|Tyrannosaurus skeleton mounts (double entendre!) at a museum in Spain.|
Sexual Dimorphism is when the male and female of one species of animal look different from one another. We can easily see this in many animals around us today. One really common form of sexual dimorphism that we can see a lot of especially with birds and non-avian reptiles is color. Typically males of many species will be more brightly colored than females. This is because in these species of animals, the females have the choice of which male she want's to father her offspring. Therefore the boys need to do everything in their power to impress females if they want to ever get their genes over into the next generation. As if it wasn't competitive enough, many animals exhibit a sexual practice called polygyny where one male mates with many females. So its not like they just partner up and the less desirable guys get with the less desirable ladies. Instead, there is usually only one male who gets to mate with ALL the females in the area and the rest go home alone at the end of the day. This drives evolution to make some species of male animals quite crazy looking.
|Mandarind Ducks (Aix galericulata)|
|Male Red-headed Agama (Agama agama). Females are just brown.|
|Male and female peafowl (pavo cristatus)|
You may be looking at some of these male animals and saying to yourself "How do they not get picked off by predators? They have no camouflage!" Well you are right. Peacocks for instance are a favorite meal of predators like leopards and tigers because they stand out so easily and when they fan their tails out they completely cut off their field of vision making it easy for the a large cat to sneak up on them from behind. Nobody ever said getting a mate would be risk free! In fact that is sort of the whole point. Think about it. Ever hear the expression "People do crazy things when they are in love?" Ever watch a friend who was dating someone he/she was head over heels for and think they were acting like a total lunatic? Its true! I, for instance, consider myself a pretty logical thinking guy for the most part but put me in front of a girl I'm after and my brain somehow manages to just shut off. Its the same thing for animals in a way. When a male looks over-the-top flashy with his colors and/or wacky mating dance, what he is essentially saying to all the females around is "I have all this going on AND still manage to avoid getting killed! I am definitely the one you want to father your babies because I'm a strong survivor!".
|Female and male Unenlagia life restoration by Christopher DiPiazza. Colors and behavior based on speculation.|
So what about extinct dinosaurs? With the exception of a small number of individual fossils, dinosaur color is a mystery. There are clues in some of the fossils that may help indicate if there was bright color going on, however. In some kinds of dinosaur's it is almost certain that males at least had more extravagant display parts like crests, frills, horns and feathers than their female counterparts. Hadrosaurs like Corythosaurus are known from adult individuals with two kinds of crests; one larger and one smaller, suggesting a male and female. If they were anything like their modern relatives there is a solid chance the males were utilizing these crests like billboards for colors as well.
|Parasaurolophus was possibly sexually dimorphic. Restoration by Christopher DiPiazza.|
We also know that these hadrosaurs could use their crests (which were hollow) to make loud noises by pumping air from their mouths and noses through them. We know that the sounds produced this way will sound different depending on the shape of the crest the air is traveling through (somewhat like how wind instruments play for any musicians out there). Therefore it is also not unreasonable to suggest that males and females were also using sound to attract each other much like many modern animals do.
|Lots of modern animals attract mates using sound. Some mate for life. Others can't maintain a relationship for more than a few months before getting dumped (I'm looking at you, T. Swizzle!).|
Color and morphology dimorphism could be true for some of the ceratopsid dinosaurs that have frills and horns. Protoceratops, for instance, is known from many specimens including eggs, babies and what is believed to be two adult forms. The one that is believed to be male has a larger frill and more robust-shaped snout.
|Protoceratops mating life reconstruction by Christopher DiPiazza. Do I need to specify which is the male and which is the female for this one? The frill is even heart-shaped! CUTE!|
The small Chinese bird/dinosaur, Confuciusornis, is also a well studied animal known from many fossils. The males and females can be easily distinguished from each other by the male's long tail feathers.
|Confuciusornis male and female fossil.|
So we already know that dinosaur males and females may have been different colors and some of them probably sounded different vocally. What about size? Sexual size dimorphism is when one sex is naturally larger than the other within a species. Most people tend to associate males with being larger since us humans happen to be that way (unless you belong to Honey-boo-boo's family) but there are lots of female larger species out there too.
|Examples of sexual size dimorphism in extant animals. Males are blue and females are red. As you can see there are both male-larger and female-larger examples within the same class. Image by Robert Cox.|
When I was working towards my degree in Animal Science at Rutgers. I had the great experience of assisting a study on Eastern Fence Lizards (Sceloporus undulatus), a female-larger species. Its interesting because even different species of animals within the same genus can vary greatly with regards to sexual dimorphism. Within Sceloporus, more than half of them are male-larger. In the case of the lizards i worked with, however, the males didn't grow as much as their female counterparts because they tend to be invested in patrolling their territory and searching for mates in addition to finding food. The females on the other hand, not being territorial, have more time to eat more food, and thus tend to become larger as adults. In fact, the study showed that testosterone, a hormone found in vertebrates commonly associated with large body size, was actually a growth inhibitor in this species of lizard.
|Eastern Fence Lizards also have striking color sexual dimorphism.|
Other female larger animals like some species of frogs and horseshoe crabs are the way they are because multiple males will often try to mount the same female at once to fertilize her eggs. In others, like spiders and scorpions, the female produces many offspring at a time and thus, needs a larger body to grow them inside her (it also makes it easier to eat the father after).
|Nothing says romance like some good old post-sex cannibalism.|
What about dinosaurs? Modern dinosaurs, the birds, can vary with sexual size dimorphism. Galliformes like chicken and turkey, for instance, tend to be male larger while birds of prey like hawks and eagles are female larger. One extinct dinosaur that has been proposed to be a female-larger species is Tyrannosaurus rex.
I have heard rumblings of this in popular dinosaur documentaries like "Walking with Dinosaurs" and "Jurassic Fight Club" but I have never actually seen or read of the hard evidence that causes some scientists to think that so before writing this article i decided to do some digging (paleo-pun!). I briefly spoke with paleontologist, Dr. Thomas Holtz about his thoughts on sexual dimorphism in Tyrannosaurus. As you may know Dr. Holtz has been on this blog a few times before and is an expert on Tyrannosaurus and its relatives. He directed me to some papers suggesting that a Tyrannosaurus's sex can be determined by the number of chevrons it has on the underside of its tail. A chevron is a small bone on the underside of the vertebrae in most reptiles and a few mammals. It is believed by some that females had fewer at the base of their tails since they needed a wider space to pass eggs through when laying them. This difference can also be seen in modern relatives to the dinosaurs like Alligators.
|Chevron differences between Tyrannosaurus sexes. From scientific journal, "Zoology", volume 4.|
He also told me that at least one larger specimen of Tyrannosaurus was found to have medullary bone. Medullary bone is where calcium is stored for eggshell production in reptiles. That being said, if a dinosaur has this, chances are its a female!
|Tyrannosaurus rex reconstruction by Christopher DiPiazza.|
So does this mean that Tyrannosaurus rex was definitely a female-larger species? No. The number of Tyrannosaurus specimens available to work with is too small to make a definite call but it certainly is a possibility. Assuming it's true, however, what does this say about the animal's behavior and lifestyle? Were the females larger due to a slower growth rate and higher food intake like the Fence Lizards? Perhaps its because only the mothers guarded the young (if they did at all) and could more easily protect them from danger? For all the new exciting information we are constantly discovering about dinosaurs, there will always still be a lot we can only guess about. We may not know exactly how dinosaurs did the deed or what exactly females may have been attracted to in a mate but we do know for sure that baby dinosaurs had to come from somewhere!
C.A. Duncan,H.B. John-Alder, Testosterone Reduces Growth and Hepatic IGF-1 Message but not Plasma IGF-1 in a Female-Larger Lizard, Sceloporus undulatus. In preparation.
C.A. Duncan, Nutritional and hormonal modulation of insulin-like growth factor-1 with respect to growth in sexually dimorphic lizards. New Brunswick, NJ: Rutgers University; 2011.
Bonnan, Matthew F., James O. Farlow, and Simon L. Masters. "Using Linear and Geometric Morphometrics To Detect Intraspecific Variability and Sexual Dimorphism In Femoral Shape In Alligator Mississippiensis and Its Implications For Sexing Fossil Archosaurs." Journal of Vertebrate Paleontology 28.2 (2008): 422-31. Print.
Chinsamy, Anusuya, Luis Chiappe, Jesus Marugan-Lobon, Gao Chunling, and Zhang Fengjiao. "Gender Identification of the Mesozoic Bird Confuciusornis Sanctus." Nature Communications 4 (2013): n. pag. Print.
Cox, Robert M., Stephanie L. Skelly, Angela Leo, and Henry B. John-Alder. "Testosterone Regulates Sexually Dimorphic Coloration in the Eastern Fence Lizard, Sceloporus Undulatus." Ed. S. J. Beaupre. Copeia 2005.3 (2005): 597-608. Print.
Cox, Robert M., Stephanie L. Skelly, and Henry B. John-Alder. 2005. Testosterone inhibits growth in juvenile male Eastern Fence Lizards (Sceloporus undulatus): implications for energy allocation and sexual size dimorphism. Physiol. Biochem. Zool. 531-608. Print.
Erickson, G., A. Kristopherlappin, and P. Larson. "Androgynous Rex – The Utility of Chevrons for Determining the Sex of Crocodilians and Non-avian Dinosaurs." Zoology 108.4 (2005): 277-86. Print.