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Brief thoughts on theropod predation



With the stunning paper by Farlow/Holtz soon-to-be
published on theropod predation, some thoughts.
It is observable that the formidable "scavengers"
griffon vultures can scan over 200 square miles a day
with telescopic vision, swooping down on dead or dying
animals with incredible speed. In the air, they also
release a scent, so that other griffon vultures can
follow to the food. Watching below, hyaenids -- when
necessary, loping for miles to where the vultures are.
Missing out on the prey quite often: lions. However,
unless there is a noticable tear in the flesh,
enabling them access to nutritious organs, the griffon
vultures's beaks are unable to puncture the skins
(which can often be an inch or more thick). Much study
remains, but these vultures can kill when necessary,
swallowing very small animals. The hyaenids, to be
sure, bone-crush and create for the theropods meals.
Another interesting behavioural fact. The "impact
tremors" which made for the quivering glass in 1993
probably did not happen with tyrannosaurs. An
acquaintance of mine, who has devoted several years to
studying Felis concolor, notes that he has witnessed
from a distance of 60 m one such carnivore walking on
dry oak leaves and fallen limbs, making no sound. In
fact, among most of these taxa, their rare
vocalizations are what frighten potential prey. The
function of the vocalizations during pursuit (and let
us substitute theropod for "mountain lion") may have
had four purposes: 1) vocalizing so that young could
locate the sound and follow; 2) vocalizations to
"excite" the offspring to learn hunting skills; 3)
vocalizing to flush prey from hiding; 4) vocalizations
at the beginning of the chase to startle/confuse the
prey ("freeze" behaviour), giving the cryptic hunter
needed moments to capture their meal as the prey
either tried to locate the approaching hunter or find
a flight avenue.
Those who have worked around tigers have documented
how they will grunt and lunge -- the sheer size of the
animals and the levels of sound frequency of the
vocals often cause the human to become incontinent of
urine and bowels.
Another thought. Much has been made of tyrannosaur
olfactory ability based on inferred anatomy, that they
were slow dolts barely able to eat. One is familiar
with the argument, its nonsensical parameters
dismissable on the basis of biomechanics. I personally
think  predation among the tyrannosaurs (and other
hunters) indeed consisted, in part, of olfactory
perception...not to smell a carcass, but to detect the
presence and social status of conspecifics (likely
"bachelor" males who would be keeping a distance from
an existing social unit).  As among extant hyaenids
and lions, cohesive social structure may have entailed
there being "bachelor" males hunting together but
apart from the familial unit ruled by a dominant male
(or female). And I have seen a male with his females
chase down and nearly kill a "bachelor" who challenged
the unit. (Months later, the "bachelor" became larger,
more skillful, fought off the first ruler, killed the
offspring, and became the new leader.) Extant
predators also use scent-marking for territories and
home-ranges (does the secretary"bird"?), and, as my
friend writes to me, this is really quite
sophisticated, not a haphazard spraying, and olfactory
perception (while not as acute as the turkey vulture
with its enormous olfactory bulbs, it is comparable)
is important because most predators have low
population densities.
Thus, it is highly probable that tyrannosaurs spaced
themselves in group patterns (the young in packs,
cornering potential prey so that larger parents could
kill, is one scenario favoured by Phil Currie), and
scent-marking would be detectable, giving information
as to reproductive status, territorial status, etc.
Moreover, there could have been, logically, a
relationship between body mass of these theropods and
the territory size to maintain a breeding population.
There is, among extant predators, a regression between
brain mass, body mass, home range sizes, so that one
can take a good guess at the minimum areas required to
sustain their metabolisms (habitat quality for
herbivores, similarly, could = meal quality for a
predator). A small theropod, thus, would use more
energy to hunt, breed, and eat than a larger taxon,
and would need to have a proportionately larger
territory to find meals.
Another point to consider. Basal metabolic rate
(BMR)of a predacious theropod would be closely linked
to brain mass (parental care systems and sensory
perceptions, e.g.) than body size, and the ecological
allometry of these theropods may have been more
important than body mass and population density
relative to prey number. In a 1995 paper (The
persistence of old designs for perception, in volume
11 of Perspectives in ethology, ed. N.S. Thompson),
R.G. Coss and R.O. Goldthwaite show how neuron-based
environmental perceptions are built in many animals
during ontogeny, persisting in populations even in the
absence of the necessity of having them. Among
theropods, then, brain mass would be constrained
spatially by time periods of conspecific interactions,
foraging or hunting,  engaging in predation or evading
predators. Population density and body mass would be
more variable, as a result of interacting with actual
areas of home ranges (and their social behaviours
derived from the environments). Hence, could it be
that theropod body mass was a result of responses of
environmental stresses rather than being causative in
theropod evolution? Smaller theropods would have
larger number of offspring than larger theropods.
Threshold home range areas of theropods, and their
population densities, would be related to female brain
mass (ecological allometry of body mass + BMR +
parental care systems + spatial scale + sensory
perceptions), as noted above, as well as the hourly
volume of oxygen consumed which would increase among
larger theropods having  larger territory sizes.


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