Scavenging and Sexual Dimorphism in T. Rex

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

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 Yankee 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 reassure 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 [of the] Cretaceous 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 Elephants 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). [It's] 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!)

Girl tyrannosaurs are bigger than boys (tyrannosaurs, 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 vertebrate 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 mating with the smaller. Generally, this means big males, with lots of low cost sperm, competing for the opportunity to fertilize the female's precious eggs (the female, having invested much more in each egg, can afford to be fussy about who gets to fertilize it). If she chooses (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 fertilization (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 fertilization 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, other times the female - and there are quite a few examples in birds where the males look after the young (phalaropes are one). With mammals it is very rarely if ever possible because of the huge initial imbalance in investment - not only do 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 tendency 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 support 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.

Colin McHenry