18 Thermal environments of dinosaur nestlings: Irnplications for endothermy and insulation GREGORY S. PAUL Abstract ronrnent in hadrosaur, hypsilophodont and small thero- Very small, altricial hadrosaur nestlings probably lived pod nests (Horner & Makela, 1979; Homer, 1982, 1984, in open nests exposed to the weather. To survive and grow 1987, 1988; Homer & Gorman, 1988; Homer & Weis- rapidly.theyneeded to be insulated endothermichomeotherms. hampel, 1988; Coombs, 1989; Kurzanov & Mikhailov, Altricial young of small ornithopods and small. theropods 1989; Russell, 1989; Winkler & Murry, 1989; Currie, neededsimilar adaptations unless they were brooded by their 1990; Lambert, 1991; Homer & Currie, Chapter 21). parents.If the parents did brood the hatchlings, they probably neededan elevated metabolism and soft insulation to insulate Conditions within the open nests of these dinosaurs and warm their young. were probably often harsh. If this is correct, then the hatchlings that inhabited these nests may have needed "sophisticated" nonreptilian physiologies in order to Introduction thrive. Alternately, if small ornithopod and theropod The dominant and most abundant dinosaurs in adults were endothermic, then they could have used many Upper Cretaceous faunas are the large bodied their warm bodies to brood their young. duckbilled hadrosaurs. They grew from diminutive hatchlings with a body mass 5,000 to 10,000 times less than the adults (Fig . .18.1). These little creatures needed Thermal strategies of modern juvenile to thermoregulate, and how they managed this may have vertebrates as much to tell us about dinosaur physiology as do ·stud- Reptiles ies of the adults. Until recently a lack of juvenile spec- Reptilian hatchlings are precocial ectothermic imens, and the preferential interest shown for the adults heterotherms. They are not brooded by their parents, and have combined to keep the physiology of dinosaur ba- ..iinmediately disperse from the nest (Bellairs, 1970). bies from receiving much attention. .. Thermoregulation occurs by behavioral modification Thulbom (197.3) and Reid(1978) have expressed and the use of refuges. Reptiles have a narrow preferred the opinion that small size, naked skin, and endothermy body temperature range (30°_39°C in most species, Bel- would be a lethal combination for nonbrood~d hatchling lairs, 1970) and a broader tolerance range (up to 45°_ dinosaurs. Russell (1980) suggested that dinosaur 47°C in desert forms; Bellairs, 1970). When the upper hatchlings preferred densely vegetated, wet areas with or lower limits of the tolerance range is reached, reptiles minimal temperature fluctuations. In contrast, Hotton seek refuge in shade, burrow, or water, for death may (I980) suggested that dinosaur hatchlings were large result from exposure. In addition, at extreme body tern- enough to be incipient inertial homeotherms. While peraturesgrowth stops. MCGowan (1979,1984) believed that juvenile dinosaurs should have been weII protected in their nests and good Birds thermoregulatorS. Homer and Makela (l979)and Hor- Bird chicks practice a wide variety of metabolic ner'and Gorman (1988) suggested that juvenile ornitho- and thermoregulation strategies to achieve ideal condi- pods were endothermic, and-Lambert (1991) argued that tions for survival and growth. There are, however, some the altricial nature of some ornithopod nestlings is sug- generalized patterns. Many altricial nestlings begin life gestive of endothermy. as poikilotherms, and metabolism slows with falling am- Studies on dinosaur nesting habits and growth bient temperatures (Dawson & Hudson, 1970; O'Con- rates provide tbr·~asis for examining the thermal envi- ner, 1975; Whittow, 1976a; Steen et al., 1989). They are Gregory S. Paul 280 naked upon hatching and have lower resting metabolic .may leave the nest immediately or soon after hatching. rates than adult birds of the same mass. Consequently, They seek optimal thermal microenvironmems in the the very survival of the altricial chicks depends upon landscape for refuge. the sheller provided by the nest and/or brooding by their Some birds form nesting colonies in open areas parents. Brooding adulls provide feathery insulation and having little or no vegetation to screen the nests from warmth at night, and shade and insulation against ex- the sun or rain (Dawson & Hudson 1970, 1972; Whit- cessive heat during the day. tow, 1976a; Perrins & Middleton, 1985). Thermal con- Most altricial chicks experience difficulty ther- ditions can become severe due to sunlight, darkness, Or moregulating outside a narrow body temperature range storms, Most chicks in such habitats are brooded. One of 30°-40°c. Growth in this temperature range is rapid, exception is the r 'dicolous chicks of some gulls. They but it slows or stops when this range is exceeded (Whit- remain in or near the nests and, because they are not tow, 1976b). Some species become torpid and stop brooded, seek shelter in the surrounding terrain when growing when their parents leave the nest on long for- the nests become too hot (Dawson & Hudson 1970; aging trips (Steen et aI., 1989). Air temperatures that Dawson et aI., 1972). drop much below 10°C can be lethal to an unprotected Among all bird chicks, there is a general, positive chick (Steen et al., 1989). At high temperatures, a few correlation between metabolic rate and growth rate taxa can use evaporative cooling (Whittow, 1976a), but (Whiuow, 1976b). On the other hand, growth ofpoikil- they remain vulnerable to direct sunlight as long as they othermic altricial nestlings is very energy efficient be- are naked. As the chick grows, metabolic rates rise to cause most of the energy is devoted to growth; adult levels, and downy insulation appears. Thermoreg- metabolic heat is obtained from the brooding parents. ulation improves (Whittow, 1976b) and growth slows As a result, altricial chicks often grow more rapidly than down, but it is still fast. precocial chicks (Case, 1978). Both altricial and pre- Precocial chicks are tachymetabolic while still in cocial nestlings benefit from a steady and abundant sup- the egg, and hatchlings are good thermoregulators ply of food from their parents. This allows them to (Whittow 1976a, 1976b; Dawson & Hudson 1970; Daw- convert energy into growth rather than use it for for- son, Hudson, & Hill, 1972; Freeman, 1971). The body aging. has an insulation of down feathers to maintain a high, constant temperature. Furthermore, the metabolism rises Mammals in response to declining external temperatures, and tem- Mammalian young also exhibit diverse thermal peratures below freezing can be tolerated for a short strategies. In addition, when they are about one- time. Heat production in the cold includes both shiver- quarter grown,' their metabolic rates are about one ing and nonshivering thermogenesis. In addition, ele- third higher than those of adults of equal size (Brody, vated temperatures up to 4rC can be tolerated for brief 1974). The young of most ungulates are precocial, periods (Whittow, 1976a, I976b; Dawson, & Hudson, have fairly large bodies, are endothermic, and are in- 1970: Dawson et. aI., 1972). Growth, however, slows or sulated with fur. These ungulates are thus able to cope stops when body temperatures exceed 40°C or drop be- with a wide range of thermal extremes. Sus piglets, low 30°-35°C (Whittow, 1976b). Nidifugous chicks with a mass of 0.4 kg, have an insulation of light Figure 18.1. Comparison of a 2.5 metric IOnadult Maiasaura peeb/esorum, O.3-kghatchling, and 20-kg juvenile. Adult stands next to a schematic cross section of an open-mound nest containing two hatchlings. while a juvenile only a few weeks old leaves the nest. Nest restored after data in Horner and Makela (1979) and Coombs (1989). Scale bar = I rn. Thermal environments of dinosaur nestlings 281 body fat and fur. They are able to cope with still, dry Structure of dinosaur breeding colonies air temperatures as low as 5°C (Hull, 1973). Other and nests mammals have altricial young. These are often naked Hadrosaurs and hypsilophodorus apparently and with underdeveloped endothermy. Consequently, nested in colonies consisting of a half dozen to perhaps the young receive close attention and warmth from hundreds of nests (Homer & Makela, 1979; Homer, their parents. 1982, 1984, 1987; Horner & Gorman 1988; Horner & Weishampel, 1988; Coombs, 1989; Russell, 1989; Winkler & Murry, 1989; Currie, 1990; Homer & Currie, Growth Chapter 21). These nesting sites appear to have been Modern vertebrates used repeatedly for many years. The colonies may have Juvenile reptiles in the wild grow only about one resembled those of ground nesting birds being devoid tenth to one thirtieth as rapidly as mammals and birds of shade trees (Homer & Makela, 1979; Homer, 1982, (Case, 1978). Even in captivity where food is more 1984, 1987, 1988; Homer & Gorman, 1988; Winkler & abundant, they grow much more slowly than poikilo- Murry, 1989; Currie, 1990; Homer, Chapter 8). Such an thermic altricial bird chicks (Case, 1978; Coulson, Coul- interpretation is supported by the apparent absence of son, & Hernandez, 1973). These facts suggest that large plant roots in the colony sites. The presence of fast-growing animals must either have high metabolic plant material within nest structures suggests that the rates and high, stable body temperatures or else be lack of fossilized plant material between the nests is not brooded by warm-bodied parents. an artifact of preservation. The close spacing of the Speculations by Dunham et al. (1989), Houck, nests, about one adult body length apart, suggests that Gauthier, and Strauss (1989), and Reid (1990) that un- the nests were placed in open areas. brooded juvenile vertebrates with reptilian or "inter- Without the protection of plants, the nests were mediate" physiologies could grow as rapidly as birds exposed to the weather. Some protection, however, and mammals remain unproven. For unbrooded juve- might have been provided by the nests themsel ves. The niles to grow rapidly in a nest they need adaptations to hadrosaur nest appears to have been a conical depression cope with thermal extremes.