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This is Chris Brochu's response to my previous letter. He inadvertently
sent it to me privately and asked me to forward it on to the list.
>If I understand it correctly, extant phylogenetic bracketing can be of
>little use here. First, behaviors (being more plastic) are more likely to
>show convergence than morphologies.
Not necessarily. There have been several papers in recent years on the use
of behavioral characters in phylogenetics - they aren't necessarily more
likely to converge (or, in the lingo of cladistics, they don't show
significantly higher levels of homoplasy) than do morphological or
molecular characters, at least for some groups.
The real problem is whether or not a behavioral trait is heritable, which
is one of the basic assumptions of phylogenetics (and the primary reason
stratigraphy and biogeography cannot be used as prima facie evidence in a
cladistic analysis). Some behaviors are inherited, and others are learned
- one can be included, the other can't. Distinguishing one from the other
is not always a simple matter.
With birds, some behaviors for some groups are demonstrably heritable and
stereotyped - but not all, and as the general intelligence of a bird
increases, the relative proportion of learned behavior to stereotyped
increases. This is why pigeons are more useful for that kind of work than
parrots or crows.
Second, the specific behavior (nest
>parasitism) is, I believe, more likely to be found only on the dinosaur
>branch after any shared ancestor of crocodiles.
Two problems with this conclusion:
1. It makes a frequency argument with very lopsided species diversity.
Sure, there's more parasitism among birds than among crocs - but we also
have 9000+ species of birds versus 23 living crocodylians, many of which
are not sympatric with another croc species and therefore cannot
parasitize. The percentage of bird species that parasitize is very, very
small. Perhaps more behavioral variation would arise if crocs were more
diverse - we'll never know.
2. At what node do you draw the line and say "It's more likely from here
on up."? As far as I know, no living paleognath parasitizes - one could
make a case that parasitism is restricted not to birds, but more
specifically to neognaths. Moreover, IIRC parasitic taxa are not found
among galliforms, loons, or waterfowl, the lineages thought to be toward
the root of Neognathae, and so parasitism may be more restricted still
One has to consider the two kinds of monophyletic group we recognize -
node-based and stem-based. It is safer to restrict certain attributes,
such as behavior, to a node-based group, since we can optimize the feature
to a most recent common ancestor. You appear to be approaching this issue
with stem-based groups in mind - you see some birds doing something, and
project it back in time. This isn't entirely invalid - it is, after all,
the reasoning that lies at the heart of uniformitarianism - but it makes
the conclusions more speculative, as you acknowledge below.
>Here are my reasons, hypothetical though they are.
>If bringing up baby is cheap you might as well do it yourself. However,
>if it is expensive, why not get someone else to do it and put your energy
>into additional reproductive effort! I think this might be the reason why
>you see it in birds but not crocs. Crocodiles do not manipulate
>microenvironmental optima as much as birds do.
Actually, some do it more than birds! Seriously.
In crocs, sex is determined by incubation temperature, not by chromosomes.
Above a certain temperature, the hatchlings will be one sex; below it, the
opposite. Female crocs of some species will actually manipulate the amount
of nesting material (either vegetation for a mound nest or soil for a pit
nest) to regulate the incubation temperature. At some level, they appear
to be regulating nesting temperature to maintain the appropriate sex
balance in the clutch.
As such bringing up croc
>baby requires less investment than bringing up bird baby.
This may be true for a single hatchling, only because female crocs lay many
more eggs in one nest than do birds.
>If there is a continuum between nest attendance in crocs, and nest
>attendance in birds--that is, between nest attendance which involves no
>environmental manipulation and total manipulation, non-avian dinosaurs
>would be more to the bird side. Evidence for this is scarce but
>includes: Likely rapid growth rates in dinos required greater access to
>surface oxygen (and subsequent protection from wider optima swings near
>the surface). Likely rapid growth rates of dinos in vegetation mounds
>needed ventilation ala megapode "taking the temperature" (heat may be the
>stimulus for that bird's behavior. But carbon dioxide concentration is
>directly related to heat in these nests). Some dino nests are known to be
>shallow (Troodon, Oviraptor, and Maiasura).
So are some croc nests - not all of them build mounds of vegetation.
>Also, if altriciality were more common in non-avian dinosaurs than crocs
>this would be a greater selective incentive to parasitize.
>An argument against dino parasitism is the logistics of even shallow
>burial, ie, it's easier to plop your egg into an open nest.
Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL 60605 USA
phone: 312-922-9410, ext. 469