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Avian endothermy: birds do it differently from mammals
Important new work:
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.