Bigger dinosaurs had warmer blood.

[Allan Edels <edels@msn.com>]

> From Yahoo:  http://news.bbc.co.uk/2/hi/science/nature/5166518.stm

Bigger dinosaurs had warmer blood.

The bigger a dinosaur was, the warmer its blood, a study of the big beasts' 
fossil remains shows.
Dinosaurs were long considered to be cold-blooded reptiles.
More recently, some researchers have proposed that the extinct creatures 
actively regulated their body temperature like mammals.
A study in the journal Plos Biology now suggests this is not the case, but 
that bigger dinosaurs may have lost heat so slowly that they stayed warm 
anyway.
Reptiles tend to be cold-blooded ectotherms, whose internal body temperature 
is dependent on the outside environment. For example, lizards and snakes 
will sun themselves on rocks in order to heat themselves up.
Birds and mammals, on the other hand, tend to be warm-blooded endotherms. 
They can regulate their internal body temperature regardless of external 
influence.
Their body temperatures tend to be more constant than those of reptiles and 
higher than the outside environment.
Growth rings
James Gillooly of the University of Florida in Gainesville, US, and 
colleagues started with an equation showing the relationship between body 
size, body temperature and growth rate of an animal.
They then applied this equation, which has been shown to be valid across a 
variety of living species, to dinosaurs.
Gillooly's team used measurements of annual growth rings in the bones of 
eight dinosaur species to estimate the animals' development rates and body 
size at full adulthood. This information in turn can be used to calculate 
the dinosaur body temperature if the equation is re-arranged.
The scientists found the smallest dinosaurs had temperatures of around 25C, 
close to environmental temperatures and similar to those observed for living 
reptiles. In other words, they did not actively regulate their internal 
temperature like mammals and birds.
But as dinosaurs got bigger, they became less efficient at dissipating heat 
and this helped to keep them warm anyway. This is known as inertial 
homeothermy.
According to the scientists' equation, the enormous sauropod Apatosaurus - 
which at 13,000kg was among one of the biggest dinosaurs - had a body 
temperature of just over 40C.
Huge variety
Most animals cannot tolerate body temperatures of above 45C, so Apatosaurus 
is both near the upper limit of dinosaur body size and the more general 
limits for body temperature. This, the authors say, could suggest that the 
maximum size a dinosaur could grow to was limited by body temperature.
"The question of whether dinosaurs were warm-blooded or cold-blooded just 
doesn't have a simple answer," said Dr Angela Milner, associate keeper of 
palaeontology at London's Natural History Museum.
"There's a huge spread of physiological states, from things that were more 
at the ectothermic end and had no problem keeping warm because they were so 
large, right through to small meat-eating dinosaurs that were not far short 
of the endothermic biology seen in birds."
Dr Milner, who is not associated with the authors, pointed out that a group 
of dinosaurs known as the hadrosaurs apparently switched from being 
endotherms like mammals and birds in youth to being inertial homeotherms 
when they were adults.

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Allan Edels