DINOSAUR METABOLISM

Why is the debate over dinosaur metabolism so important?

The debate over dinosaur metabolism is important because it determines the way we visualize the behavior of these animals. If dinosaurs were cold-blooded (similar to todays reptiles) they were probably mostly sluggish, with only occasional bursts of quickness. Plus, they would not have been very smart. They probably would have spent most of their time basking in the sun, moving only to obtain more food, similar to the behavior of modern crocodiles. On the other hand, if dinosaurs were warm-blooded (similar to todays mammals) they were probably active, social animals. They would have been quick, alert, and intelligent. They would have spent much of their time actively grazing, similar to modern antelope, or hunting in packs, similar to lions and wolves.

Who first proposed that dinosaurs were warm-blooded?

In the late 1960s and early 1970s, paleontologists John H. Ostrom (1928-2005) and Robert T. Bakker (1945-) first suggested that dinosaurs were not sluggish, stupid, cold-blooded animals. Their work paved the way for the theory that many of these animals were actually agile, dynamic, and smart. In 1969, Ostrom published a description of the Deinonychus, a Cretaceous period carnivorous dinosaur. Based on his study of the creature, he theorized that dinosaurs may have been warmblooded. In 1975, Bakker summarized his ideas about dinosaur endothermy in an article published in Scientific American. This set off a new era in dinosaur paleontology that continues today, especially in advancing ideas on how dinosaurs truly regulated their bodies metabolism and heat.

Why did Robert T. Bakker believe dinosaurs were endothermic homeotherms (warm-blooded)?

Some of the reasons paleontologist Robert T. Bakker (1945-) gave for dinosaurs being endothermic homeotherms, or warm-blooded, are:

1. Dinosaurs had complex bone structures (with evidence of constant remodeling), a feature of modern mammals, not reptiles.

2. Dinosaurs had an upright structure similar to birds and modern mammals.

3. Dinosaurs, at least the small theropods, had, based on the evidence to date, active lifestyles.

4. Predator-versus-prey ratios were closer to that of modern mammals than reptiles.

5. Dinosaurs have been found in polar regions, a climate that cold-blooded animals would not likely inhabit.

What can dinosaur bones tell us about their internal heat regulation?

Dinosaur bones, like modern reptile bones, often show signs of not growing as if these animals went through periods of little or no growth. One of the reasons for a lack of growth could be hibernation during periods of seasonal cold, indicating that the animals used an ectothermic method of heat regulation. Mammals and birds, on the other hand, are endothermic, and show no lines of arrested growth. Thus, the presence or absence of these lines give paleontologists possible clues about the way dinosaurs regulated their internal temperature.

What kind of heart did dinosaurs have?

Fossil evidence of the soft internal parts of dinosaurs, including the heart, is sadly lacking. But based on indirect evidence and one heart from a dinosaur called Willo paleontologists have extrapolated that dinosaurs had a divided heart capable of keeping the blood at two different pressures.

Dinosaur bone tissue also shows evidence of blood vessels. Therefore, a heart was necessary to drive the circulatory system, sending blood around the body. Dinosaurs with extremely long necks (such as the large sauropods) and those with heads held upright needed a high blood pressure. This would allow the blood to pump all the way to the brain when the animal was reaching for food. But such a blood system would have too high a pressure to safely circulate to the lungs for oxygenation. Thus, scientists believe dinosaurs probably had a divided heart capable of supplying blood at two different pressures into two separate circulatory systems.

Hunting theropods like this Tarbosaurus required larger brains and more active metabolisms in order to catch their prey (Big Stock Photo).

What does theropod dinosaur brain size tell us about their metabolism?

Unlike most of the other dinosaurs, some of the smaller theropods (carnivores) had large brains relative to their body size. The brains were equivalent to those found in similar-sized mammals. To function properly, large brains need a steady temperature and constant supply of food and oxygen, all of which could indicate a potential for the higher metabolism associated with warm-bloodedness.

Do dinosaurs noses indicate they were not warm-blooded?

Some scientists believe that dinosaurs noses may tell us something about dinosaur metabolism. Respiratory turbinates are small scrolls of bone or cartilage in the nose covered with membranes; the absence of these bones in dinosaurs noses is probably a good indication that the animals were not warm-blooded. These turbinates are found in all warm-blooded animals, having evolved independently in mammals and birds; no known cold-blooded animals have them.

Warm-blooded animals breathe quite rapidly. The warm, exhaled air passes over the turbinates and cools, causing the moisture in the air to condense out onto the membranes. In turn, this prevents dehydration.

If dinosaurs were truly warm-blooded, they would need to have respiratory turbinates to prevent dehydration. Scientists recently used computer-aided tomography (CAT) scans to study dinosaur fossil skulls for any signs of respiratory turbinates. So far, the remains of Velociraptor and Nanotyrannus do not show any signs of these structures. Further analysis is planned to study fossil skulls from all the major groups of dinosaurs.

 
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