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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.>>
<<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
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.>>
<<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:
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.
<<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
<<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
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
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
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.
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