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Re: Phorusrhacids killing large mammals in National Geographic Channel



> 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.

You are right in both observations. However, I think temper of the
carnivore is important. For example, incisors of rodents are very
dangerous and for sure would inflict great damage, yet mustelids kill
them anyway, sometimes rodents larger than themselves. I never heard
of rat-sized rodents killing ermines, for example, although it may
have happened sometime… (lucky rodent). One anteater is known to have
killed a 19 years old zoo keeper with his claws, and I heard something
of anteaters even killing a jaguar. But most of the times, the
anteater is the prey. And it is true that modern pilosans are not fast
reproductively, yet in the Cenozoic they were very abundant, both
individually and in species number. And for a carnivore with such
thick jaws as Borhyaena, turtles and small cingulates are other
options. I am neither so sure all the small South American ungulates
were as fast as proterotheriids and hegetotheres, not sure how fast
small toxodonts, and some caviomorphs were. In addition, mustelids
hunt prey faster than them, such as rabbits, just by digging, so this
may be also the case for these marsupial predators.

Most aggresive creatures are commonly very stocky, with powerful jaws,
and resisting much injury, but that does not explain why the maximal
prey size of a pack of African wild dogs (adult zebra) is the same
than that of the much larger and more robust hyaenas. Or why a small
pack of these dogs, or the Cuon, can kill large prey way faster than
the more dubitative wolf. As you say, sociability does not imply large
pack numbers. So, it would not be rare if the canids facing
phorusrhacids have more of a wolf-like cautious behaviour, if forming
small packs. Perhaps, if these carnivores respected the large bird
when both were active, they may respect them also at night.

> 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.

True, but killing smaller or less social carnivores does not imply
extinction of one of them. Phorusrhacids may have where to hide their
eggs, or perhaps even themselves, even among bushy vegetation. There
seems to be some evidence, presented by some Argentinian
paleontologists, that some large ground sloths may have digged large
caves, where much other animals may have hided (many owls do so in
rodent-digged tunnels). Anyway, I do not know many cases of predation
of sleepy ostriches by mammalian carnivores, even when they would be
easy to subdue even by a Cape badger because of their thin neck.
Perhaps it is also matter of the awareness the birds can have.

The less dominant carnivores are not necessarily doomed. Phorusrhacids
face some danger similar to ostriches and cheetahs, so that their eggs
are difficult to protect, as their youngs, they can be killed while
sleeping, and they only can escape with velocity. In low population
numbers, they may resemble cheetahs. Yet cheetahs do not go extinct,
even with everybody robbing their food, killing their youngs, and for
sure trying to kill asleep adults. After all, small cats live in
Africa alongside the similarly agile, but larger and stronger,
caracal. Or more generally speaking, you would have to perform a test
of how much probable was the encounter between a carnivore and a
sleepy bird given an area, with which frequency, and taking in account
the population number of the bird and their predators, to see which
impact such event will have in the population of the bird. I mean, to
settle this question we should make many calculations. The same with
the carnivore-phorusrhacid egg question. That the Iberian lynx can
kill many other smaller carnivores do not imply these will be
extinguished.

> (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.

Yes, I heard the argument concerning sabertooths, anyway, I also heard
of a lioness with a broken jaw which lived from locusts she alone
hunted. Sociability is much restricted in cats, and is less
parsimonious to infer given the extant outgroups for machairodonts
(e.g., linsangs and the primitive character state for Felidae).

> "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.

Ok., this was for your assertion that large herbivorous birds were
folivorous instead of seed-eaters. More for Diatryma than
Phorusrhacos.

> 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.

Perhaps you are right, but this requires smaller parrots evolved from
larger ones (which goes against the general pattern envisaged by
Cope’s Rule; anyway, this statement has not to be unviolable, I only
would posit against it that it requires an assumption). This theme of
the explanation of the appearance of the parrot beak is also
interesting. Although I do not doubt parrots bite hard, their enhanced
cranial kinetic system suggests greater dexterity more than sheer
force. Both strength and dexterity are correlated with avoiding seed
toxic membranes, so in any case they fit the seed-eating selective
scenario. Anyway, parrots do not only eat seeds, and they include many
other fruits also, so it is also an alternative that manipulative
abilities came first and then were used in very different ways,
including nut-cracking (which also correlates well with manipulative
abilities of feet, which are even of use to grasp the same nuts). It
seems that greater manipulative capacities, along with brains capable
of imagining how to use these tools, is also a selectively favourable
trait which does not require specific sizes.

> 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 wider beak suggests resistance to greater lateral forces, so yes,
it is likely to help coping with relatively larger prey.

> "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?

As far as I know, from a 1980s paper by Marshall et al. in Science,
faunal members of both the South and North American realms went on
extinguishing all along the time the interchange continued. Perhaps
the stress for the ecosystem as a whole, instead of one-to-one (or
more) competition between species can explain that.