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Re: Segregated vs age-mixed sauropod herds
On Mar 19, 2010, at 2:25 PM, John Bois wrote:
...but isn't it fair to say milk is an amazing formula for rapid
neonate development, containing antibodies, etc., etc., and that this
contributes to the reproductive success of the lactator...that it has
adaptive value which is probably why it is highly conserved.
Sure, but it's also fair to say that limiting parental investment and
producing large numbers of offspring (or, in some cases, even limiting
parental investment and producing a small number of offspring at a
time) also contributes to reproductive success in many taxa, and that
this may be why such strategies are highly conserved in many
lineages. It's also worth noting that life history traits need not be
conserved through selection - they can also be conserved through
constraint. Perhaps too many systems of placental mammals now rely on
the specific hormonal cascades involved in their reproductive
physiology for any viable variants to arise (just an example, not
really a favorite hypothesis).
The pattern above seems like a progression on an evolutionary scale.
Is there one that goes in the opposite direction?
Yes, there are many that go in the "opposite direction" - that is,
reduction in parental care. Life history traits are plastic in both
directions. I'm also not sure the parental investment you listed is
actually as unidirectional as you think. In some rodents, for
example, there is less parental investment per offspring than in their
ancestors (or, at least, the ancestral state based phlyogenetic
bracketing). Trends in both directions show up in carnivores, too, if
I recall correctly.
Presumably, it would be possible to give up lactation if, in some
environment, it paid to
do it more cheaply.
It might not be, though - see note above about constraint.
I mean, couldn't it be possible that a mammal
would find greater reproductive success in some other environment by
delivering and leaving the babies to fend for themselves?
Some very nearly do. Furthermore, many organisms that do exactly this
compete just fine with mammals. In some environments, the low-
investment taxa would appear to have an advantage (see inner Australia).
And yet this hasn't happened, right? This suggests (to me) that
there is something
intrinsically valuable in bringing baby up to a more developed form
before pitching it onto the world stage. Is this just my mammal bias?
Probably a mammal bias. Or, to put it another way: it seems that *for
mammals* there is often (but not always) an advantage to investing
heavily in each offspring and leaving them to fend for themselves at a
more developed state.
Same thing with birds...behavior now. From stem reptiles to
theropods...comparing lizards and turtles to today's theropods...do
you believe extinct relatives of lizards and turtles fussed around
their young, built elaborate nests, provisioned the young with as many
varied and _complex_ behaviors as birds do today?
No idea - but presuming that they did not, I'm not sure I see the
point. Lizards and turtles are crown groups, just like birds. Going
from stem reptiles into subsequent nodes, we do not find a trend
towards complex care across the board. Instead, we find that a few
lineages develop these traits (and, in some cases, secondarily lose
them), while other lineages never (or rarely) express apomorphies
related to complex care. In other words, there is no single trend
from low care to high care.
...and no backtracking here as well (OK...a megapode species that
volcanic soil...but most megapodes are like fastidious bakers,
constantly testing the nest environment).
Also nest parasites, and a number of precocial birds (after all, they
don't have to "backtrack" all the way to superprecocial to count -
there are lots of semi-precocial taxa out there, and some nest near
altricial species in modern cladograms).
And incubation...it's expensive in terms of energy use and predation
risk...yet birds don't
give it up...certainly one reason for this is that they have already
invested so much in terms of yolk volume (another trend)...One would
think if these traits weren't valuable you would see some
hedging...some reversion to the primitive (in the cladistic sense)
And there is - see above. Also note that, again, there may be
constraints that prevent "hedging" - it need not be purely adaptational.
Large yolk volume, incubation, and lactation are, judging by their
almost universal retention, highly valuable traits.
Yes, but by this same logic, low yolk volume, large clutch size, and
environmental incubation are also valuable traits, judging by their
almost universal retention in other clades.
Overall, you've identified that parental care is common in crown
birds, crocodilians, and mammals. No arguments there, but I don't
"trend" you're arguing for.
I'm arguing that the crown groups are the (current) end of that
Lizards, anurans, etc. are crown groups, too.
Assistant Professor of Biology
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