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Trexie and whales..?! (fwd)



                               
A drizzle of consciousness precipated by recent discussions concerning 
Trexie:-

1.  Tyrannosaurus is a scavenger because...
(a) Its arms are too short.  
Right.  This agrees nicely with the recent suggestion that Crocodylus 
porosus isn't the nastiest thing with teeth after all, but merely 
scavenges the bodies of Yankie tourists who are cooperative enough to 
pass away (in shock?) just in front of it.
(b) Its thigh bones were too long.
He's right again, you know.  There's this bipedal primate with a 
femur/tibia ratio > 1, and it can't manage anything faster than a sexy 
saunter.
(c) Predators need good vision, and therefore large eyes.
Next time I'm in the water, bleeding profusely from coral cuts, I shall 
reasure myself that the tiger shark circling me can't eat me because its 
vision is not as good as its sense of smell...

And as for the keen sense of smell?  Well, my recollection of N. American 
vegetation types during the end Cretinaceous is dim (I was only young at 
the time) but wasn't there something  about all these big dense forests?  
With these elephant sized ceratopsians (the 's' is for snack, as in 'rex 
food) trotting around in them?  For those of you who have not undergone 
the bowel loosening experience of being close to some Elephas in the 
middle of dense forest, I can assure you that you can smell and hear them 
a long time before you can see them (and Asian elephants aren't that much 
smaller that African elephants).  A skill you pick up very quickly.  If I 
was trying to dine on pachyderm in such a habitat a good snozzler would 
be very useful - but I'd still need binocular vision for the final attack.

Other factors which are relevant - does the energetics of the situation 
permit such a large animal, or even any terrestrial animal, to exist on 
scavenging alone?  It's not as if your Triceratops is going to be dying 
very often if the T. rex isn't killing it (can you image any other beast 
capable of preying on Triceratops?).  If it's such an exclusive 
scavenger, then why aren't the teeth and forelimbs adapted to 
manipulating bones to get the most out of them (it can't be a scavenger 
because it's arms are too short!)

2. Girl tyrannosaurs are bigger than boys (tyrannosuars, that is) because...

Ignoring for the moment the dismal sample size and the uncertainty 
surrounding good skeletal indicators of gender, there are some 
interesting points in this.

Whilst the reasons for males being bigger than females are well 
understood (and are usually to do with males competing for mates, 
especially amongst mammals), reasons for the opposite condition (what Jay 
refers to as reverse sexual dimorphism) are less well so (surprise, 
surprise).  Outside mammals, examples of larger females are to be found 
in all vetebrate groups (with well known e.g.s amongst the birds, and am 
I right in thinking crocs too?).  The exceptions to larger (or same 
sized) mammalian males that I can think of off the top of my head are the 
mole rats (which are eusocial, and therefore odd, and ugly, in many 
ways), and the baleen whales.  Since no mammal except the mole rat can be 
polyandrous (a possible reason for the female being bigger, see below), 
the reason for the larger female roquals is probably due to the fact that 
they give birth in the lower latitudes (with not much food there), and 
have to suckle a 5 ton baby until they get to their Antarctic feeding 
grounds (note that in toothed whales the males are larger).

As Jay says, the possible reasons for sexual dimorphism in size include 
the exploitation of different niches (as can be witnessed in some 
spiders, where the male is a small fraction of the female's size), and 
energetic considerations connected with rearing young.  Personally, I 
don't think either  is involved in  the case of T. rex.  So for  Raptor 
Rachel, who seems to view Trexie as a good role model (and why not?) I 
float this possibility...

The classic reason for one sex to be larger than the other is competition 
within the larger sex for matings with the smaller.  Generally, this 
means big males, with lots of low cost sperm, competing for the 
opportunity to fertillise the female's precious eggs (the female, having 
invested much more in each egg, can afford to be fussy about who gets to 
fertillise it.  If she choses (directly, as in the classic 'female 
choice', or indirectly, as with joining a harem) bigger males, then males 
will get bigger.  If she chooses brighter males, then males get gaudy.  
If she chooses males which hang around and help bring up the young, then 
males get faithful and even helpful.  These are the general patterns.

In those circumstances where parental care is necessary to ensure the 
offsprings' survival, a male being sucked in to helping look after its 
brats can get left holding the baby.  In animals (e.g. fish and 
amphibians) with external fertillisation (and where the female has lots 
of low cost eggs) the very nature of the act means the male is often the 
last to have contact with the eggs, giving the female the opportunity to 
scarper and leaving him with the job of parental care (and there are many 
examples of good daddy fish - sea horses are one).  Where fertillisation 
is internal (e.g. amniotes) the female can't do it this way, but males 
can be made to be better parents than females if the species goes through 
a situation where the care of both partners is essential to successful 
reproduction.  If conditions then get less harsh, then each will  have an 
equal chance (in evolutionary time) to run off first, leaving the other 
with the sole responsibility for care of the young.  Sometimes it will be 
the male, othertimes the female - and there are quite a few examples in 
birds where the males look afetr the young (phalaropes are one).  With 
mammals it is very rarely if ever possible because of the huge initial 
imbalance in investment - not only to females produce the egg, but they 
gestate and suckle the young as well.  Male mammals have it easy.

In those situations where the male is the sole carer, it is now he, 
rather than the female, who has the most to offer a prospective romantic 
interlude.  And it will thus be she who must compete for his compliance.  
Leading, perhaps, to selection for larger females...   (I think there was 
even an example of this in raptors - I'll see if I can find the ref.).  
And of course the female, freed from the burden of maternal 
responsibilities, can now indulge in polyandry - i.e. mating with a 
number of males, leaving each mate to look after the resulting 
offspring.  Again, phalaropes are good examples (I think the female 
manages up to three broods, each with different mates, in the short 
Arctic summer).  With polyandry the selective pressures for larger 
females are even stronger. It all means, of course, that T.rex should be 
T.regina.....

As for hunting in pair groups... well, that wouldn't happen if the 
reasoning above is correct, and anyway I shudder at the thought of having 
to supply food for two hungry tyrannosaurs.  They would only pack if they 
needed to in order to hunt/forage effectively, and I can't imagine why a 
single T. rex isn't effective enough on its own (especially in forest).  
Most animals show a strong aggressive tendancy to other conspecifics, 
which is overcome only during mating seasons.  My hunch is that the 
ability to hunt together requires a fair amount of brain size, which only 
began to appear with carnivorous mammals.  But I don't have the data to 
supoort this, and I've gone on for long enough.

And I've just remembered  that female fruit bats are larger than males, 
but I've no idea why.

Bye for now, 

Colin McHenry

P.A. Swamps Inc.
aka Vertebrate Palaeontology Lab
Dept Zoology
University Queensland
OZ