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RE: Nocturnal crocs?
>I meant "aggressiveness" interms of a winning strategy. Clearly a mouse
is better practicing concealment, and escape rather than aggressiveness.
But, if it is in the jaws >of the cat, it has only one option. The real
question is: what were/are the effective strategies of a cassowary? And
what were the animals--ghost species, perhaps
>which selected for its unique behavioral repetoire? I am sure no one
would claim they were identical to ostrich selective forces. General
wisdom has it that predatory >forces are harsher on large continents,
relative to small ones (Ed Wilson and Tim Flannery). Is there any truth
to this? If so, this gives the cassowary a broader range >of
options--rather than compete with the emu, it could enter the forest
niche, provided, perhaps, that it could run fast, hide well, and kick
like a mule to defend against >a critical percentage of predators. This
is not an unreasonable speculation, I think.
Cassowaries are fairly quick on their feet but definitely not fast
runners like ostriches, rheas or even emus. However according to the
literature (my own encounters with cassowaries in the field have
fortunately been amicable) they have a techinque of delivering their
kick/stab on the run which is apparently quite effective. They can
certainly kill dogs so I suppose they might well have been able to deter
Thylacines, Marsupial Lions and Megalania.
Generally speaking prospective prey will often be "aggressive" when
flight is not practicable. This is often successful since predators
will, generally speaking, avoid attacking any prey which is even
moderately dangerous. There is a basic asymmetry here since the prey
risks its life while the predator only risks missing a meal and is
therefore probably unwilling to take more than a small risk.
However we must make a distinction between defending Your life and Your
nest. In the latter case desperate measures would normally not be
selected for, at least not in long-lived species that can lay many
clutches during their lifetimes.
Generally speaking it is probably true that predation-pressure is
greater on the "mainland continents" than in Australia. However when it
comes to nest-predation differences between habitats are probably more
important. Rain-forest is probably worst of all in this respect.
Rainforest birds go to extreme lengths to conceal or protect their nests
but still suffer very heavy nest-predation (there is a good chapter on
this in D. W. Snow's "The Web of Adaption" which I strongly recommend).
Unfortunately nest protection/defence has not been systematically
studied but extant birds use a large variety of strategies for this:
Concealment, in an infinite variety of ways
Nesting in inaccessible places (also with many varieties)
Colonial nesting (a large number of birds can often "mob" a predator
badly enough to drive it away, and a single predator will probably only
plunder a small fraction of the nests)
Nest parasitism (leaving the chore of protecting the eggs and young to
Nesting unpredictably in time and space (this is not usually considered
as a form of nest defence, but it must certainly make life harder for
Nesting close to somebody dangerous (preferably more dangerous to nest
predators than to Yourself). This is a quite common strategy, for
example many wildfowl prefer to nest in or near gull colonies even
though the gulls will take eggs and young because of the protection the
colonies give against other predators. The neatest example I can
remember was a Wagtail that actually built it's nest _in_ the nest of a
pair of eagles.
Direct physical attack on predators (not that frequent but certainly
used by a number of birds (e. g. some owls, raptors, gulls, terns,
Decoying predators (the classic "broken-wing" dodge, with variations)
Of course not all strategies used by birds would have been available to
non-volant dinosaurs (nesting in trees, inaccessible cliff-ledges or
small islands for example, though most of these should have worked
quite well for pterosaurs) but many would have been quite practical. For
example burying Your clutch close to a Tyrannosaur nest might be a good
idea. Nesting in burrows would be quite practicable for small dinosaurs.
Colonialism definitely did occur. Extand crocodiles use both concealment
and/or direct defence of the nest. It should also be noted that Marine
Turtles have used the simplest strategy of all (bury them at night and
forget them) quite successfully for 65 million years, even on
continental beaches and using traditional sites year after year.
Also if nest predation became really bad one might expect that some
dinosaurs would have become viviparous. Ichtyosaurs definitely were
viviparous and many extant snakes are ovoviviparous (the eggs hatch as
they are laid). Admittedly no birds have ever been viviparous as far as
known (though it has been suggested that Hesperornis may have been). The
reasons for this is obscure, but the most plausible suggestion is that
their highly specialized breathing system makes the abrupt changeover
from a fluid-filled to a gas-filled system at birth impractical.
Also it seems to me that at least large dinosaurs would be able to
tolerate a heavier degree of egg/young predation than just about any
other tetrapods. They were almost certainly quite long-lived so they
would have had time to lay many clutches. Also their eggs were small
relative to their body size so clutches could be large without putting
too much strain on their physiologies.