METABOLISM

["Matthew Troutman" <m_troutman@hotmail.com>]

This post should clear up some things that have been discussed on the 
list during last week when I was gone.  


<<The link applies only to animals that spend the majority of their time 
walking on dry ground. >>

Even erect-legged and near erect-legged crocodylotarsians?

<<Energy cost of locomotion is pretty much the same regardless of limb 
posture and design, it is primarily a function of size.>>

Noted...

<<For most animals, energy cost of locomotion is pretty much a simple 
multiple of speed (humans and elephants are unusual in having a U-curve 
relationship).>>

Possibly because of the straight knee?

<<A few lizards do keep standing and walking many hours at a stretch.>>

Hummingbirds are also capable of flying for short bits of time while in 
a metabolic torpor (they can't fly like hummingbirds fly though).

<<Only animals with high aerobic scope (these days birds and mammals) 
can sustain walking speeds over 1-2km/h. This is true regardless of limb 
form or body mass. No reptile has been shown to do so (even the most 
aerobically capable monitors). The teiid lizards that stand and walk for 
many hours move at only a fraction a kilometer per hour.>>

varanids are capable of doing so (see below).  

<<Sprawling limbs are good for slow walking (below 2 km/h), because the 
provide a stable platform that will not allow the animal to tip over as 
it walks slowly. Sprawling legs are also suitable for high speeds.>>

Noted...

<<Long erect legs work under a strong pendulum effect. They are 
therefore ill suited for slow walking, and tend to force walking speeds 
to be above 3 km/h. You can try this ourself, walking slowly is not 
quite comfortable, we "feel better" walking at 5 km/h. Because walking 
so fast is "dynamic" in that tipping over is prevented by rapid foot 
placement, a narrow trackway is acceptable.>>

I agree wholeheartedly.

<<A sample of many hundreds of erect legged animals, including a few 
hundred dinosaurs, found that 95+% walked at speeds a of 2-3+ km/h.>>

Because of the erect legs of course.

<<Because long erect legs probably force land walking speeds to exceed 2 
km/h, and reptilian aerobiosis cannot sustain such high speeds, the 
evolution of erect legs probably forces aerobic scopes to be elevated 
above the reptilian level.>>

Though I used to believe that dinosaurs were endothermic I do not now 
because of the evidence that is at hand.  The erect leg correlation to 
dinosaur metabolics sounds strong, but its weakness is an 
underestimation of the reptilian metabolism.  What makes 1km/h so 
different from 2km/h?  Not much metabolically in energy used.  I agree 
with Farlow (1990 in the Dinosauria) in his points about walking speeds 
as metabolic indicators:
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Bakker argument is interesting, but not yet compelling.  His basic 
premise-that endotherms walk at faster speeds than ectotherms-is 
plausible, but competing hypotheses come to mind.  First of all Bakker 
implicitly assumes that all, or at least most, bouts of walking reflect 
foraging behavior, which may or may not be true.  Second, ectotherms may 
forage at the same speeds as endotherms of the same size but less often.  
If ectotherms indeed generally walk more slowly slowly than endotherms, 
this might be as much a function of their limb mechanics as their 
metabolic rates (although these variables may themselves be 
correlated)...  Activity levels and metabolic rate may be correlated in 
other ways, however.  In modern reptiles, the capacity for sustained 
power input at high activity levels is rather limited (Bennett 1982, 
1983; Coulson 1984), and several authors have championed the theory that 
a primary factor in the evolution of tachymetabolic endothermy in birds 
(and perhaps dinosaurs) and mammals was selection for greater aerobic 
endurance (Bennett and Ruben1979, 1986; Taigen 1983; Carrier 1987-but 
see Pough and Andrews 1984)...  Auffenberg (1981) reported that Komodo 
dragons (oras) generally walk and forage at speeds of about 4.8km/.  
Estimates of the walking speeds of medium-sized to large bipedal 
dinosaurs generally fall in the range of 5 to km/h (Bakker 1987; Farlow 
1987a); given the larger size and longer legs of most bipedal dinosaurs 
as compared with oras, these estimated walking speeds would not seem 
beyond the capacity of hypothetical ectothermic dinosaurs. 
     The maximal aerobic speeds of modern endotherms are considerably 
greater than those of living reptiles (see above references), and during 
sprints at top speed or other intense activities, modern reptiles are 
forced to rely on anaerobic power sources.  However, this does not 
necessarily preclude impressive burst-or sometimes rather extended 
bouts-of activity.  Garland (1984) reported that a 230 g  _Ctenosaura 
similis_  sprinted at 34.6km/h.  Large saltwater crocodiles vigorously 
resisting capture can struggle vigorously for a half-hour or more, 
developing high levels of blood lactate; most crocodiles recover at 
least partially from acid-base disturbance after two hours of rest 
(Bennett et al. 1985).  Webb and Gans (1982) reported that  _Crocodylus 
johnstoni_  gallop at speeds of as much as 17 km/h.  Komodo dragons will 
trot at speeds of as much as 8 to 10km/h and when frightened at 14 to 
18.5 km/h; one ora maintained a speed of about 14km/h over a distance of 
somewhat over half and kilometer (Auffenberg 1981).  
     Although the advantages of tachymetabolism for vigorous, sustained 
activity are clear, there seem to be other ways (even if less effective) 
of accommodating a respectable activity level.  Some varanids and other 
lizards have a relatively high factorial aerobic scope, even though 
their standard metabolic rates are no higher than those of other lizards 
(Taigen 1983; Bickler and Anderson 1986).  The activity levels of 
varanids are made possible by a sophisticated heart structure (Burggren 
1987), a high hematocrit, and a capacity for efficient oxygen transfer 
from lungs to blood to body tissues (Bennett 1973; cf. blue marlin 
[Dobson et al. 1986])...  the observations on large living reptiles 
suggest the possibility that ectothermic theropods could have moved at 
faster than maximal aerobic speeds for some time before having to stop 
and rest.  Although the adaptations fro vagility seen in theropods are 
consistent with the hypothesis that these animals had high aerobic 
capacities made possible by tachymetabolism, the known trackway evidence 
does not demand such an explanation.  
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So there!
  
<<Because dinosaurs had long erect legs, and because trackways show that 
they almost always walked faster than 3km/h, they should have had an 
aerobic capacity above that observed in reptiles.>>

But not varanids and oras.  

The increased activity levels of varanids because of their several 
physiological adaptations brings up some possibilities with dinosaurs.  
Would a ectothermic, bradymetabolic basal dinosaur with a four-chambered 
heart be capable of high walking speeds?  The case of the varanids makes 
this possible.  Would a ectothermic, bradymetabolic theropod with a 
four-chambered heart and a primitive airsac system be capable of high 
walking speeds?  The varanid evidence makes this possible.  Because an 
animal is ectothermic does NOT mean that it could have been aerobically 
active.  

<<Chameleons have erect legs. They evolved in order to allow stalking at 
extremely slow speeds along narrow branches. Because normal speed is 
slow rather than fast, the erect legs do not force an elevation in 
aerobic capacity.>>

1)  Chameleons do not "stalk".  Chameleons take a perch and stay still 
until their prey comes by.  Seldom do they stalk.
2)  It is true in this case that the erect posture was not evolved for 
high speeds.  

turbinates

Allow me to take this instance to bring up the turbinate evidence for 
dinosaur metabolisms.  Ruben et al. (1996) have extended this idea far 
beyond where it has been before.  "The presence or absence of nasal 
respiratory turbinates in fossilized tetrapods may be used to infer the 
metabolic status of long-extinct groups" (Ruben et al. 1996; 1205).  
"Respiratory turbinates (respiratory conchae) are epithelially lined, 
scroll-like, ossified or cartilaginous structures located in the 
anterior nasal passages of more than 99% of all extant birds and mammals 
>snip<; their presence increases the surface area of the nasal passage.  
During inhalation and exhalation, respiratory turbinates act as 
countercurrent heat exchangers.  By this process, they function to 
reduce the otherwise dramatically accelerated rates of respiratory heat 
and water loss that would accompany the high lung ventilation rates 
typical of endothermic taxa" (Ruben et al. 1205).  "The nasal passage in 
extant archosaurs and mammals consists of an anterior vestibular region, 
typically adjacent to the nostrils.  Immediately posterior to the 
vestibule is the nasal cavity proper (cavum nasi proprium); the boundary 
of the two is usually denoted by ostia through which various nasal gland 
ducts communicate with the nasal passage.  The nasal cavity proper is 
broadly subdivided into a main respiratory passageway and a "blind" 
posterodorsal or posterolateral olfactory region.  Posteriorly, the 
nasal passage is continuous with the nasopharyngeal duct.  In addition, 
a variety of pnematized cranial cavities (sinuses) communicate with 
portion of the nasal passage in many amniotes.  Cartilaginous or osseous 
conchae (or turbinates) lined with olfactory sensory epithelia are 
housed within the olfactory regions of the nasal passage.  Additionally, 
in birds and mammals, sheets of osseous or cartilaginous, often coiled, 
respiratory turbinates (the middle turbinates of birds and the 
maxilloturbinates of mammals) project into nasal cavity proper.  
Respiratory turbinates are oriented with their long axis parallel to the 
main path of airflow and are lined with well-vascularized respiratory 
epithelia.  Birds generally possess an additional, anterior set of 
respiratory turbinates located within the rostral vestibular region" 
(Ruben et al. 1996; 1205-06).  

The purpose of all this quoting is to make it clear what the functions 
of turbinates are and also to make sure that nobody misinterprets the 
facts.  

G. Paul (frequent posts on the list and an SVP abstract) has argued that 
in Apteryx and many seabirds that the nasal passage proper and the 
anterior vestibular region is narrow as in ectotherms and dinosaurs 
(further discussed in Ruben et al. 1996) and still contains middle 
turbinates and the anterior set.  It is also claimed that the region of 
the nasal cavity proper is large enough to house turbinates.  

The faults with this correlation is that it ignores the specializations 
of kiwis and seabirds.  Seabirds (most notably the tube-nosed seabirds 
of the Procellariiformes) have small nasal passages and tiny turbinates.  
However, the reason for this is not applicable to dinosaurs; tube-nosed 
seabirds are predominantly aquatic and use their specialized bills to 
capture food and reduce drag in seawater.  A large rostrum with 
typically sized turbinates and nasal passages would be disadvantageous 
because seawater would be more easily breathed in.  Frontal salt glands 
are present that allow the birds to drink seawater; this may be an 
adaptation to both the reduced turbinates and to get seawater.  Hence, 
no water is lost and the metabolism of these birds is not effected.  
Another point that should be made is that in Apteryx the turbinates in 
the nasal cavity proper are of relatively normal size, it is the rostal 
turbinates that are small.  
The nasal cavity proper in dinosaurs does not seem big enough to house 
turbinates.  This particular bit of wishful thinking ignores that the 
whole nasal passage proper in the dinosaurs studied in the Ruben paper 
was measured.  (Read the caption in Fig.3; "the relation of nasal 
passage (cavum nasi proprium) cross-section area to body (M) in modern 
endotherms (mammals and birds), modern reptiles (lizards and crocodiles, 
and three genera of Late Cretaceous dinosaurs").  In fact, a bird with a 
relatively narrow and tubelike bill was studied and was found to have a 
greater cross section than any dinosaurs; the great blue heron.  

Matt Troutman

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