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

Re: Sundry responses of Bois



More (sigh!) on eggs:

At 10:16 AM 29/05/00 -0400, John Bois wrote:
I disagree.  Anything that affects the reproductive success of another
animal must _a priori_ be considered a potential factor.

I'm afraid that this reminds me of W. S. Gilbert's observation in The Gondoliers that "When everyone is somebodee / Then no one's anybody". Saying that any factor that could possibly affect reproductive output must be considered as a potential cause of extinction says nothing whatever about whether one factor is likely to have been more important than another. You could make exactly the same argument for a massive outbreak of sunspots.


Yes. Varanids must also be considered. But my argument is that the guild
membership increased such that the predation load became overwhelming.

I know that is your argument. The problem is that you have failed to provide any evidence that it is true.


you know, most oviparous--especially large oviparous creatures--suffer
very high predation.

But they survive nevertheless. That is the point. If they were incapable of surviving the normally higher levels of nest predation that such animals face, they would never have evolved in the first place. The fact that they existed almost requires that they must have had some way of overcoming this difficulty.


We are talking about reproductive failure, right?  Or does the Alvarez
model posit a clean wiping out of all non-avian dinosaurs without
regard to their reproduction?  I mean, it is in these details that the
Alvarez model has no clothes.

Why? A catastrophic event does tend to cause extinctions without regard to the survival strategies that served animals perfectly well before the catastrophe took place. Thus, to give a specific example, the extinction of an undescribed species of megapode on the Kermadec Islands that resulted from a volcanic eruption blanketing the crater where the species occurred had nothing whatever to do with the reproductive strategies of that bird. The catastrophe eliminated its entire habitat.


So am I.  Yet, varying abilities in placental vs. marsupial body plans
remains a subject of modern research.  And a reasonable assumption is that
the two taxa are not equally gifted in this regard.

Once again, why? Both the placental and marsupial body plans have produced a very wide range of mammalian types, with extensive convergence. As others have pointed out, the North American opossum is a superlative egg predator. I see absolutely no reason to assume that marsupials and placentals, taken as a whole,differ in this area.


My hypothesis is that the phororhacids were protected by grasslands--in
the same way that ostriches are (for concealment);

As I pointed out in an earlier post, this is simply not true of ostriches across the whole of range. In many areas they nest in quite open situations.


Take a big Nile croc.  Situate its nest in the middle of the
Serengeti.  Do you think that nest would survive jackals, hyenas, lions?

Crocodiles do nest in the middle of the Serengeti, though admittedly along rivers. Do you think that jackals, hyenas, and lions are incapable of walking down to a river bank in search of a meal?


Large tortoises can exist only where there are no mammals
(Galapagos).

this is not true. The following is taken from Pritchard's Encyclopedia of Turtles:


"... in prehistoric times tortoises fully as large as those of Galapagos or Aldabra lived in many parts of the world. Geochelone crassiscutata, for example, was a four-foot long tortoise from the Pleistocene of Florida and Texas; Geochelone cubensis was a giant tortoise from the Pleistocene of Cuba; Geochelone ammon was a giant tortoise from Egypt; G. pespiniama another giant species from the south of France ; while the largest of all, G. sivalensis (or Colossochelys atlas) was a six-foot tortoise from the Pliocene af northern India, Even today, the distinction between island 'giant' tortoises and mainland `small' tortoises is not as clear-cut as many people believe. For example, some Galapagos tortoises are certainly huge; on Albemarle Island males may reach a straight carapace length of over fifty inches; on the other hand, on Hood Island even males do not grow longer than about thirty inches, while females are mature at about 21 inches. By comparison, the tortoises in adjacent Peru on the mainland of South America may on occasion reach a carapace length of about thirty inches, and in extreme cases reach a weight of well over 100 Ibs. (though on the mainland, interestingly, it is the female that is by far the larger). In Africa too the spurred tortoise (Geochelone sulcata) may exceed a carapace length of thirty inches. "

But let's have some alternate hypotheses
which explain the following: non-random distribution of large egg layers
in Oz.

First of all, I do not see that the distribution of these animals is any less random in Australia than anyplace else, especially if you compare areas with similar temperature and moisture regimes. But even if it is, it does not follow that this has anything to do with nesting behavior. It may relate to metabolic factors, food availability, or even vicariant events.


 Non-random distribution of very large egg layers on mammals-free
islands.

To some extent this may be an artifact of the spread of human beings. As noted above with respect to tortoises, many of these animals were more widely distributed a comparatively short geologic time ago.also, we should remember that Madagascar, which had both giant flightless birds and giant tortoises not that long ago, is hardly a mammal-free island.


  Non-random distribution of large egg layers in either grasslands
(emu, ostrich), or semi aquatic niche (crocs).

Once again, this may be an artifact of recent extinctions. The Madagascar elephant birds were apparently forest dwellers, for example, and there were certainly animals in Madagascar that could have acted as predators on eggs or newly hatched young.


  The lack of re-evolution
of non-avian dinosaur body form and the ability of mammals to fill this
vacant niche (this _must_ require a better hypothesis than "chance").

What, exactly, do you mean by non-avian dinosaur body form? I would argue that the ostrich, for example, does indeed represent the re-evolution of at least one such form, barring only the retention of highly modified wing and tail structures. Further, it seems worth mentioning that the phorusracids may have represented a re-evolution of the predatory dinosaur form, and that the last of these successfully invaded North America where it survived until the Pleistocene.
--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:ornstn@home.com