In many modern animals, there are distinct differences between males and females. This is called sexual dimorphism. Which of the following is an example of sexual dimorphism in modern animals? Multiple answers may be correct. A, Older male buffalo are bigger than younger males. B, Male caribou have antlers. Or C, male umbrella birds have elaborate feathers to display to females. Check all the answers you think are correct. The correct answer is C, when it comes to A, we can't just look at different sizes of the younger and older buffalo. The different is one of age, not sex. In various species of deer, it is often only the males that grow antlers. However in Caribou, both sexes can have antlers. This example serves to remind us that while a structure like antlers in one group of animals, may be a sign of dimorphism, the same might not apply to all species in the group. Finally, the correct answer is C. In umbrella birds, the males have amazingly complex feathers that allow them to dance and display to the females, in hopes of finding a mate. This is a good example of sexual dimorphism. Do you know which of these two bird is the male, and which is the female? Peacocks are relatively easy to tell apart, as the males have elaborate feathers and coloration that sets them apart from females. The proper terminology for this Sexual Dimorphism. All this means is that the males and females look different. Even among humans, it is typically easy to tell the difference between a male. And a female. Although human males don't take this to quite the extreme that a peacock does. So some things are relatively obvious when it comes to sexual dimorphism. In these peacocks feather coloration is one of the most obvious things we can think of when it comes to dimorphism. However there are a lot of other ways that dimorphism might manifest itself. For example, we see that in male hoofed animals such as moose and most other deer grow enormous antlers shortly before mating season. There have been many proposed cases of sexual dimorphism in dinosaurs. But so far almost all of them have been fairly ambiguous. Male and female ceratopsians, for example, have been suggested to have different horn shapes and sizes. However, because many ceratopsians are known from single specimens, it becomes hard to say anything definitive about sexual dimorphism. One case of differences in males and females that is much more obvious. Is that of the cretaceous bird Confuciusornis. This small bird from China often has preserved feathers. And there have been several dozen specimens found. Some specimens, thought to be the males, have a long pair of tail feathers. While the other form, thought to be female. Lack these very distinctive feathers. Although we can't be certain at this point as to which is the male and which is the female, the pattern of gaudy males and more demur females is relatively common among modern birds. Although cases of obvious sex specific structures in dinosaurs are very rare to find there is another way. That we can tell if a dinosaur is female. This once again, relies on bone histology. A chicken is a good example of a modern day dinosaur. It lays eggs just like dinosaurs did. However, laying eggs, just like any form of reproduction, is a costly endeavor. The female dinosaur would have had to put a lot of energy and resources into producing eggs. In particular, eggshells require a lot of calcium carbonate, the same basic mineral found in the skeletons of all vertebrates. This eggshell forms inside the body cavity of the dinosaur, meaning that the dinosaur had to have had enough extra calcium in its body to create the shell. Now,. Where might a dinosaur get this extra calcium from? >> Where do you think a female dinosaur get's the calcium it needs for it's eggs? A, by eating calcium rich foods. B, from it's bloodstream, or C, from it's bones? Check all the answers you think apply. The correct answer is C. Although a dinosaur gets calcium from the food it eats, and the calcium does travel through the bloodstream, most of the calcium that goes into eggshell creation comes from calcium stored in the dinosaur's bones. >> Egg laying presents a serious trade-off. When a dinosaur produces an egg, it can steal calcium from its bones to make the eggshell as hard as possible, which in turn, makes its bones much weaker. Or it could retain more calcium in its bones, keeping itself healthier, but producing an egg with a thinner shell that is more likely to crack. Killing the embryo inside. The way dinosaurs manage to get around this problem, essentially keeping its own bone strong or producing strong eggshells, is through the development of a unique type of bone called Medullary Bone. In the months before a dinosaur lays its eggs, it will store up calcium in the hollow spaces inside its bones. If you remember from the module on anatomy, most dinosaur bones had spaces inside of them. We call these spaces Marrow Cavities. Dinosaurs would accumulate the main bone in this space, storing it up for future use. When the time came for dinosaurs to lay eggs, instead of stealing calcium from the parts of the bone supporting the animal. They could up this stored calcium, creating strong egg shells while maintaining healthy bones. Of course, only females would need this calcium store. In living birds, we only find this type of calcium store in egg-laying females. This evidence in the bones of dinosaurs can then be used to tell males and females apart. Of course, the technique is not foolproof, as only female dinosaurs that are preparing to lay eggs, not juveniles or ones that just recently laid eggs, show medullary bone. Nevertheless, it is one of the best methods we have for telling males and females apart. We do have an example of one other type of bone that likely aided in the production of eggshells. This enormous osteoderm was found with a Sauropod called Rapetosaurus. We believe that this bone formation in Repetosaurus' skin was used to store minerals like calcium. Previously, osteoderms like these were thought to have only evolved for defense, but this osteoderm is incredibly thin and hollow. Making it an unlikely candidate for armor-plating. But since it could have been used to store medullary bone, we think that the repetisaurus this osteoderm came from was female. >> Finally, in the past decade, there have been two separate fossils found that we can tell are females. And in both these fossils, the eggs are still inside the mother, and they're ready to be laid. Dinosaur eggs have different shapes, but this particular kind of dinosaur egg is elongate. And that's so the eggs could be laid two at a time. And we know that in modern birds, there are two functional oviducts and that modern birds can also lay two eggs at a time under some circumstances. Most of the time, though, birds do not lay two eggs at a time, whereas dinosaurs most of the time were laying two side by side. Then they would lay a separate pair beside those and a separate pair beside those. >> Now we've skipped over one very important step on our way from a baby dinosaur hatching out of an egg to a sexually mature dinosaur laying an egg. And that is dinosaur sex. Because dinosaurs laid eggs with hard shells. We know these eggs would have had to have been internally fertilized. But how did two sauropods, or worse, two stegosaurus, manage to, well, have sex? Here again, because we can't observe dinosaur behaviors, we have to take our best guess, based on the animals around us. We can certainly observe how crocodiles and birds mate. And this gives us insight into how dinosaurs may have made it. And although we don't know exactly how a male and female stegosaur would have gotten together to fertilize their eggs, we do know that they were very successful at it. They, like species of other odd and dangerously shaped dinosaurs were around for millions of years, generation after generation. Thanks to the use of a number of novel techniques to study dinosaur fossils, as well as some pretty incredible finds, our understanding of dinosaur birth and reproduction has grown immensely. We can only wait and see what the future will bring us. New discoveries in paleontology are being made at an incredible rate.
