Dinosaur smarts

How can we measure the intelligence of dinosaurs?1 The short answer is "Not easily." However, it is clear that, at a very crude level, there is a correlation between intelligence and brain : body weight ratios. Brain : body weight ratios are used because they allow the comparison of two differently sized animals (that is, brain : body weight ratios allow comparison of chihuahua and St Bernard dogs). The correlation suggests that, in a general way, the larger the brain : body weight ratio, the smarter the organism. Indeed, mammals have higher brain : body weight ratios than fish and are generally considered to be more intelligent (Figure B12.4.1). But how smart could a very large dinosaur with a miniscule brain be (for example, see Box 5.2)?

It is well known that organisms change proportions as they increase in size; this is allometry. And it turns out that brain : body weight ratios follow allometric principles as well: brains do not increase in size proportionally to the rest ofthe animal. For example, the brain of a 0.5 m rattlesnake is proportionally larger than the brain of a 3 m anaconda. Does this mean that the anaconda is significantly stupider than the rattler? Obviously not. So, when considering how big or small a brain is in an animal, there has to be a way to compensate meaningfully for size. A quantitative method of doing this was first proposed by psychologist H. J. Jerison, who, in the early 1970s, developed a measure called the "encephalization quotient" (EQ). Jerison constructed an "expected" brain : body weight ratio for various groups of living vertebrates (reptiles, mammals, birds) by measuring many brain : body weight ratios among these animals. Jerison noted that, on the basis of EQ, living vertebrates cluster into two groups, endotherms and ectotherms. The endotherms show greater encephalization (higher EQs) and the ectotherms showed lower encephalization (lower EQs). Thus, for Jerison, living endotherms and ectotherms could be distinguished by brain size. Having constructed a range of expected brain : body weight ratios, he could account for size in different organisms (and accommodate what might at first seem like an extraordinarily large or small brain). Noting that some organisms still didn't exactly fit in his ectotherm or endotherm group (by virtue of having brains either larger or smaller than expected), he measured the amount of deviation, and then termed this EQ.

Dromaeosauridae

Dromaeosauridae

Encephalization Quotient Dinosaurs

Figure B12.4.1. EQs (Encephalization Quotients) of dinosaurs compared. The line at 1.0 represents the crocodilian "norm," and suggests that many groups of dinosaurs had larger brains than would be predicted from a conventional reptilian model (the crocodile). Note also the break between 2.0 and 5.8; if these measures mean anything, apparently coelurosaurs significantly outdistanced other dinosaurs in brain power. (Data from Hopson, 1980.)

Figure B12.4.1. EQs (Encephalization Quotients) of dinosaurs compared. The line at 1.0 represents the crocodilian "norm," and suggests that many groups of dinosaurs had larger brains than would be predicted from a conventional reptilian model (the crocodile). Note also the break between 2.0 and 5.8; if these measures mean anything, apparently coelurosaurs significantly outdistanced other dinosaurs in brain power. (Data from Hopson, 1980.)

Paleontologist J. A. Hopson, now knowing what he could expect for living vertebrates, measured how much the estimated brain : body weight ratio EQ of extinct vertebrates deviated from the expected brain : body weight ratios of their living counterparts. Figure B12.4.1 shows the EQs for several major groups of dinosaurs as reconstructed by Hopson. Because dinosaur are "reptiles," he measured the deviation ofvari-ous groups relative to a "reptilian" norm, in this case a living crocodile. Significantly, many ornithopods and theropods show a brain : body weight ratio that is significantly larger than would be expected if a modern reptilian level of intelligence is being considered.

1. A more fundamental question is: what is intelligence? As applied here, intelligence refers to the ability to learn, and perhaps the capability for abstract reasoning. The measurement of "intelligence" in humans is freighted with a notoriously racist history and consequently much emotional baggage (see Gould, S. J., 1981, The Mismeasure of Man. New York: W.W. Norton, 352pp.) Here, we are discussing intelligence in far cruder terms; that is, at the level of the comparison of the intelligence of a crocodile and a dog.

Figure 12.4. Cross-sections (solid shading) through the nasal regions of (a) an extinct dinosaur (Veloaraptor) and (b) a living bird (Rhea); skulls and positions of the cross-sections are shown to the left. The nasal cavity of the bird shows convoluted respiratory turbinates, while that of the dinosaur does not.

Figure 12.4. Cross-sections (solid shading) through the nasal regions of (a) an extinct dinosaur (Veloaraptor) and (b) a living bird (Rhea); skulls and positions of the cross-sections are shown to the left. The nasal cavity of the bird shows convoluted respiratory turbinates, while that of the dinosaur does not.

bone. Secondary bone is deposited in the form of a series of vascular canals called Haversian canals, and resorption and redeposition of secondary bone can occur repeatedly during remodeling. When remodeling occurs, a type of Haversian bone known as dense secondary Haversian bone is formed. This bone has a distinctive look about it (Figure 12.6).

Dense secondary Haversian bone is found in many mammals and birds - all, of course, endotherms. Among extinct vertebrates, dense secondary Haversian bone has been observed in dinosaurs, pterosaurs, and therapsids (including Mesozoic and Cenozoic mammals). Given the distribution of dense secondary Haversian bone in vertebrates, it was not too great a leap to suppose that dinosaurs, too, must have been endotherms.

The significance ofsecondary Haversian bone. Although this idea was initially promising, it turns out that dense secondary Haversian bone is due to a variety of factors, one of which is endothermy. Secondary Haversian canals are known to be correlated with size and age, and possibly with the type of bone being replaced, the amount of mechanical stress undergone by the bone, and nutrient turnover (the metabolic interaction between soft tissue and developing bony tissue).

And what does this mean for the possibility of endothermy in dinosaurs? It means that, assuming that dense secondary Haversian bone formed in dinosaurs at rates comparable to those in mammals, dinosaurs probably lived for lifespans approximating to those of living mammals, and dinosaurs likely had rates of bone growth similar to those found in mammals. If such growth rates really occurred, they would be in good agreement with conditions that might be expected to be found with an endothermic metabolism.

Growth. But what is really known about rates of dinosaur growth? Work on the hadrosaurid Maiasaura suggests that it grew at an astounding 3 m/year. If so, this makes such a growth pattern distinctly different from that seen in living non-dinosaur reptiles, and much closer to that seen in modern birds (Figure 12.7).

Further work shows that, uniquely in both young birds and other dinosaur juveniles, developing bone has a distinctly porous quality. The porosity has been linked to vascularization (the extensive network of blood vessels carrying nutrients), itself linked to the rate of deposition of the bone. The message is clearly one of bone morphology and growth rates closer to those seen in modern birds than those seen in lizards, snakes, turtles, and crocodiles.

As we have seen, the paleontological evidence - dinosaur juveniles in nests with differing stages of development (see Chapters 6 and 7) - suggests that parental care was involved in raising at least some dinosaurs. Such behavior contrasts with that generally seen in snakes, turtles, crocodiles, and lizards.

The sauropodomorph Massospondylus and an early theropod, the small, light-bodied theropod Syntarsus, both from South Africa, were studied to estimate rates of growth. Massospondylus took 15 years to reach 250 kg (17 kg/year), while Syntarsus took 7 years to reach an estimated 20 kg (3 kg/year). Although the appearance of secondary bone in the thighs of these organisms more closely resembled that of modern birds than that of a crocodile, the rates were somewhat slower than J. Peterson and J. Horner's estimate of Maiasaura growth rates (Figure 12.8).

LAGs. Concentric growth rings have been observed in the bones and teeth of dinosaurs. Among modern tetrapods, such growth rings are typically found in ectotherms, where they are believed to represent seasonal cycles. During times of slowed metabolism (such as dry seasons in the tropics, or cold seasons in more temperate latitudes), growth is stymied - hence the term "lines of arrested growth," or LAGs (Figure 12.9). So bone records a pattern of ring-like deposits representing

annual cycles of growth and stasis. Among endo-therms, on the other hand, such patterns are rare, because the relatively constant, elevated metabolic rates ensure growth at a constant rate.

The results from LAGs have been somewhat inconsistent. LAGs found in dinosaur teeth were very much like those found in crocodilian teeth from the same deposits. Here then was evidence that seemed to suggest that dinosaur growth rates fluctuated, as might be expected if they had an ectothermic metabolism. Morevoer, LAGs occur in many different kinds of dinosaur (notably among the best candidates for endothermy, Coelophysis, Allosaurus, and Troodon), suggesting to researchers that growth in dinosaurs was more susceptible to external climatic influences than had been predicted by the simple homeothermic endothermic view of dinosaur metabolism.

In two small flyers and one large flightless enantiornithine (Patagopteryx) bird, the presence of LAGs lead to the conclusion that the early birds' metabolism(s) were subject to seasonal growth, even though the birds clearly bore feathers. The presence of feathers in these early birds could mean that, ultimately, they had not quite attained the level of endothermy seen in living birds (see below).

Enantiornithine bone histology contrasts with that of ornithurine birds (for example, Hesperornis and Ichthyornis). In these, the bone tissue looks very similar to that in modern birds. So too is the bone tissue of the primitive, Early Cretaceous Confuciusornis.

But are LAGs truly seasonal? Indeed, one hadrosaurid fossil is reported to have different numbers of LAGs on its arms than those on its thighs! Moreover, the appearance (or not) of LAGs has never been tightly correlated with climate. We can be certain that the last word about LAGs has not yet been spoken.

Figure 12.6. Primary bone in the process of being replaced by Haversian bone in the leg of a hadrosaurid. Longitudinal canals (at the top of the figure) in primary lamellar bone (a) are resorbed (b) and then reconstituted as Haversian bone (c).

Ecology

Of predators and their prey. Endothermy is much more costly in terms of energy use than ectothermy. It has been estimated that it costs 10-30 times as much energy to maintain an endothermic metabolism as to maintain an ectothermic metabolism, in part because so much energy is expended on maintaining a constant body temperature.

Given that fact, paleontologist R. T. Bakker reasoned, if predators are endothermic, they should require more energy than if they were ectotherms, and this should be in turn reflected in the weight proportions of predators to prey, or predator:prey biomass ratios.

Bakker calculated that predator:prey biomass ratios for ectothermic organisms are around 40%, while predator:prey biomass ratios for endothermic organisms are 1-3%. Here then was an order of magnitude difference in the biomass ratios, which ought to be recognizable in ancient populations.

Now, by counting specimens of predators and presumed prey in major museums and by estimating the specimens' living weights, Bakker was armed with a tool from modern ecosystems that he believed could reveal the energetic requirements of ancient ecosystems.

His results seemed unequivocal: among the dinosaurs, the predator:prey biomass ratios were very low, ranging from 2% to 4%. He interpreted this low number to indicate that predators and prey in dinosaur-based food chains were endothermic (Figure 12.10).

This study, for all its creativity and originality, had some problems. The assumption that there is an order of magnitude difference between ectothermic predator : prey biomass ratios and endothermic predator : prey biomass ratios - may not hold in all cases.

Figure 12.7. Femur of a Maiasaura hatchling compared to that of an adult. Note the size ofthe human hand.

Crocodile

Syntarsus / Homiens

Crocodile f

0strich Ma\asaura f

0strich Ma\asaura

10 15

Time in years

MassosspondyUL

Adult size

10 15

Time in years

Figure 12.8. Estimated growth rates of some dinosaurs, Alligator, and a human. The graph is based upon guesses of how long it takes for the tetrapods to reach adult size. Note that because the sizes of these organisms vary extensively, the growth rates also vary. Unlike the other tetrapods presented, Alligator grows continuously throughout its life; hence, it has no fixed "adult size." Sexual maturity usually comes within six to eight years. (Estimates for Syntarsus and Massospondylus from the work of A. Chinsamy; estimates for Maiasaura from the work ofJ. Peterson and J. Horner.)

Tyrannosaurus Fibula
Figure 12.9. Lines of arrested growth in a Tyrannosaurus fibula. Arrows indicate LAGs.

Bakker's assumption that prey are approximately the same size as the predators is clearly not correct (consider a bear eating a salmon), and has drastic effects on the resultant ratio. Most significantly, the predator : prey biomass calculation assumes that all deaths are the result of predation; that there can be no mortality due to other causes. This is simply not the case.

I I Predators I I Prey

There are serious and legitimate problems with using fossils. Most obvious are difficulties in estimating dinosaur weights (Box 12.5). Moreover, the preservation of dinosaur material is subject to a variety of biases. How can one ever be sure that the proportions of the living community are represented? Because we can't, paleontologists commonly talk about fossil assemblages, which, as we've seen (Chapter 1), may have nothing to do with the proportions of the same animals in the living community in which those animals lived.

Finally, Bakker obtained his data by counting specimens in museum collections, specimens that were likely collected because they were rare or particularly well preserved. Museum collections thus tend to represent assemblages of well-preserved organisms with a higher percentage of rare organisms than was present in the original fauna.

Ultimately, too much uncertainty for the results to be definitive was introduced through the brilliant, but flawed, ideal of predator : prey biomass ratios.

Zoogeography

The distribution of dinosaurs around the globe far exceeds the current distribution of modern ectothermic vertebrates, which are generally not found above and below, respectively, latitudes 45° north and 45° south. Large modern ectotherms rarely occur above latitude 20° north and below latitude 20° south (Figure 12.11). Correcting for continental movements, Cretaceous dinosaur-bearing deposits have been found close to latitudes 80° north and 80° south of the equator. Both the northern and southern sites experienced extended periods of darkness, and we can be reasonably sure that, at least occasionally, air temperatures in winter fell below freezing.

The Arctic assemblage, from North America, includes hadrosaurids, ceratopsids, tyr-annosaurids, and troodontids. The Antarctic dinosaur assemblage, from Australia, includes a large theropod and some basal euornithopods (including many juveniles). Along with these dinosaurs are fish, turtles, pterosaurs, plesiosaurs, birds (known solely from feathers), and, incredibly, an improbable late-surviving temnospondyl (an amphibian group that apparently went extinct in the Early Jurassic everywhere else in the world; see Figure 13.6).

Equator

Prehistoric Life

Equator

Figure12.11. The latitudinal distribution of ectothermic tetrapods on Earth The larger terrestrial tetrapods do not get much beyond about latitude 20° north and south. These include large snakes and lizards, crocodilians, and tortoises.

Equator

Equator

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