Avian endothermy: birds do it differently from mammals

[GUY LEAHY <xrciseguy@q.com>]

DMLers,
 
Important new work:
 
http://jeb.biologists.org/cgi/content/abstract/212/15/2328
 
This research suggests a couple of interesting items. The molecular
basis for endothermy in birds appears to differ from mammals, thus the 
selective pressure which drove endothermy in both groups may not have 
been the same.  The coupling of avian metabolic heat production with
oxidative metabolism would, at least in birds, support the "aerobic 
capacity" hypothesis of Bennett and Rubin (1979) in Science, where they 
proposed that selection for increased stamina resulted in a corresponding
increase in resting metabolic rates.  
 
This suggests all sorts of interesting possibilities:
 
1. Morphological evidence for elevated aerobic power (air sacs, erect posture) 
might precede evidence for heat conservation in archosaurs ("dinofuzz", 
feathers) in the fossil record.
 
2. Endothermy in pterosaurs was more likely to be similar to avian endothermy 
than mammalian endothermy, thus elevated aerobic power was probably 
characteristic of all pterosaurs.
 
3. Seemingly paradoxical evidence of elevated exercise metabolism in modern 
crocodilians (four-chambered heart, decoupling of locomotion from respiration) 
suggests instead the ectothermy characteristic of extant crocodilians is a 
physiological reversal from the ancestral condition.
 
4. Provides a possible explanation as to why structural adaptations for heat 
conservation in protomammals (turbinates) appear to occur earlier in the fossil 
record than locomotor adaptations suggestive of elevated aerobic power. This 
also suggests the absence of turbinates in dinosaurs may not be diagnostic of 
the absence of endothermy.
 
5. Provides a possible reason for the ecological rise of archosaurs over 
protomammals in the Triassic (elevated exercise metabolism). Throughout their 
history, protomammals exhibited a more conservative locomotor morphology 
relative to archosaurs. By contrast, multiple archosaur lineages (rauisuchians, 
crocodiliformes, ornithodirans) were sprouting long running limbs and erect 
posture all over the place).
 
6. Could explain the proliferation of coelurosaurs into a variety of 
non-predatory ecological roles in the Cretaceous, at the possible expense of 
some ornithischians and sauropods.
 
 
Guy Leahy