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Tyrannosaur hunting strategies (was Re: How Did Hadrosaurs Survive?)



Warning! Warning! My sensors detect rampant speculation ahead!

Brett Booth wrote:

> But they appear to have more in common with modern birds and
> mammals than crocodilians. So they can be compared to those animals, we know
> of no reptiles that hunt in packs.

Crocs can hunt in groups, and even specialise their tasks. Smaller ones
chase mammalian prey into deeper water, while larger individuals wait
down stream to finish them off. They usually do this when herds of
herbivores cross rivers during annual migrations. I doubt it is true
"pack" hunting, but it does the job quite nicely. Even when scavenging,
they work together and specialise tasks. Large crocs clamp down on the
carcass, while others twist pieces off. They'll even work together to
chase lions off a kill. There's not complex communication involved. Just
a combination of instinct and experience.

> If you are going to postulate that rex
> was a pack hunter, it pays to look at modern pack hunters, since we are
> unable to look at extinct ones.

There are no modern predators that even come close to a Tyrannosaur
analogue. Thinking modern may not be the way to go, and may simply limit
our ability to postulate new theories (like this one).

> > Young juveniles (say, a year old) could have concealed themselves and
> > watched, or been used to scare up prey to determine which ones were
> > easiest to hunt. Animals that are several years old, but not full
> > adults, could have guided them, since they probably had several years
> > experience themselves.
> 
> And where do these young juveniles, just a year old, learn which animals
> will make good prey? 

That's the point. Their hunting skills would be crude, so they'd charge
about making lots of excited noise. The prey animals recognise them as
juvenile tyrannosaurs (just as crocs recognise juvie hippos and know
that adults can't be far away), and start to panic. In the panic it
becomes clear which animals are fit, fast and smart, and which ones lag
behind for whatever reason. Then the sub-adults take over, persuing prey
items accordingly.

> These are
> skills that are learned over YEARS of active hunting. Even young modern
> animals that have been allowed to join the hunt are still guided and taught
> by the older, more experienced animals. 

An older, more experienced tyrannosaur need not be a 12m full adult.
Predators have to learn fast. It's a jungle out there.  :)

> I seriously doubt that animals just several years old would have the
> experience to teach to what amounts to a baby how to take on such a huge
> responsibility. 

Cheetahs learn how to hunt within a couple of years, then go it alone.
As do leapards. Animals may hunt quite successfully before they reach
full sexual maturity.

> To have the entire pack depend on the young ones for
> sustenance makes absolutely no sense. 

Not really young animals. Just the "teenagers". And they may not
have relied on them, just taken full advantage of what abilities
their group members may have had. If sub-adults were faster
and had better endurance, then why not utilise the fact? Why do
anything if you're big enough to force others to do it for you? :)

Besides, alpha lion males, and elderly lionesses, hunt less often
in a pride. The large size and impressive mane of the male is
great for defence or intimidation, but they slow them down a
bit as predators. That doesn't mean that adult male lions can't
hunt at all. They can survive by themselves if need be. However,
when in a pride, it is the younger and smaller females that do
most of the active hunting stuff, since they're better at it.
The larger male concentrates on defensive behaviour - whether
it be a carcass, territory, or others in his pride. Any social
animals will take full advantage of the best attributes of
their members. Those that hunt best, hunt. Those that have
better defensive capabilities defend. Of course the defenders
can hunt when they have to, and the hunters defend, but if
there's someone else about who is better at a certain task,
then it makes sense to let them do it.

> Pack hunters also do not just 'scare
> up' prey to determine which ones could be readily killed; by the time they
> make a run at a herd, they have already picked out an animal and are testing
> their assumption. If their assumption is wrong, they fall back and regroup
> to try again.

That's one theory. Based on some modern animals. Besides, I've seen
dingos hurl themselves into a flock of magpie geese. The fit animals
react quickly and fly off. Sick or injured animals become apparent
immediately. I expect that arctic foxes do something similar, since the
geese can generally see them coming with not much cover about. 

> A male (or female) grows to adulthood and leaves it's pack
> to start it's own family. It meets up with a mate and they produce young. 

Outside of an established group, sub-adults may be able to breed. A one
year old wolf can breed, but doesn't get the chance if the alpha pair
are around. Send it out by itself and it gets the chance.

> If
> the adults are incapable of the 'flushing the prey' behavior, who teaches
> the babies? And how do the adults eat until the babies are old enough to
> participate in hunting? How do the adults even feed their babies in this
> case?

They pick off smaller prey, until their offspring are old enough to join
them. The complex pack-hunting of really large prey is worked up to
gradually. Arctic wolves raise young in pairs, hunting mostly rabbits.
Other wolves hunt in packs, and can bring down items larger than
themselves. Even lone wolves manage to survive. Being a predator means
being flexible.

Besides, in social animals (and we're assuming the tyrannosaurs were
social - a BIG assumption), it's not just the adults that help feed
young. Offspring from previous years also help out. Year-old wolves
will regurgitate for the pups. Magpie geese actually nest in threes -
the breeding pair, and one of their offspring from the previous
year. They all help in raising that years offspring.

> T.rex was as big as most of the food items it hunted. 

Most? How do we know what tyrannosaurs habitually hunted?
There is no rule that says a large predator needs to always hunt
large prey (wolves mostly eat things like mice).

> What would the advantage be in having such an extended
> family together? 

Complex pack-hunting of really large prey. Share the risk about.

> Why would you need anything more than a small family pack?

You wouldn't, necessaarily. A large group can do things that a smaller
group might not be able to, but the smaller group can still get by using
other hunting strategies. I'm not suggesting that tyrannosaurs had just
one strategy. A successful predator needs to be flexible.

> You're also assuming that they would produce young every year, so there
> would be an endless supply of juveniles to scatter prey animals. What
> happens if there's a bad year and the babies die? 

They don't have to breed every year. I'm guessing a tyrannosaur took
several years to grow into a 12m monster. In fact, young juveniles may
have learned more from their siblings that are two or three years older
than themselves than they would from last year's hatchlings.

> Who feeds the adults then,

Adults simply employ other hunting strategies, if required. Why not
scavenge for a while? 

> .. and who would the babies learn from if there were no sub-adults around? 

If tyrannosaurs had an alpha pair, and they took many years to reach a
size capable of challenging them for breeding rights, then they may have
remained sub-adults for several years. Amongst mandrils, males only
reach sexual maturity if there is no dominant male around. When the
alpha male dies, one of the younger males suddenly has a growth spurt,
becoming larger and developing his "war paint". It actually happens
quite rapidly. 

> What
> if there's a small litter or clutch or whatever you want to call it and the
> single surviving juvenile is killed or injured during hunting? Hunting is
> dangerous. What about disease? Young animals tend to be more vulnerable to
> disease than adults, what happens when disease sweeps through the pack and
> kills the young ones? Do the adults just roll over and die of starvation? 

No, because... yep, you guessed it - they can employ other food
procuring strategies. Ambush animals along a game trail. Steal from
other carnivores. Turn on each other, if need be. There's more than one
way to feed a tyrannosaur.

> It's a huge risk and makes the forming of new packs or
> family groups totally unlikely, for the adults to depend so heavily on the
> juveniles for hunting success. 

They wouldn't depend on them. Just utilise their greater speed and
agility. If it's not there to be utilised, then do something else. But
if they do happen to have a bunch of spritely sub-adults, why not
maximise their hunting strategy by assigning specialised tasks
(flushers, chasers, killers, defenders of the carcass).

> In that scenario 'd have to assume that rex
> was mainly a scavenger (which I totally disagree with,) 

Most modern carnivores ARE mostly scavengers. Why turn down a free meal
if you can get it?

> and the juveniles
> were used for hunting only when the adults were unable to find a carcass or
> steal one from another predator.

Or the other way around. They scavenged, or resorted to other food
procuring strategies, if they didn't have a group structure that allowed
for more complex hunting strategies.

> No, that's called special rules for T. rex, who was evidently so intelligent
> and such a good hunter that it was successful despite the welfare of the
> entire group being placed on the shoulders of the young'uns.

Adult male lions do less hunting than their offspring. Besides, maybe
that was why they layed lots of little eggs. Any given ecosystem can
only support a given number of carnivores, even fewer if they happen to
be large animals. If only a couple of animals survived from each clutch,
in a few years you'd have a sizable group. So what it a few get killed?
That's the life of a carnviore, and since they probably layed lots of
eggs at any given time, there would have been plenty of young'uns to
spare - probably too many, actually.

Of course what you have to keep in mind is that is is merely a theory.
I'm not suggesting that this is definitely what tyrannosaurs did, or
that they only used this strategy. They may not even have hunted in
"packs" at all. Hell - they may not even have hunted, period!

In fact, here's another theory: sub-adults chased prey into one of
several waiting adults (which are scattered about in strategic places).
The adult grabs the prey (in this case an Edmontosaurus) by the tail,
and uses its bulk to hold it steady (or as steady as a terrified
hadrosaur can be). The sub-adults arrive. They would have been too small
to subdue the prey themselves, but with it held securely by the tail by
a heavier adult, the more agile sub-adults can nip in and deliver quick
well-aimed bites to kill the animal. The larger adult is able to hold
the prey, but may have lacked sufficient manoeuverability to dispatch it
safely without being injured (and a badly injured tyrannosaur is in
serious trouble). By working together, and utilising the various skills
of different tyrannosaur size classes, the predators make a successful
kill. 

Of course, if the adult was unable to hold the prey long enough for the
sub-adults to arrive, then it may have escaped, bearing the scars of its
close call. Now if only we could find an Edmontosaur skeleton with a
healed bite mark on its tail... :)

-- 
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Dann Pigdon                   Australian Dinosaurs:
GIS / Archaeologist         http://www.geocities.com/dannsdinosaurs
Melbourne, Australia        http://www.alphalink.com.au/~dannj/
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