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OUR FRIEND, THE KIWI



Of course all birds are theropod dinosaurs phylogenetically speaking, but
some birds are more theropod-like than others. These include predatory birds
that hunt entirely on the ground, such as the roadrunner, and in Africa the
secretary bird and the big headed ground hornbill. Then there are the big
flightless ratites, so similar in form to the ostrich-mimic dinosaurs. But
for living dinosaur of the week, I nominate the little earth-bound ratite,
the kiwi - of which there are a few species ranging in mass from 1.5 to 3 kg.

Of course wingless kiwis have a theropod-like locomotary apparatus,
suggesting that their locomotary energetics were similar. Kiwis have fur-like
feathers similar to those reported on the Chinese theropod, suggesting a
similar need to retain internally generated body heat. 

Kiwis also grow slowly, it takes them 18 months to go from a big, 400 g
hatchling to a 2-3 kg adult. The growth rate is much slower than in any other
bird, and is somewhat faster than reptiles of the same size. There is
evidence that some theropods may have had a similarly intermediate growth
rate. 

Less appreciated is that kiwis have a respiratory apparatus that appears to
be similar in many regards to that of theropods. The nose consists of a long
and EXTREMELY slender, tube-like anterior nasal passage (containing a long
simple conchae) and a short posterior nasal passage (containing more complex
conchae) that is narrower than those of other birds, but is broadly similar
to the condition observed in theropods. Also as in theropods, pneumatic bones
are associated with air-sacs, but the sternal plate is so short and set so
far forward that it cannot strongly ventilate the rather poorly developed
posterior trunk air-sacs (in particular, the abdominal air-sacs so large in
most bird are rudimentary in this and other ratites [as shown by good old
Thomas Darwin's Bulldog Huxley back in 1882, Proc. Zool. Soc. London
64:560]). A similar condition probably existed in theropods. 

Despite their narrow nostrils and not so well developed air-sac system, kiwis
are true homeothermic endotherms with resting metabolic rates far above those
of reptiles, and only a little below those of other ratites and some raptors.
As far as I know no one has measured their aerobic exercise capacity, but it
is far above the reptile level. A few years ago W. Calder in Scientific
American proposed that the kiwis be made an honorary mammal (because it is
"furry", nocturnal, has poor vision and excellent olfaction and hearing, and
a lower metabolism than other birds), but he missed the true archosaurian
nature of these worm eating avian dinosaurs. Kiwis appear to be a much better
model for theropod energetics and growth than any reptile. 

If kiwis with short sterna can have elevated respiratory capacity, then
pterosaurs with larger sterna and pneumatic bones could have had better
developed air-sac driven lung ventilation. Combined with a large heart and
high oxygen capacity blood (as in bats), the aerobic flight capacity of
pterosaurs should have matched that of flying birds and bats. This is
contrary to the view that pterosaur energetics were reptilian in nature (J.
Ruben, NY SocVertPaleo meeting). 

GSPaul