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Internal Testes and Endothermy [don't hold your breath]



Martin Human wrote:

<<So, (tongue firmly in cheek) can anyone say a) why
the need in mammals vs. any other endothermic
creature, or b) can we deduce birds are actually
ectothermic or c) did dinos sport a scrotum?>>

Dann Pigdon wrote:
 
<I suspect (and it's no real big gamble) that
archosaur and mammalian physiology is quite different.
Mammalian testes are at their most fertile when a few
degrees cooler than the body, and should they get too
warm (such as failing to drop from the body cavity) it
can lead to testicular cancer.>

[snipped]
 
<That said, some species of mammals seem to have
internal testes (the cetaceans for example). Perhaps
early on in archosaur evolution there was a need to
have them internally, to reduce the bulk or perhaps
the aero/aquadynamic characteristics of the body.>

  There is another explanation. Ectothermic,
poikilothermic animals, including extant non-avian
reptiles and so forth, have internal testes, but their
body core is at a lower mean temperature than mammals
or birds. The testes can operate at efficient fertile
levels quite easily, I could imagine. Let me not
speculate on this too far. However, a paper in either
_American Midland Naturalist_ or _Paleobiology_
[recent? latest issues? I forget, and didn't copy the
article] on the question of some mammalian
phylogenies, studied the internal testes of some
dolphins (spotted?) in regards to this very question,
and found a counter-current heat-exchange system that
cooled the organs to a few degrees less than the core
temp. This said, the condition in afrotherian mammals
(elephants, sirenians, hyraxes, and fossil
desmostylians) of internal testes is a key feature,
and may be further explained by this means but not, I
believe, tested. This was suggested as further
comparison in the paper, and when the library as ACofI
opens again, I can check. However, it requires the
animals be endothermic, and possibly homeothermic, to
support the counter-current system in this fashion.
Could be wrong.

  Core temp in small endothermic dinosaurs will be
lower than larger animals based on mass estimates
versus surface area (Henderson, 1999). They will not
require this, but larger endotherms will, and this
might suggest that hyraxes evolved from larger
afrotheres. However, larger dinosaurs, such as
sauropods exceeding a couple of tons, or the largest
theropods, could have lower mean temps than their
smaller kin (Farlow, 1986, etc.) and internal testes
would not need to be counter-current or external to
operate well, unless afrotheres operate by a different
structure or system.

  Birds, as I understand it, may not need a
counter-current, but could use heat-exchange through
the air sacs instead. I don't know, but if any
ornithologists out there know, please.... This could
be derived through descent, as both basal reptiles and
crocs have internal genitalia, so....

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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