Re: Unidirectional gator breathing in Science

[don ohmes <d_ohmes@yahoo.com>]

--- On Sun, 1/17/10, David Marjanovic <david.marjanovic@gmx.at> wrote:

> If you exhale ( = deflate your airsacs so you can stay
> down) when you enter deep water, you can't inhale again when
> you are in. Either the water pressure won't let you, or
> you'll float up because your density shrinks -- avoiding
> this would have been the whole purpose of deflating the
> airsacs.

Given that 'dive time' is proportional to body size (for obvious reasons), and 
that birds apparently are pound-for-pound the best divers amongst the 
endotherms, it could well have been 20 or 30 minutes until another breath was 
needed. 

That said, you seem to be thinking about a mammalian system. Given  the 
constant flow avian-type system, it is obviously at least theoretically 
possible to shrink total air sac volume, yet continue to maintain a constant 
air flow.

Taking the temporary assumption that sauropods were so buoyant that they could 
not maintain their footing in hip-deep water, the sequence would be; "exhale" 
to shrink internal air volume, wade into deep water, then re-start the 
inspiration/expiration cycle (if necessary at an increased frequency), thus 
maintaining aeration w/out affecting buoyancy. 

Hence the question, "Is assuming that controlling buoyancy by voluntary 
deflation of air sac volume could not be done by sauropods a safe assumption?" 
Or re-stated, "Do any extant avians have the ability to significantly shrink 
air sac volume?"

There is of course, another and even simpler scenario: as sauropods waded into 
water, the increase in ambient pressure around the abdomen deflated portions of 
their air sac system, particularly the posterior air sac, decreasing buoyancy 
automatically. Logically, this would also move the center of buoyancy downward. 
I do not believe that Henderson addressed either of these factors in his "tipsy 
punter" paper, but am working from only memory. So corrections appreciated...

As an aside -- given that the avian system has 'spring-loaded' inspiration, 
driven by the elasticity of the ribs among other things, the theoretical 
threshold beyond which even minimal inspiration cannot occur is logically 
defined by that pressure at which _expiration_ cannot occur. In other words, 
the ribs are so compressed by ambient pressure that the muscles cannot compress 
them further. It seems likely that it would take significant pressure to 
compress a sauropod's abdomen to that point... 

> You're arguing sauropods stood (some of the time) in fairly
> deep water (hip-height or something) and breathed. Right?

As previously stated, I argue what sauropods were capable of, and what would 
have been advantageous to them, not what they actually did. Most of my 
scenarios involve swampy terrain and therefore mud or muddy bottoms overlain by 
water. But I do indeed argue that sauropods _could_ stand hip-deep in mud 
(and/or water) and breathe.