Re: Theropod bluff & hunting (was re Dilophosaurus Forelimb Bone Maladies

[Mike Habib <biologyinmotion@gmail.com>]

> On Mar 4, 2016, at 3:59 AM, Darius Nau <dariusnau@gmx.at> wrote:
> 
>> 
> For a lion, this involves grappling the buffalo and applying pressure to its 
> trachea until it asphyxiates. Its bite isn’t sufficiently damaging to 
> reliably bring down something that large by other means.
> 
> A carnosaur (or a komodo dragon, or shark) would have had no such 
> restriction, and consequently no need for much precision or getting into a 
> direct strength contest with its prey. Their jaws are proportionately larger, 
> and more importantly, are filled with serrated teeth that would cause 
> proportionately much larger wounds and greater blood loss that are dangerous 
> to even a very large prey item all by themselves (demonstrated by extant 
> animals with this tooth morphology).

I would flip this on its head: big cats, as high power precision biters, are 
unusually well adapted to kill large prey. That's why cats do so much more 
often than other living mammalian predators.

Most varanids, some of which have teeth that are also good matches for 
theropods, are obligate small prey hunters (they cannot cut effectively with 
their teeth). Komodos are the exception, and their ability to carve muscle is 
not so much a matter of their teeth as it is a special form of cranial kinesis 
that allows them to saw with the jaws (see work by Domenic D'Amore). Most 
sharks are also small game hunters. Even large adult great whites mostly hunt 
juvenile pinnipeds that can be consumed in a bite or two.


> Yet in some regions Cape Buffalo are the preferred prey species of African 
> Lions, such as Hwange where in males and females respectively they represent 
> 56% and 33% of all prey items by count of kill sites. I don’t think that 
> qualifies as "almost never".
> https://urldefense.proofpoint.com/v2/url?u=http-3A__&d=CwIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=x82f3Wlkwtmbr1z8IAt9jA&m=6w6lLukAL0Gn6JEFTMTmpj5PMwPdREM815r_S988QSc&s=6Y1VP2Ff78K9UTFl6Bc3_miAQS6hkggIumJxrl6_TMA&e=
>   journals.plos.org/plosone/article?id=10.1371/journal.pone.0055182

Yes, but the prey are mostly juveniles (see Hone and Rauhut, 2009 and 
references therein).


> Other large animals are also a common part of their diet, we can potentially 
> extend the same arguments to several other large mammals; Giraffes, Young 
> Elephants, Wildebeest, Zebra and Greater Kudu, which are all at least similar 
> in size to an adult lion. None of these animals is particularly unusual prey.

The adults are unusual prey


> There is enough footage demonstrating that they hunt and kill adult cape 
> buffalo, despite the risk. Tigers accomplish similar feats, in their case to 
> gaur.

Yes, there is enough footage to show that this occurs, but that doesn't mean 
it's common. And that's for big cats, which as already noted, are unusually 
well adapted to kill large prey. (Much more so than a theropod).

> 
>> The capacity for rapid neck motions noted by Snively et al. also fits what 
>> we would predict to see from a small prey specialist.
> Yes, but not exclusively. What does not fit what we would predict to see from 
> a small prey specialist though is that the skull is also tremendously strong, 
> and most small-prey specialists don’t have jaws that could easily be mistaken 
> to be adapted for brontophagy.

That depends on how we measure strength. Tyrannosaur skulls are truly strong in 
the way I think you're referring to. Other theropods have high bending strength 
in the dorsoventral direction because the skulls are tall. But they don't have 
a great number of particularly specific reinforcements to twisting or even 
compressive loads along the dorsal margin of the skull.

The jaws of small prey specialists can actually look a lot like those of 
related large prey capable taxa. Varanids are a good example, as are cats.

Actually, this raises an interesting question for discussion: what do we 
predict that a small prey adapted theropod skull should look like, and what 
should a large prey specialist skull look like? I suspect we have different 
predictions.

> 
> Now it is obvious we have different views regarding the relative macrophagous 
> capabilities of a large theropod, but that some were very well-adapted for 
> taking large prey is not exclusively my view, and would certainly provide a 
> consistent explanation for a fair number of trace fossils (numerous bite 
> marks, the paluxy track-site…) and pathological evidence of interactions 
> (thagomizer pathology, healed bite marks on ceratopsians…).

True. But bite marks are often interpreted as carrion feeding traces. The fact 
that predators received injuries supports the view that attacking adults was 
excessively risky. The small prey hypothesis is supported by the tooth size, 
jaw mechanics, and forelimb proportions in most theropods. It further matches 
the apparent life history structure of Mesozoic communities (extremely numerous 
vulnerable juveniles, very rare large adults, adaptations to rapid growth in 
juveniles, and likely sexual maturity well before maximum size).


> Another point, this _Dilophosaurus_ is obviously not the only known theropod 
> to have sustained serious trauma (think BHI 3033, FMNH PR 2081, SMA 0005, MOR 
> 693…). Another thing that would be hard to explain if all theropods were the 
> ecological equivalent of overgrown storks, but very easy to bring into accord 
> with predation on large and dangerous prey.

I disagree; living birds often injury each other  during intraspecific combat, 
for a start. It would also match what is seen in living systems, which is that 
young predators make mistakes and sustain injuries attempting to attack prey 
that is beyond their ability. Finally, those injuries could easily be sustained 
by a predator attacking prey say, 25-50% of its mass. That's actually within 
the range of "large prey", but not so large as to be unreasonable. I'm fine 
with a model in which theropods occasionally took prey half their own mass. But 
this "charge the equal mass armored monster with your face" model leaves me 
highly skeptical.

> With inter- and intraspecific competition for prey (as well as the difficulty 
> of a multi-ton predator to catch 50kg animals)

I'm not sure why that would be difficult. Large living predators don't have 
trouble catching things much smaller than themselves. Regardless, a multi-ton 
predator might prefer prey around 100-300 kg. That's still plenty small to be 
tractable.

> it would be strange if predators didn’t tend to evolve to fill every 
> available niche.

I know that's a popular sort of way to look at niches, but I challenge this 
view. Living taxa don't fill every ecology that we can come up with. True 
niches are not all available permutations. Dave Weishampel used to make some 
great arguments against the classic view of the niche as a set of conceived 
vacancies. I'll see if I can find some of his notes/papers that mention it.

> In the extant world there may be fewer juveniles, but they undoubtedly exist, 
> and yet there are a variety of predators for which killing large and 
> dangerous adults seems to be worthwhile.

That's a good point, but since juveniles are still preferred in the modern 
world, I'd consider that to be the dominant trend.

Cheers,

--Mike H