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CARNIVOROUS SLOTHS



WARNING: NO DINOSAURS. GROUND SLOTHS ONLY.

I'm happy to see that some of you are somewhat sceptical of Farina and Blanco's
theory about _Megatherium_ being a carnivore. I got hold of their paper when it
came out (published earlier this year in _Phil. Trans._ B) and, while I find
much of what they say very interesting, their case does not necessitate
carnivory in the megatheres. 

Basically, they are saying that, because these animals could do a quick strike
with the forelimbs, they must have been using their forelimbs offensively. OK...
offensive forelimbs can be used for self-defence. Why aren't megatheres
defending themselves from, say, sabre-toothed cats or dire wolves? Farina and
Blanco only considered _Smilodon_ as a potential megathere predator. But,
amazingly, they did not think that this animal would pose any threat to a
megathere as (this is their argument) the _Smilodon_: _Megatherium_ size ratio
is the same as _Felis_: _Homo_, and, after all, you don't get domestic cats
ripping people to shreds. 

This is an invalid argument. _Smilodon_, as you may have noticed, is not just
a big domestic cat. It's rather nicely adapted for killing big animals - err,
doesn't it have some sort of, err, sabre-shaped teeth in its mouth? Not only
that, but _Smilodon_ was probably social - ground sloths would have to contend
with prides of these horrible beasts. Farina and Blanco's size ratio comparison
is further invalid as many predators are well adapted for killing prey many
times their own size. Why choose the ratio of _Felis_ to _Homo_ when you could
have chosen _Mustela_ to _Oryctolagus_? Weasels kill rabbits regularly - yet
rabbits are sometimes more than 10 times bigger than weasels. And wolverines
have been known to kill moose for god's sake! Similarly, _Smilodon_ may have
been adapted for, and used to, tackling prey several times its own size. A
domestic cat is not a suitable analogue.

_Megatherium_ lived in a fauna that was also populated by Dire wolves (_Canis
dirus_). Its is probable, but unprovable ASAIK, that these were social too.
Carnivoran expert Dr. David McDonald has said that dire wolves may have been
more intelligent that any living carnivoran - they do, apparently, have
proportionally bigger brains (this is in contrast to what Bjorn Kurten said but
let's ignore that for the time being). They are also big and there is every
indication that they predated on ground sloths. Rancho La Brea also reveals
_Canis lupus_ - the Pleistocene variants were up to 25% bigger than extant ones
and may have hassled ground sloths too. And there was another major predator -
 _Panthera atrox_ (or, as I suspect, _P. spaelea atrox_): the American lion.
Extant lions are social and can, on occasion, take down animals up to the size
of Cape buffalo (about 500 kg). They *have* killed adult giraffes and baby
elephants (but these are very rare occurrences). American lions may or may not
have been social. They were very much larger than modern lions (big living lions
reach 180 kg and recently extinct Cape (_P. leo melanochaita_) and Barbary lions
(_P.l. leo_) reached 230 kg or more) and weighed it at between 300 and 500 kg!
A cat of this size could have killed a megaherbivore single handedly, let alone
if it hunted in a pride!!

Now, are you going to tell me that megatheres _don't_ need to defend
themselves?? I hope not. The quick-acting megathere forelimb - with its large,
hooked claws - looks like a suitable anti-predator device that was probably
employed in self-defence. Remember, also, that megatheres cannot run away as can
ungulates. Megatheres would _need_ to defend themselves: believe me.

Quick-acting forelimbs, as McNeil Alexander noted in _New Scientist_, do not
necessarily denote carnivory - especially when megathere skulls, dentitions and
even coprolites indicate that they were herbivores and quick-acting forelimbs
appear highly beneficial in an animal threatened by a number of predatory
species. So I don't think Farina and Blanco's observations were, in the case
of _Megatherium_, valid. However, it is still possible that some ground sloths
were carnivores or omnivores. Particularly noteworthy are those Argentinian late
Tertiary ecosystems where ground sloths exist at very high taxonomic diversity
and dominate their communities. Yet some of these environments are apparently
very low in plant biomass and it is a mystery how all these 'herbivorous' sloths
got by. The answer may be that they were not all herbivorous and some may have
eaten insects, small vertebrates or carrion. Perhaps - perhaps - some were
active carnivores. Armadillos do eat other vertebrates on occasion and some
traditional herbivores like giant tortoises have been known to eat carrion.
Hippos have been known to kill and eat impala and one individual at Manchester
zoo ate flamingoes and a tapir. I can therefore see some of the smaller, more
generalised ground sloths as being as adaptive in their diet as, say, pigs,
peccaries or hippos. But I do not believe that _Megatherium_ was killing
glyptodonts.

And the correct version is:

"Ah! Do that again!"

I had to watch it again last night to make sure.

DARREN NAISH