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Re: Phorusrhacids killing large mammals in National Geographic Channel



"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";

Ah, OK. So even the bigger mammalian predators of that time and place
were only about dog-sized. I can see why the idea of phorusrhacids
being the top predators is so intuitive (aside from simple
energetics).

"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 would just say that there's a good set of selective reasons to not
attack really durable prey animals that could put multi-inch claws
through your face.

Modern pilosans only produce one offspring at a time, right? So I
suppose hunting juveniles isn't too attractive an option either. But
juveniles of other critters would be feasible.

"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."

I dunno, hawks hunt other hawks and owls despite being high up in the
trophic levels, and large mammalian predators will often go out of
their way to kill each other without even eating each other, so I
think any flexible, opportunistic mammalian (placental?) predator
would take advantage when it could, if only to remove competition.
Size and aggression would have been deterrents, of course.

"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)."

That's interesting, but IIRC dholes tend to travel in packs that are
much larger (on average) than wolf packs, making them threatening
through force of numbers instead of their coyote/collie size range.
(Having that many more jaws is harder to deal with than facing a
smaller number of bigger ones). There's no real reason that social
canids and social large cats of that time would not (or at least could
not) have been as social as those today, which live with very
well-defined group hierarchies. (Also, there's good evidence that at
least *Smilodon fatalis* was social in some capacity - worn-down
broken sabers in some individuals - so even machairodonts could have
been coordinated hunters.) Of course, all of this assumes that these
sorts of carnivores were present, and at least pressuring the
phorusrhacid population.

"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."

Well sure, I'm just making an observation that phorusrhacid beaks are
closer to raptorial beaks than granivorous/folivorous ones.

"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."

The use of the beak for manipulation and climbing is a good
observation (multi-functional selection, I assume), although as to
smaller birds - which someone else noted but to which I'll add -
larger seed probably scale differently than small ones, with a
propotionately thicker shell necessary to provide support relative to
their surface-area/volume ratio. That requires a stronger beak (and
better leverage, which is where the superior manipulative abilities
would indeed be handy), and circles are of course the strongest (2D)
shape from an engineering standpoint. I assume smaller parrots that
eat little seeds evolved from larger ancestors who already had some
proficiency with the bigger kinds.

But again, just noting that terrorbird beaks look way better for ripping flesh.

"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";

The difference in shape between the large and small species is
intriguing. I know that for mammalian predators, there is a particular
threshold of body mass at which hunting small prey becomes too
inefficient and hunting big prey becomes adequately safe to justify. I
think the size is somewhere around that of a large lynx/somewhat
bigger than a large coyote. Roughly. It would be interesting to see if
that size or something close defines the split between thin- and
broad-beaked phorusrhacids - it might constitute evidence that the
larger ones were specifically adapted for grappling with and downing
prey approximately as large as (or larger than) themselves, or at
least some kind of behavioral differentiation in hunting style.


"The reason this fails is that no extinction of phorusrhacids is known
to coincide with the Great American Interchange."

Alright then, what does it coincide with? Pleistocene glaciation
started about that time (~1.8 mya), right? I guess I'd just be
surprised if such a widespread and reasonably diverse group got taken
down by the glaciation in such a comparatively stable and ice-free
climatic zone (especially with such a long history of coping with
South America's other habitat shifts). I know the GAInterchange
started with limited dispersals around 8 mya, with a few more
significant ones around 4-5 ya, and then the floodgates opened about 3
mya. It seems possible, though by no means necessarily probable, that
the presence of placental predators may have been pressuring
phorusrhacids for some time, and that this *coupled* with climatic
change allowed the placentals to either directly outcompete the birds
in some way, or simply breed faster in and adapt more effectively to
the altered conditions. It's a hypothesis, but maybe it's worth
investigating (or already has been, what do I know?).