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Re: Subject: Bird hibernation/torpor
Ron Orenstein wrote:
> >> Their ability to excrete uric acid allows them to conserve water quite
> >> efficiently - but I would hardly call the desert an avian "refugium" but an
> >> area they have successfully invaded.
John Bois replied:
> >Who can adjudicate the distinction you have made? One would have to be
> >present at the moment of desert colonization.
Tom Holtz wrote:
> Although arid conditions may have been responsible for the origin of this
> adaptation, it was much more likely the arid conditions around the time of
> the Synapsid-Sauropsid split (ACCTRANS) or at least by the Anapsid-Diaspsid
> split (DELTRANS) than anywhere within the bird radiation itself!!
I now relpy:
But I don't understand how this addresses my point. In
birds and reptiles there is a continuum of resilience to this.
Some can take it, some cannot. Ostriches are extraordinary in
this respect. They expel urine as a crystal in mucus; they are
facultative water-drinkers; they have a long neck, sharp eyes,
razor reflexes for picking off scarce and sparsely distributed
prey items; they have feathers which they manipulate to cool
their skin--this allows them to stay shadeless and fast, i.e.,
they would be heavier with scale protection (is it possible that
feathers evolved to keep dinosaurs _cool_ rather than warm?).
This particular suite of adaptations, including their
reproductive tactics, predator defences, age/sex structure and so
on, is unique to ostriches. And they are all shaped by the
ostrich's niche. The question remains: are ostriches ostriches
because they, as Ron says, "successfully invaded" the desert, or
is the desert a refugium from predators? The answer: ostriches
are ostriches because they are reptiles, is true but, for this
discussion, too simple. How do we know, for example, which of
these adaptations might not be secondarily converged?
Tom Holtz says: "Given that all reptiles have (the capacity for
uricotely), the deserts are no safe refuge from reptilian
According to a thorough study by Brian C.R. Bertram, (_The
Ostrich Communal Nesting System_, (1994) 196 pgs.) ostriches
don't have much to fear from reptiles. Although they are
practically immune from adult predation, their nests and chicks
are targeted by birds and mammals. Egyptian vultures patrol from
the air and search for the white eggs (they are white to reflect
light when unshaded by the parent). Then they flick rocks at
eggs, returning day after day until complete destruction is
achieved. At night the mammals take over, small jackals nip the
ostriches off their nests and apparently bang eggs against each
other to break them open, and hyenas crack them with their teeth
and may even carry them away to their dens! These nests are
probably found by smell. Indeed, the desert is not prime ostrich
habitat. "Semi-arid, open, short grass plain is usually
associated with the highest ostrich densities." The mammals must
sniff them out because they are impossible to see--they have
stealthy nests! And this is afforded by grass: "Just finding
nests was the greatest problem of the whole study...Grass cover
conceals most of the bird." "Eggs in nests...(are)...invisible
to the terrestrial observer except within perhaps 10 meters."
Dinosaurs did not have even a grass analogue.
(By the way, phorusrhacoids declined when placental mammals
reached S.A. Is it possible that this was due to their greater
faculty of smell--did they have a better sense of smell than
Tom Holtz and Betty Cunningham both note that dry conditions did
support dinosaurs in the Gobi. I attack this argument on two
fronts: first, that they became extinct because they had nowhere
to hide, i.e., there was no grass; and, second, in its
productivity grass provides a kind of cline-extending buffer
between forest areas and desert, between humid and arid
environments. Ostriches have a marginal existence and it is due
to the productivity of the grasses in the margins of the desert!
I don't (now) question the drought-resistant abilities of dinosaurs.
And I am certain they exploited this to lay their eggs in desert
refugia just as ostriches do. For starters, (at least in Tsavo,
Bertram's study site) ostriches only nest in the dry season.
This gives them a margin over the vertebrates who would eat their
eggs but are not as gifted at tolerating the heat and dry. This
very complex interplay of factors is summarized by Bertram: "The
productivity of an area, and therefore the vertebrate biomass
which it can support, is clearly affected by the climate. In
more productive areas, the ostrich food supply would be expected
to be high, but so would the number of other vertebrate species
competing for the same food supply. Therefore, the relationship
between productivity and ostrich population density is not
straightforward. BUT A HIGH VERTEBRATE BIOMASS RESULTS IN MORE
VERTEBRATE PREDATORS, IN PARTICULAR (IN RELATION TO OSTRICH
NESTING) HYAENAS, JACKALS AND LIONS. IT IS LIKELY THAT AS THE
NUMBER OF PREDATORS INCREASES, THE CHANCES OF AN OSTRICH NEST
REMAINING UNDETECTED...REDUCES DRAMATICALLY" (my emphasis).
Now, we can't know how this played out in the Gobi, but one
thing is for sure, the productivity of the vegetation was
critical influence. And, inasmuch as the grasses provide a
hideaway and a food base for the ostrich, they also provide a
margin for preventing the extinction of this reproductive
anomaly--a large, egg-laying non-flying avian dinosaur. Maybe
similar creatures in the Gobi would have survived if they had