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Trexie and whales..?! (fwd)
A drizzle of consciousness precipated by recent discussions concerning
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
(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
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,
P.A. Swamps Inc.
aka Vertebrate Palaeontology Lab