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Re: Phorusrhacids killing large mammals in National Geographic Channel



2009/7/28 Raptorial Talon <raptorialtalon@gmail.com>:

> My cursory check of Wikipedia claims that *Borhyaena* reached 100 kg,
> which is about the size of a large cougar.

Darren Naish claims in his blog Tetrapod Zoology that this was the
older estimation, and that the one i gave before was based on work by
Argot (2003):
http://scienceblogs.com/tetrapodzoology/2008/07/borhyaenoids_part_three.php


> Sloths are slow, true, but they're also armored in osteoderm
> chainmail, besides the weaponry you mentioned. I'm guessing that
> critters like *Borhyaena* hunted more conventional prey, and that they
> were the predominant predators in regions with heavy cover, where they
> could hunt from ambush.

Depending on the boldness and agressiveness of the predator. Perhaps
if tempered like a Sarcophilus or a mustelid, they could defeat those
apparently slow prey. I suppose they were not so brave, because they
are not any more related to these as they are to the more timid
Didelphis.

> I'dd add that, specifically, nesting higher up makes a nest harder to
> detect, even if an otherwise ground-based predator is capable of
> easily reaching it. The ability to climb means little if the would-be
> predator doesn't know the target is there. Phorusrhacids would have
> been worse off due to their nests being more exposed to a greater
> diversity of predators, large *and* small.

True, but many birds fare well with nests on the ground, including
many flying ones. Of course, phorusrhacids, because of being large and
carnivorous, should have very small populations and be thus more
sensitive to predation. However, perhaps because of being rare, in
addition to any other defensive device, they would have the advantage
of not being a food item to which other predatory animals got
accustomed easily.

> We can't rule out big cursorial hunters simply eating the adults at
> night, either. Dire wolves and sabertooths would have been all too
> eager to take advantage of any big stubborn bird that had bedded down
> in the open and was adapted for fight rather than flight.

That may also go the other way around, however, if we consider your
hypothesis that the bird can deliver a killing bite to the neck, which
in carnivores is more slender than an human thorax. Perhaps if these
carnivores were social, this can be more difficult for the bird. But
their sociability seems to be relatively contentious. Or the more
unlikely gregariousness of the bird (well, in packs they may have been
much more terror birds). Even so, they may depend on temper of the
sociable animal. As far as I read (not remembering the source), tigers
avoid Cuon alpinus in India, so the canid may be ecologically
dominant, but tigers are dominant upon the proportionally larger wolf
in Siberia, which avoids the large cat (the disproportion in size
between Cuon and the Siberian wolf is larger than between the
geographic varieties of tigers).

> Macaw beaks have a strong curvature all along their length for maximum
> strength/leverage against large, thick-shelled nuts. Raptors have
> beaks that tend to be straighter proximally and only really curved
> distally, at the "business end" so to speak, which is the pattern we
> see in phorusrchacids. Even in keas, the only parrots to eat meat, the
> curvature is weaker than in nut-cracking parrots and somewhat closer
> to the condition seen in raptors. Macaws also have very short, robust
> lower jaws, again not a good match for phorusrhacids (although raptors
> aren't exactly perfect either, as you've mentioned). I would assume
> that a big herbivorous ground bird would be mainly
> frugivorous/folivorous anyway, rather than granivorous, so we wouldn't
> expect to see a strong curve in those forms (as is true for elephant
> birds, moas, takahes, etc; the kakapo is an exception, but that's just
> a plesiomorphy from its parrot heritage).

I neither believe of giant seeds being so common to feed a population
such a large endotherm (of course, this is no scientific statement,
lacking any math). Also believe the differences observed between
phorusrhacids and parrots suggest different diets.

Perhaps parrot beaks primarily represent a nut-cracking device, as you
say. There are many other birds, of which I mostly mind passeriforms,
which possess thick beaks, but not so curved, which seem to break
relatively big nuts, so perhaps the explanation for parrots is
something else, related to the better manipulative capacities of these
birds, which may be related to their greater "intellectuality". They
can perform other tasks as well, such as helping in climbing, and use
a wide repertoir of vocalizations, carrying large branches and food
items while flying, which may also be related with the
"intellectuality". Among these, the downwards tip is also of use,
overall in climbing and grasping.

Now, I have to correct one (of the many) unintentional lies I drop
here :). Small phorusrhacids have laterally compressed beaks, but for
large ones, the only one complete skull known (Kelenken) show no
lateral compression. Former reconstructions of the skull of
Phorusrhacos are wrong according to Chiappe and Bertelli, and based on
much destroyed remains. So perhaps their beaks of the larger
phorusrhacids were more resistant than I thought to lateral
compression, and depart a little from those of the Steller's eagle.

Something similar to the lateral shaking of dogs can be observed in
this gull, admittedly in not so great frequency, so it may not be so
unlikely to occur in other predatory birds:
http://www.youtube.com/watch?v=xXKwQzYLPHE