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display structures (was Re: Homo sapiens idaltu)



----- Original Message ----- 
From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
To: <dannj@alphalink.com.au>; "DML" <dinosaur@usc.edu>
Sent: Friday, June 13, 2003 7:29 AM
Subject: RE: Homo sapiens idaltu


> > From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> > Dann Pigdon
> >
> > As an archaeologist, I can let you in on a secret: if you don't know
> > what a certain object or behavioural strategy was for, then it's always
> > 'ritual'. That way we don't look stupid by saying "we don't know". :)
> >
> The equivalent on the paleo side: display structure.  (Which is pretty
much
> a ritual, when you think about it...)


Couldn't agree with Tom more.  Everytime I see a 'wierd' structure in
palaeontology 'explained' as a display structure I feel like screaming.
Invoking display as an explanation is tantamount to an open admission that
you don't understand what use the animal might have made of it, except
without the honesty.

I reckon we should coin another acronym for this, like Tom's 'Wait For The
Paper'....something like Don't Invoke Display, or Use Display As A Last
Resort.  Demonstrating that a structure is even used for intraspecific
display in living species - let alone establishing that the primary role of
the structure is display - is hard enough, and you can actually watch them
behaving.  In extinct animals it is nearly untestable, to my mind.

Which is not to say tht animals don't display, or don't use display
structures, but I think a little perspective is needed.  Most display
interactions between members of the same species simply involve 'ritualised'
behaviours and vocalisations.  Basic body colouration is also used, although
body colour also is important in crypsis, thermoregulation, aposematism
('frightening' displays directed at potential predators) and mimcry.

The nature of intraspecific display structures varies between the different
aminote groups.  In mammals fur and skin colouration is often used
(especially in primates, with their colour vision), but basic body size
(usually in males) is also important.  Birds commonly use feather shape and
colour, and reptiles make use scale shape and colour (especially in the
display crests, often seen in agamids and iguanids, that are formed by an
arrangement of scales, often brightly coloured.  Note that these are all
'soft' (i.e. non-bony) tissues of the integument.  In some cases the
colouration and integument structures differ between males and females, in
others they don't.

Most instances of hard parts (i.e. structures made of bone or horn) being
used in display involve mammals, although there are some examples from
reptiles and birds.  In mammals, hard structures that are used for display,
and that are not sexually dimorphic (except as a function of body size)
include the paired horns of bovids, the tusks of walrus, some pigs, hippo's,
and (African) elephants, and the horns of giraffes.  As far as I am aware,
all of these structures are also used in feeding and/or defense against
predators.  Sexually dimorphic hard parts that are used in display include
the deciduous antlers of deer and the tusks of Asian elephants and some
suids.  In the case of deer antlers, these appear to be used primarily for
intraspecific display (in the form of male-male combat), but are also used
to defend against predators.  As far as I am aware, Asian elephants that
possess tusks are just as likely to use them in feeding as Afrian elephants
are.  Sexually dimorphic hard parts which might be used in display include
the tusk of narwhals and the large teeth of the Strap-toothed beaked whale
(_Mesoplodon layardii_), but we simply don't know enough about the ecology
of these animals to say what role these strange teeth might play in the
lives of these beasts.

With birds the only display structures that I can think of, that are
composed of hard parts, are the cranial crests of the cassowary, and the
'nasal' crests of some hornbills.  I think it unlikely that the large nares
of petrels and albatorsses fall into this category.  I am not aware of any
sexual dimorphism in the crests of hornbills or cassowaries.

With reptiles, hard parts that are used for display include the 'horns' of
some chameleons (again, important in male-male combat). I can't think of any
other examples off the top of my head.

There may tend to be a difference in the type of 'display' structure used,
depending on whether the intra-specific display focuses more on male-male
interactions, or upon female choice.  I know there has recently been a large
body of work on this in brids, but I couldn't tell you offhand what the
overall conclusions might be.  Anyone?

So, there are structures that are used in display, in living animals.  Does
this then allow one to state that 'The function of structure X is for
intraspecific display'.  I have a couple of issues with this sort of
statement.  The first is semantic - I find that the use of the phraseology
'function....for' is very determinate.  It emphasised one aspect of the
possible use of a structure, and seems almost to swamp any other 'possible
uses'.  This is one of the reasons I try to avoid using 'function' in this
context.

The second issue is obvious - as we all know, most (all?) structures in
biology have more then one use for the animal.  Feathers on a parrot are
used by the bird to assist with thermoregulation, flight, crypsis, and
intraspecific display (and probably play a few other roles besides).  I
don't think anyone would suggest that these structures (the feathers) are
'for' display.  One could say that the colour of the feathers is important
in intraspecific display, but the latter is a very different statement -
much more cautious, and to my mind more likely to be accurate.

One could say that 'function' could be used as an equivalent term to
'primary use', but I'm not sure even about this.  Take, for example, the
feathers on our parrot - they are so integrated into the biology and
physiology of the animal that it would be hard to distinguish a 'primary
role' for these structures.  Granted, a denuded parrot in the wild may not
enjoy much success with potential mates, but it might die of being easily
seen by predators, unable to fly away from predators, unable to fly to a
food source, or being unable to thermoregulate, before it had a chance to
worry about not looking sexy.

Identifying a 'primary role' for other structures may be possible, if only
because they are not absolutely essential to the physical survival of the
animal in the wild.  Female deer, as well as young male deer and even adult
males during the winter, are perfectly capable of surviving without antlers.
Experimental removal of antlers in some adult males may not, therefore, be
automatically fatal.  Careful experiementation may reveal if the antlers are
used by normal males in feeding or defense against predators, and it may
even be possible to work out selection coefficients on the importance of
antlers in these aspects of the deers' ecology.  At the same time, it would
be possible to look at the relationship that mate choice and territory
defense have with the presence/absence of antlers in adult males.  If you
did all of this work (and it may have been done - does anyone know?), then
you may be in a position to state with confidence what the 'primary role' of
antlers in deer might be.

But you would only have looked at the selection pressures involved in the
evolutionary maintenance of antlers in deer. The selection pressures
involved in the evolutionary origin of these structures need not be the
same.

It gets more complicated when you look at structures that are not sexually
dimorphic.  For example, consider horns in bovids.  You only have to watch
adult male gazelles to see that their horns are used during intraspecific
display - but then, so are a few other parts of their bodies (legs to jump
and prance, glossy fur to look good, etc.).  Horns are used by those same
gazelles in defense against predators, by males and females.  I think that
to say that the horns in these gazelles are 'for' display is therefore
misleading - yes, they are used in display, but they play other roles in the
animal's life.  I don't think you can tell, just by looking, what the
'primary role' of the horns might be.

Thus it is with fossil species.  The head crests on hadrosaurs probably
played a role in all aspects of the animal's vocalisation, rather than just
in sexual display.  Remember that elephants use ultrasonics to keep the
members of a dispersed herd in contact with each other, even though they may
not be able to see each other (they also use ultrasonics in sexual display,
especially between males).  Horns on Triceratops probably had a role to play
in defense against predators.  Plates on stegasaurs would have affected the
thermoregulation of the animal, as would sails on spinosaurs and
pelycosaurs.  As for head crests on pterosaurs, there are biomechanical
interpretations of these also.

Enough of this.  Can we construct a set of criteria to determine whether or
not a structure on a fossil animal might have been 'for' display?  Possibly,
but the first thing to remember is that any part of the organised can
potentially be used as a display structure. Structures that are actually
used in display are often on or near the head, or positioned so that they
make a large difference to the profile of the animal. However, the relevant
question seems to me to be, not 'could it have been used for display?', but
instead 'what else could it have been used for?'.  In my opinion,  many
palaeontologists do not come near to exhausting biomechanical and
physiological explanations for bizarre structures before the old 'display
structure' clause gets invoked.

To my mind, marked sexual dimorphism (beyond what can be attributed to any
dimorphism in body size) of a structure is very suggestive of a structure
whose use in intraspecific display is very important to the animal.  In such
cases, the structure should be most extremely developed in the largest
adults (or the largest of one gender, if this can be distinguished).  And
the effects of the structure upon the animal's physiology - usually, a
simple consequence of the large change to the animal's surface area:volume
ratio - need also to be considered.  If you looked at all the other possible
explanations - biomechanical, physiological, etc - and exhausted these,
then - and only then - might you be justified in suggesting that the
structure was primarily used for intraspecific display.

Would be interested to know what others make of this.  Off the soapbox...

Cheers
Colin

Colin McHenry
56 Gaskill St
Canowindra NSW 2804
+61 2 6344 1009
0428 131 858
cmchenry@westserv.net.au