfect the mimicry be
stronger than the selection pressure upon _Spinosaurus_ to improve its
powers
of discrimination?

This situation then leads on to the situation, as stated by Jean Michel,
where;

> >And why not the contrary, as a strategy for the carnivore, to approach
> >planteaters within a reasonnable distance before attacking? (as already
> >suggested onlist IIRC)
> >Cheers,
> >Jean-Michel

This is certainly an interesting theoretical situation.  In this instance,
_Spinosaurus_ would become the mimic, and _Ouranosaurus_ would be both the
model
species and the dupe.  Consquently, _Ouranosaurus_ would be under selection
pressure to either 'evade' the mimic (by changing its appearance), or
improve its own powers of discrimination, or both.  It might be surprising
if, in the face of these selection pressures, _Spinosaurus_ was able to
successfully maintain the mimicry.  As Silvio states below, this situation
is more likely to work for _Spinosaurus_ if it is a very uncommon predator
of
_Ouranosaurus_, and therefore minimises the selection pressures acting upon
_Ouranosaurus_.  Of course, if this is the case, then it is hard for the
mimicry to be an important selection pressure in the evolution of
_Spinosaurus'_ sail/hump - at best, the mimicry might then be a (for the
predator) fortuitous result of a sail that has evolved in response to other
selection pressures.

 >
> Good point,
> Since it is still hot and my brain doesn't want to work on usual stuff,  I
> try to go on with the guess and explain better my point of view:
> I was guessing that spinosaurids preyed close to/in rivers (some large
fish
> or other aquatic ad semi aquatic vertebrates) and rarely on Ouranosaurus.
> Otherwise it would have been difficult for the prey to avoid the similar
> shaped predator and your idea (spinosaurs are the mimics) becomes the
right
> one.

> If spinosaurs were not the main predators, the mimicry in ouranosaurs was
> instead a device against other, non spinosaurid predators, small enough to
> feel that dealing with a spinosaur would have meant business for them. Who
> knows, might the spinosaurid claw  have been a deterrent even for a
> carcharodontosaurid ?
> Before shutting up, I wonder:
> Is there today a similar example of "reversed mimicry" that is, a predator
> that looks like a prey to get closer to it with more ease?
> Putting aside insects, where I think you can find nearly to everything,
> does it happens among terrestrial/flying vertebrates?

An example of predators using mimicry is the use of 'lures' by anglerfish
and death adders  to attract their prey.  Remember. however, the lure is a
small part of the predators body (anterior dorsal fin ray in anglerfish,
tongue in death adders), although I don't think that this necessarily a
theoretical limit of this strategy.  Then you have the flower mantids, which
mimic flowers in order to catch pollinating insects.

There are some deepwater predators which mimic the bioluminensce of their
prey's prey (again, another form of lure).  It would not surprise me if
their are instances of such predators mimicing the light displays of the
prey species themselves - I think that dragonfish might do this (or possibly
they might be the dupes of some predator that mimics the unusual red light
that dragonfish use for intraspecific communication - I seem to recall
seeing something about this recently, but my memory fails me).

A well known example of what you are talking about is the species of
predatory fireflies that mimic the mating displays of other firefly species
in order to attract prey.  An insect example, I'm afraid - I can't think of
any instances in vertebrates.  Other examples involving invertebrates
include a spider that
mimics its ant prey, and some caterpillers that mimic the 'scent' of ant
larvae - I think there have been examples of each featured in National
Geographic in
the last few months.

> Within snakes both the deadly coral snake Micrurus and the inoffensive
milk
> snake Lampropeltis represent mimics of a moderately poisonous genus.

Well, sort of.  In this case _Micrurus_ and the 'moderately poisonous genus'
(do you know what it is?) are displaying Mullerian mimicry, where two
'model' species undergo selective pressure to become more similar in
appearance (the selection pressure is stronger on the less 'distasteful'
species, so the resulting phenotype should, all other things being equal, be
closer to the original appearance of the more distasteful model.
_Lampropeltis_ would be considered a Batesian mimic of the other two.  This

situation of a 'mimicry ring', with more than one model and (sometimes) more
than one Batesian mimic, may be more common than the more straightforward
pairwise relationships, and the dynamics can get very complicated.  The
mimicry ring of which the Monarch butterfly is a member is one of the better
known examples.


Going back to the situation with the dinosaurs, I think that you might be
able to conclude that;
1. Because they shared a similar, unusual phenotype, and (if) they are
sympatric, there existed the theoretical potential for mimicry of some sort.
2. Theoretically, it is perhaps more likely that _Ouranosaurus_ was the
mimic
of the spinosaur.
3. Whether or not the selective advantage of this mimicry was enough to
drive
the evolution of the sail/hump is a different question from the existance of
mimicry at all.  It is quite possible that the sail/hump evolved in both
species in response to other selection pressures, and that any mimicry
involved only a similarity in colour pattern, or a minor modification of the
sail/hump in the mimic to more closely resemble that of the model.

I didn't ever think that an honours project on the evolutionary ecology of
mimicry systems would ever have an application to vertebrate palaeontology!
However, enjoyable as it is to indulge in such speculation, I cannot think
of a single way to test any of this from anything but a perfect fossil
record.  Nevertheless, if anyone is interested in pursuing any aspect of
this, there are some experimental simulation techniques which could be used
to investigate the theoretical evolutionary dynamics of this scenario.  Some
of then make great teaching tools for high school / university students.  If
any teachers are interested in these, let me know and I'll dig out some
info.

Cheers
Colin