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Re: The Very Very Latest Paper From 2006!!!



Comments on David's comments on my reply to his reply to my post on his post 
interspersed below.

Don

----- Original Message ----

Don: > Restating-- Large animals have low surface mass ratios, and therefore 
lose 
> heat more slowly on a mass specific basis than smaller animals. It follows 
> that direct ambient temperature effects on growth are more pronounced in 
> small animals more than large, and LAGs would therefore be less defined in 
> larger animals, and would disappear entirely at some size.

David: If any temperature has an influence on bone growth, then it's the 
temperature of the bone itself. Small mammals work harder tthan large ones 
at maintaining their body temperature constant and high -- but despite the 
higher cost they still succeed. Therefore we don't have any reason to expect 
more LAGs in small than in large mammals.

Don:--------
Logically, part of the relatively high energy cost for smaller animals
of maintaining body temp is a relatively larger (negative) variation in
growth rate when it is cold, as growth requires energy, and the energy
available is finite. Therefore, LAG _definition_  increases as body
size decreases. I would expect the _rate_ of LAG formation to correlate
w/ the rate of temp variation, which presumably correlates w/ season,
therefore being equal to all body sizes in a given location, especially
within lifestyles. But in larger animals the LAGs may not be
well-defined enough to easily measure, and as body size (per
thermo-regulatory system) increases, eventually disappear entirely.

David: However, despite this expectation, the largest mammals apparently _have_ 
fewer LAGs than smaller ones. So this must have another reason; I guess the 
speed of growth.

Don:
-------- If the total temp variation required to create a measurable
LAG is larger for larger animals (as the thermal/energetic implications
of size demand), in a given time and location large animals will have
fewer (visible) LAGs than small animals.

> Due to the obvious seasonal effects on food intake of herbivorous animals,

David: Good idea, but doesn't explain why elephants have fewer LAGs.

Don:----
In my model, lack of (or fewer) LAGs in larger animals is explained by
the thermal/energetic implications of size (eg, surface/mass ratio,
relative cheapness of locomotion, increased resource utilization
capabilities). Cattle, etc, still have visible LAGs because they are
smaller, and have larger surface/mass ratios... elephants may have
reached a large enough size (ie, have small enough surface/mass ratios)
that they don't. In ectotherms, the bodysize at which LAGs become
poorly defined is obviously larger than in ectotherms, as their
metabolisms are more strongly affected by ambient temperature
variation. Therefore, IF the size at which dino LAGs become poorly
defined (ie unmeasurable) is larger than it is in mammals, this size
differential has relevance to the thermo-regulatory debate. Especially
if seasonal nutritional effects can be ruled out.

> Also-- To change the subject slightly, would endotherms show a 
> proximal/distal limb bone LAG definition gradient?

David: I'd expect that endotherms with long, slim legs living in a seasonal 
climate 
would show more LAGs in distal bones. That's because such legs have inbuilt 
heat exchangers: the distal parts, which contain tendons but more or less no 
muscles, approach ambient temperature, so I'd expect them to stop growing in 
winter, while the proximal parts don't even stick out from the body wall and 
approach body core temperature.

However, this expectation, too, is wrong AFAIK. 

Don:
------------ Maybe the distal/proximal gradient is there, but not
measurable. More likely, I think, is that the overall metabolic rate of
the animal controls bone deposition, not the "local" temp of a specific
body part. But I am now far enough off of my personal lilypad to where
I can't even see land, so... back to my real job.