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Re: CLIMBING CLAWS
On Sun, 13 Dec 1998, Ronald Orenstein wrote:
> Bright colours can serve purposes other than sexual display, including
> establishing dominance relationships. According to the Handbook of the
> Birds of the World, "It is thought that this array of gaudy colours acts as
> a social signal in the dark rain forest; the colours of the bare parts can
> change with the bird's mood."
And it could serve as warning signal, i.e., a former attacker is more
likely to remember an unpleasant experience it had with the brightly
colored thing with the nasty claws.
> >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).
> I don't think anyone accepts that this difference translates into
> "competency". Snakes are remarkably "competent" at raiding bird nests -
> look at the effect the Brown Tree Snake has had on Guam, where it has
> eliminated almost the entire land avifauna in a few years.
The central question is whether the
difference in computational (if it exists--and it does at least in
respect of hearing) power is a result of a less-demanding niche,
i.e., marsupials don't need it, or as a result of constraints of their
reproductive system. It turns out their reproductive system must make
"choices" which don't favor bulking up on neurons. Now, of course
placentals don't have lots of extra neurons for nothing. The question
becomes: what are they for? This question is not answered yet. Regarding
your "competency" points. I agree that other species are extremely
competent. My point has to do _only_ with comparison.
> >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.
> This won't do. The diversity of flightless ostrich precursors in Asia, for
> example, was once pretty high. Certainly it is true that flightless ground
> birds are more common in areas lacking predators, such as islands, but I
> cannot believe this applies on continents. For example, both Australia and
> Africa have birds of prey that smash ratite eggs with rocks (the Egyptian
> Vulture and Black-breasted Buzzard, respectively).
Yes. The avian members of these predator guilds should be equivalent
because of their dispersal ability. But land-bound mammals may have a
differential effect based upon competency--should it exist. New Zealand
also has birds of prey and yet it enjoyed very high diversity of large,
flightless birds. I don't know if there could be a gradient in diversity
dependent upon the composition of specific guilds in a particular region.
Would such a survey be based on area (eg., how many big bird species in a
given continent per sq. hectare)?
Should it only compare biomes _outside_ of traditional low predator
density areas, eg., grasslands and wetlands. I suppose that leaves mainly
forests of different sorts?
I mean, what sort of evidence would convince you, since you are so
strongly convinced of the opposite?