[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


On Sat, 12 Dec 1998, Ronald Orenstein wrote:

> Any hypotheses for that?  I have one, of course.
> >It is the biggest bird to nest in a forest.  As such it depends upon
> >cryptic coloring. 
> There is nothing cryptic about a cassowary.  All three species have
> brightly-coloured facial and neck skin (in fact they are the most colourful
> of all the ratites) and I can tell you from personal experience that they
> stick out like a sore thumb.  Their behaviour may at times be cryptic, but
> their colouring certainly is not.

According to David Wescott who works with these birds, they can, in the
rain forest be amazingly cryptic even away from the nest.  Certainly their
coloring is quite "loud".  But I'm not sure you can say it does not, in
the rainforest, add to their behavioral crypticity.  Firstly, bright
colors are not bright in dull light.  Secondly, (I'm guessing) a greater
diversity in vegetational colors might give the cassowary a blending
effect.  Thirdly, since these birds are not particularly dimorphic, it is
doubtful that it is a sexually selected trait.  Then, what is it for if
not crypticity.

> >However, because forests support high predator density
> >such nests are prone to discovery.  In Australia this is not so
> >bad because a significant proportion of would-be nest predators can be
> >easily bested.  But "naughty birds" don't seem to do well elsewhere.
> Remember that we really have no idea what the conditions were,
> predator-wise, under which cassowaries evolved because of the extinction of
> many of Australia's predators, including Thylacoleo-type marsupials,
> terrestrial crocodiles, giant pythons and oversize monitor lizards.  And
> just because a bird is incapable of killing a human being does not mean
> that it cannot fight off the types of animals who do raid its nests.

These are good points.  However, this guild was probably not as competent
at locating forest nests as guilds which included eutherian predators.
All known marsupials have a smaller brain/body ratio than placentals
(except, I think, insectivores).
This no doubt had some bearing on perception and behavioral adaptivity and
size of repetoires.  Certainly auditory sensitivity was probably less in
all marsupials.  No extant marsupial can match a cat either in detection
of high sounds (such as that made by a prey walking through grass or
fluttering wings) or in threshold (the volume needed to trigger a response
in an animal).  And, although specific studies have not been done yet, it
is likely that such deficits (relative to eutherians) exist in other areas
as well (but probably not smell, which I grant is important).  It is now
known (ref. if needed) that neuronal development is
permanently delayed in marsupials by a rush to develope the motor and
skeletal equipment (probably for the trek to the pouch).  At the same time
marsupials are doing this, placentals are "focused" on laying down the
most time consuming, energetically expensive cells: neurons.  And then,
once the marsupial is in the pouch it cannot catch up because the pouch
is inferior to the womb in both nutrient supply and ability to provide
developmental optima.

I argue, as a valid hypothesis, anyway, that the slightly higher diversity
of large birds on marsupial continents relative to placental continents,
is due to this relative competency of guilds.