The thermal physiology of the dinosaurs and of other large reptiles has been the subject of animated discussion, not least because it is relevant to their extinction at the end of the Cretaceous period (Chap. 12). Temperature regulation in animals is either behavioural,autonomic (self-governing) or,more usually, a combination of the two. Like most day-active animals, Mesozoic reptiles would have basked in the sun when they were cold and sought the shade if they were too hot. Nocturnal species, on the other hand, would have taken shelter during the daylight hours and emerged at dusk when the air was cooler. The Nile crocodile (Crocodylus niloticus), for example, maintains a relatively constant body temperature by spending the night in water, sun basking in the morning and evening, and retreating into the water or shade during the heat of the day (Cott 1961).
The rate of metabolism and, therefore, of heat production increases with body size in most of the larger homeothermic animals, but not in proportion to it. When the logarithm of metabolic rate (MR) is plotted against the logarithm of body weight (BW), however, a straight line is obtained. The slope of this line indicates that MR is a function of BW2/3. Since the surface area of a sphere varies with mass2/3, as explained above (Sect. 7.4), this relation is an indication of the influence of surface area on MR. The maintenance of a body temperature above that of the environment and at more or less the same level irrespective of size requires that the rate of heat production per unit mass of tissue must be proportional to the rate of heat loss. There is no way of estimating the metabolic rates of Mesozoic reptiles but, if the larger species had metabolic rates as high as those of extant birds and mammals, their surface areas might well have been insufficient to allow for adequate cooling of their bodies. Using tortoises (Testudo graeca) as model dinosaurs, David Butt and the present author made simultaneous measurements of metabolism and transpiration at different ambient temperatures. Up to about 24 °C the metabolic heat gained and heat lost by evaporation kept approximately in pace with one another. At temperatures above this, however, there was an increasing excess of heat production over heat loss (Cloudsley-Thompson 1978,2001). It seems inconceivable that the metabolic rates of dinosaurs should not have been at least as high as those of a slow, solid tortoise!
The main points at issue are whether the large Mesozoic reptiles, in particular the dinosaurs, maintained high and more or less constant body temperatures endothermically, as modern mammals and birds do, or whether they were heterothermal - with variable body temperatures - and had lower metabolic rates like those of extant reptiles.
Small extant reptiles lack effective insulation, but exercise control over their body temperatures through a combination of behavioural and physiological body mechanisms. They are ectothermal - most of their body heat is obtained from environmental sources, and comparatively little is generated by metabolism. They are also poikilothermic and their body temperatures are not controlled precisely like those of homeothermic endotherms, most of whose body heat is generated metabolically. Thermoregulation by behavioural means is an excellent strategy for small animals that have a large surface-to-volume ratio, but it is a mechanism far less readily applicable to large animals, as we shall see.
The most important source of environmental heat is solar radiation, and most day-active reptiles today divide their time between basking in the sun and cooling themselves in the shade. Such behaviour would be far less practicable for a large animal such as a dinosaur, and there are additional complica tions as well. Colbert et al. (1946) found experimentally that, under similar conditions, whereas a very small alligator (weighing about 50 g) needed 7.5 min to warm from 27 to 32 °C, a large alligator (weighing about 13,000 g) required 30 min to heat up from 28 to 32 °C. In other words, the small alligator heated at a rate of 1 °Cper 1.5 min, the larger at a rate of 1 °Cper 7.5 min. When these results were extrapolated, it was found that a reptile the size of a medium-sized dinosaur weighing about 9,000,000 g (9 tonnes) could be expected to warm up at a rate of only1 °C per 9.6 h (5,760 min)!
Of course, this medium-sized dinosaur, having a small surface-to-volume ratio, would retain its heat overnight. Seasonal changes would, however, present a problem, and it might experience considerable difficulty in finding a cool retreat if it got too hot. Indeed, it would be dangerous for its body temperature to approach the critical maximum because of the amount of time needed for it to cool down. Again, the juvenile dinosaurs might also have been susceptible to unusually high temperature over short periods which would not have affected the adults adversely. Colbert et al. (1946) argued that upland dinosaurs might have reacted differently from swamp-living forms, from which they differed in size and shape. They would have spent much of the time in shade, whereas amphibious forms would have been able to seek relief from the sun's heat, as crocodiles do. Bipedal forms may have exercised some thermal control by facing the sun except during the early hours of the morning and in late afternoon. Alligators in a bipedal posture were found to show the slowest rate of rise in temperature when exposed to the sun: alligators flat on the ground showed the highest rate of rise, while animals held in a quadrupedal pose heated up at a rate intermediate between that found in the bipedal posture and that experienced when flat on the ground.
It is conceivable that the tremendous size of many of the archosaurs, as well as the prevailing bipedal posture, were the direct results of thermal limitation. The erect posture presents a smaller surface to insolation and removes the body from contact with the ground, while the great volume requires a longer period to heat as well as a longer period to cool off during the night (Cowles 1940). In fact, as shown by the calculations of Spotila et al. (1973), changes in environmental temperature were probably damped out over periods of several days. Axelrod and Bailey (1968) believed that the mean annual temperature in equatorial regions at the end of the Cretaceous period was probably not much above 27 ° C (80 °F). Moreover, they pointed out that most large reptiles today escape excessive heat by submergence. It is also believed that many of the larger dinosaurs were to some extent amphibious, and that others, such as the herbivorous trachodonts, were at least partially so, to judge from their webbed feet (see Cloudsley-Thompson 1978).
The 'sail' of the extinct Pelycosauria may have been a device for absorbing solar heat in the Permian mornings and for radiating it again when the animals became overheated (Sect. 2.5); the numerous grooves in the spines suggest that the skin covering it may have been well supplied with blood vessels (Romer 1948; Rodbard 1949; Bellairs and Attridge 1975). This hypothesis has recently been supported by the calculations of Bramwell and Fellgett (1973), which showed not only that the sail of Dimetrodon grandis could have enhanced heat gain, but it would also have been capable of radiating heat. They stated: "Faster attainment of the activity temperature in the morning would have been an obvious advantage to a carnivorous reptile feeding on other large poikilothermic animals. Dimetrodon grandis would have been able to reach an active state and attack prey while they were still torpid or sluggish. During the hot part of the day the sail would have acted to radiate away excess heat; this effect could be an important adaptation in a reptile which was too large to seek shade behind small stones or in rock crevices in the manner of small living reptiles. In the evening Dimetrodon could have gained extra activity time,in comparison with a small sail-less pelycosaur of the same A/W (area to weight) ratio, by restricting blood flow to the sail. The sail could therefore prolong the total time in which Dimetrodon could be active in any 24 hr."
"Our conclusions are reinforced if, as seems probable, the controlling adaptations had become highly refined. In addition to adjustments of blood supply, the degree of blackening may have been under nervous or hormonal control. Some lizards which are black when basking to catch the maximum radiant energy can change to white when oriented head-on to the sun and emitting heat. It is possible that Dimetrodon (or indeed living reptiles) may be black in the infra-red to radiate heat more effectively, while appearing white in visible light. Visible and infra-red emissivities of Dimetrodon may have been separately adjustable according to the thermal state of the animals" (Bramwell and Fellgett 1973).
In a similar way, the dorsal plates of Stegosaurus (Fig. 71) probably acted as forced convection fins, serving an important thermoregulatory function. Windtunnel experiments on finned models, calculations of internal heat conduction, and direct observations on the morphology and internal structure of stegosaur plates support this hypothesis, demonstrating the comparative effectiveness of the plates as heat dissipaters, controllable through the rate of input blood flow, temperature, and body orientation with respect to wind (Farlow et al. 1976). The dermal plates of Stegosaurus, like other bizarre structures which have, until recently, defied satisfactory explanation, may well also have been concerned with intraspecific combat and display behaviour. Such structures include crests, spines, frills, and dorsal plates (Sect. 9.3; see review by Hopson 1977.)
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