Although endothermy is characteristic of birds and mammals, it is by no means restricted to these groups. For some time, physiologists have known of plants (!) that can regulate heat in a variety ofways, the most common being to decouple meta bolism (described in Box 12.1) from respiration, so that energy from the breakdown ofATP is simply released as heat. Several snakes are known to generate heat while brooding eggs, although this is accomplished by muscle exertion. Certain sharks and tunas can retain heat from their core muscles by counter-current circulation, and a variety of insects, including moths, beetles, dragonflies, and bees, are known to regulate their body temperatures. Endothermy is not characteristic of these groups of organisms. Simply, it is known that some ofthem do maintain temperatures warmer than those of the medium (air or water) in which they are living. Indeed, it has been estimated that maintaining a temperature against an external gradient has evolved independently at least 13 different times.
This, of course, differs from the idea that endothermy is diagnostic of a particular group. Indeed, endothermy is characteristic of but two groups: birds and mammals.
among living tetrapods, the only bipeds are endotherms.
Endotherms produce more energy than ecto-therms; it takes longer, during heavy use, for the muscles of endotherms to enter an anaerobic physiological state (characterized by lactic acid production). This means that, generally, endothermic tetrapods are capable of higher levels of activity sustained over longer periods of time than are ectothermic tetrapods (Figure 12.2).
A variety of small- to medium-sized bipedal dinosaurs such as dromeosaurids and ornithomimids are characterized by gracile bones, in which the thigh is short relative to the length of the calf. This, in turn, suggests high levels of sustained running - behavior certainly not characteristic of modern ectotherms.
And what of the larger dinosaurs, especially those that were not bipedal? Here the issue becomes murkier. The walking speeds of all tetrapods can be calculated from a combination of footprint spacing (stride length) and the length of the hindlimb (Box 12.3). But, of course, the walking that produced most trackways was generally not full-tilt running.
Could quadrupedal dinosaurs have run like the fastest mammals today? Ancestry gives a hint. In mammals, a fully erect posture evolved in a quadrupedal ancestor; however, in dinosaurs the fully erect posture evolved in a
endothermic line ectothermic line
Figure 12.2. Energy output versus time in ectotherms and endotherms. The curves show that the muscles of both endotherms and ectotherms achieve their maximum energy output virtually instantaneously. In general, however, an endotherm sustains maximum energy output for more than twice as long as an ectotherm does.
15 Time in minutes
biped. Quadrupedal dinosaurs are thought to have evolved their four-legged stance secondarily (see Chapters 5 and 6) and thus the front limbs of dinosaur quadrupeds look, and likely functioned, differently from those of mammals (see Figure 6.24).
Assorted adaptations for processing high volumes offood. Remembering that endotherms require more energy than ectotherms, if it could be shown that all dinosaurs required large amounts of food to function, an endothermic metabolism for Dinosauria might be implied.
We've seen that all genasaurs had skull design features such that food must have been processed in the mouth to a far greater extent than is found in living ectotherms such as snakes, lizards, crocodilians, and turtles.
Likewise, secondary palates, because they allow breathing and chewing to take place simultaneously, are commonly associated with more efficient feeding. Indeed, all mammals possess a secondary palate. Ankylosaurs and hadrosaurids both have well-developed secondary palates.
The relationship of these diverse specializations to metabolism is by no means clear. Hadrosaurids and ceratopsians clearly had developed chewing mechanisms at least as efficient as those found in modern herbivorous mammals. Birds, however, which by and large are endothermic homeotherms, do not chew and do not have secondary palates. Ankylosaurs, though possessed of a secondary palate, had very small teeth and little of the chewing morphology characteristic of ceratopsians and ornithopods (see Chapters 6 and 7). And, secondary palates are known in modern turtles and crocodiles, so their significance in terms of endothermy is not clear. Sophisticated feeding mechanisms do not provide an absolute guide to who is endothermic and who is not.
Hearts. All living endotherms possess four-chambered hearts. The four-chambered heart system, in which the oxygenated blood is completely separated from the deoxygenated blood, may be a prerequisite for endothermy. Endothermy requires relatively high blood pressures in order to constantly perfuse complex, delicate organs such as the brain with a constant supply of oxygenated blood. Such high blood pressures, however, would "blow out" the alveoli in the lungs. For this reason, mammals and birds separate their blood into two distinct circulatory systems: the blood for the lungs (pulmonary circuit) and the blood for the body (systemic circuit). The two separate circuits require a four-chambered heart - a pump that can completely separate the circuits.
Blood from a stone. Would such a heart be possible in dinosaurs? The nearest living relatives of dinosaurs, birds and crocodiles, possess four-chambered hearts; thus it is likely that a heart with a double-pumping system was present in basal Dinosauria.
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