Re: egg meanderings while on cough medicine

[John Bois <jbois@umd5.umd.edu>]

On Thu, 23 May 1996, Tony Canning wrote:

>  However, phylogenetic analysis of reproductive strategy shows that
> live-bearing has probably NEVER evolved from a species which guards
> its eggs.  This is true even where egg-guarders and live bearers are
> close relatives. There are other explanations, but this suggests to
> me that live-bearing brings no increase in fitness over egg guarding
> in this diverse group.

I don't agree with this.  Strategies suitable for one kind of organism
are not for another.  It is unreasonable to compare, for example, the
relative fitness of flying for different organisms: for a bird it is
good but for a whale it is not.  In the same way, defending nests is
only adaptive for creatures who are capable of defence.  To say a
garter snake would increase its fitness if it defended its nest is
silly.  For the snake suffering alot of egg predation perhaps live
bearing is the _most_ adaptive strategy.

Many complex strategies have evolved, each presumably adaptive for its
organism.  Many birds hide their nest, lay plenty of eggs, abandon the
nest when in doubt, and FLY away to lay another day.  This is neither
guarding, in the sense that they don't make a stand, nor live bearing.
Indeed, live bearing must be selected against in birds because, among
other things, of the added weight in flying.  I know bats carry
foetuses in flight.  Perhaps this is one of the things which limits
their aerial agility _and_ their realized niche to the evening hours!

How does this relate to dinos and mammals?  Dinos were already
specialized nest defenders.  Big, strong, ferocious, they were
completely invested in the strategy of killing or scaring away things
which tried to eat their eggs.  Once invested in this sort of
strategy, moving to live bearing might have been impossible.  I'm not
sure about this but I claim that it is impossible to become an instant
live-bearer if you are big. I'm speculating that surface area to
volume ratios are prohibitive in a big foetus which has no specialized
blood supply (i.e., like a mammal's).  If we look at sharks, we see
three possible modes of reproduction in different species: oviparous
(where development takes place outside the body), ovoviparous (an
intermediate type in which development takes place internally but
without a placenta), and viviparous (in which the embryo gets direct
parental nourishment through an elaborate sytem of blood vessels).  It
is likely that viviparity developed in species which posses it in this
order: ovi->ovovivi->vivi.  In other words, for a large dinosaur to
become an instant live bearer may have been impossible whether or not
it was adaptive!  Attacked from all sides by mammals, birds,
crocodiles, snakes, other dinos, and varanid lizards, dinosaurs were
stuck with an extinction-driving problem: while they could resist
traditional onslaughts of big egg predators, new small agents were
nickle and diming them in a way that they could not respond to. And
they could not resort to small "resevoir" species, I mean species in
which new radical mutations were not lethal (such as instant
live-bearing), because they were all relatively big!
        
        Mammals, on the other hand, developed what are perhaps the
most stunning examples of evolutionary products, placental and
marsupial reproduction.  Developed in small creatures and refined in
the crucible of intense predation, these adaptations could
subsequently give the advantages of live bearing to big animals.
 
> ...maybe live-bearing never evolved [in dinosaurs] because it would
> have brought no evolutionary advantage, ie. fitness might have
> decreased.

Then let's compare the strategies of an open-field egg layer
(dinosaurs) with those of a modern day open-field animal (i.e., big,
live-bearing mammals).

1. Dinosaurian nest defenders must return to the nest every night to
provision the guard.  Mammals do not have to do this.  This may be
important in maintaining a big secure herd.  Mammals can range all
day, sleep, and move on the next day to ever-fresh patches.
Dinosaurs, having to start from the same place every day (at least in
breeding season), must not be able to support as big (secure!!!) herds
because they tend to over-graze the range around the nest site.  In
this regard, any environmental stress would selectively target the
dinosaurs!

2. Guarding a nest is less secure than carrying babies within an
organism.  This is true for at least three reasons, a) A parent
guarding a nets has two choices: stay and fight, risking injury and
loss of nest, ; or abandon the nest to the predator.  On the other
hand, an open-field live-bearer is just not a target for an offspring
predator.  If they themselves are attacked they simply run away with
their selves and their babies intactb) Egg guards must risk injury
from intraspecific competition for nest sites.  I have a video
(National Geog. _Amphibians and Reptiles_) of 2 Galapagos marine
iguanas fighting over a nest site while a sea bird eats the defender's
eggs!  Live bearers in the open-field avoid these risks altogether.
c) Time from laying to viability is _much_ longer than time from
bearing to viabiltity.  This gives live bearers the luxury of
developing much more complex behavioral patterns.  It also gives
offspring predators _much less_ time to find the babies!!!, d)
Agressive defence _and_ constant provisioning visits to the nest
announces to the would-be predators the location of their next meal.
 
        All of these factors must add up, in the open-field at least,
to a big payoff in relative fitness for live bearers.  Perhaps this is
why practically the only vetebrates in this habitat are live bearers,
or stealthy egg layers.  Once upon a time there were _only_ nest
guarding species.  Is this just a monstrous coincidence _or_ does it
tell us something about the relative success of these alternate
strategies?

If it does, perhaps dinosaur extinction is the _fait accompli_ of the
development of a better strategy--live bearing!