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The Anatomy of Dinosaur Sex

Despite the rarity of direct evidence, paleontologists know quite a bit about dinosaur gonads

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A pair of Tyrannosaurus restored in the act at Spain’s Jurassic Museum of Asturias. Photo by Mario Modesto, from Wikipedia.

Over the past few days I have written about the dinosaurian Kama Sutra, the idea that sauropods had sexy necks, and how to sex a Tyrannosaurus rex (Answer: very carefully). But there is one topic that I have saved for last: what the Tab A, Slot B reproductive anatomy of dinosaurs actually looked like.

Whenever I bring up dinosaur sex in conversation—which is probably far too often—questions about the anatomy of the dinosaurian penis arise almost immediately. I am not sure why this is. Maybe it’s because we expect such impressive, terrifying creatures to have equally scary gonads. Few things would be better nightmare fuel. Whatever the reason for this interest, though, the sad truth is that we don’t know very much about the reproductive organs of male dinosaurs. No one has yet found a fossilized impression or other vestige of a non-avian dinosaur’s penis, a discovery that would have a good shot at the cover of Nature or Science. Instead, restoring a dinosaur’s delicate bits requires some evolutionary context.

Male dinosaurs must have had the equipment for internal fertilization. This was a mode of reproduction passed on by their ancient ancestors. Around 375 million years ago, the first vertebrates with limbs, the early tetrapods, began to crawl along the water’s edge. These amphibious creatures had to stay wet to survive, and like their fish ancestors, they reproduced in the water. Females probably laid soft eggs in aquatic cradles and males squirted sperm over the egg clusters to fertilize them. By about 315 million years ago, however, the early radiation of amphibious vertebrates had produced a lineage of creatures capable of reproducing away from the water. These lizard-like animals, akin to Hylonomus from the Carboniferous strata of Nova Scotia, laid eggs that encompassed an internal pond surrounded by membranes and a tough outer shell. This was the amniotic egg—one of the most important evolutionary innovations of all time. But males could no longer fertilize eggs by excreting sperm over egg clusters in the water. Egg-laying on land required internal fertilization before the female deposited her eggs. All descendants of these creatures, from the dinosaurs to creatures that carry offspring inside the body (placental mammals like humans), continued this tradition.

A different set of evolutionary brackets is needed to narrow down what a dinosaurian penis might have looked like. Birds are living dinosaur descendants, and crocodylians are  the closest living relatives to dinosaurs as a group, and so we can expect that features shared between birds and crocodylians were also present in dinosaurs. One such trait is a cloaca. This charming-sounding orifice, from the Latin word for “sewer”, is the common opening for the reproductive, urinary and intestinal tracts in birds and crocodylians of both sexes. Dinosaurs almost certainly had cloacae, too, and this means that the genitals of Stegosaurus, Deinonychus, Argentinosaurus and all other dinosaurs were hidden away internally. You wouldn’t be able to watch Allosaurus walk by and see anything swinging around.

And that brings us to the thrilling details of size and shape. The difficulty is that, according to a 2006 estimate by Steve Wang and Peter Dodson, there may have been more than 1,850 genera of dinosaurs during a span of more than 150 million years. Almost any generalization about dinosaur sex organs is going to be wrong in some respect, and looking for modern analogs is a complicated task. If we look to modern avian dinosaurs for hints, we are met with a bizarre array of reproductive organs and strategies. Males of most bird species don’t have a penis at all and pass genetic material to females through a brief encounter given the cringe-inducing term “cloacal kiss.” Then again, the Argentine lake duck Oxyura vittata has the longest penis in relation to body length of any known vertebrate, and ducks in general have become infamous for having bizarre sex organs that have a lock-and-key arrangement. In general, though, it seems that the presence of a penis in male birds is the ancestral state, and that the loss of a penis is an evolutionary specialization.

Things are not so varied on the other branch of our evolutionary bracket. Male crocodylians have relatively small penises. This condition, combined with the fact that a penis seems to be the archaic state for male birds, means that male dinosaurs probably had penises as well. As paleontologist Kenneth Carpenter colorfully described, “ssuming you were stupid enough to sneak up under a T. rex and pull the cloaca open, the last thing you would ever see during the last moments of your life would be a penis if it was a male, probably similar to that seen in a crocodile.” The organ probably would have had a single head and a runnel along the top for sperm to travel down, as seen in the closet living relatives of dinosaurs.

We will probably never know the full range of dinosaurian penis variation. I doubt that such diverse and disparate creatures would have had a one-size-fits-all anatomy, although I also doubt the horrifying idea—which comes up often in internet comment threads—that male dinosaurs might have had long, prehensile organs which allowed them to inseminate at a distance. No matter what their gonads looked like, though, male dinosaurs probably had to get very close to their female partners during sex. There were only a limited number of positions which would have worked for dinosaurs.

But we actually know a little more about the reproductive anatomy of female dinosaurs than male dinosaurs. Dinosaur penis anatomy is constrained by what we know about the evolutionary relationships of dinosaurs and what we are willing to imagine, but a few significant fossils have given paleontologists a general idea of the female dinosaur reproductive tract. The most fantastic of them is a pelvis of an oviraptorosaur—one of the feather-covered, beaked dinosaurs that were relatively close cousins of dinosaurs like Velociraptor—with two eggs preserved inside. Described in 2005 by Tamaki Sato and colleagues, the hips show that the female oviraptorosaur had died just before laying those eggs. This fortuitous discovery illustrated that at least some dinosaurs had a mix of bird- and crocodylian-like reproductive features.

While female birds have only one oviduct—thought to be an adaptation related to becoming light enough to fly—the presence of two eggs in the dinosaur suggested the presence of two oviducts, as in crocodylians. But the fact that there were only two eggs indicated the dinosaur laid a small number of eggs at a time. Instead of producing a large clutch of eggs and laying them all at once, like a crocodylian, the dinosaur only laid two eggs each round and arranged those pairs around the nest. (Oviraptorosaurs have famously been found preserved on top of nests which seem to show a ring of paired eggs.) The female dinosaur did not have a reproductive system just like that of a bird or a crocodile, but a combination of traits seen in the modern lineages.

Other eggs hint that some of the largest dinosaurs might have been more crocodylian-like. No one has yet found a Diplodocus with eggs preserved in the hip region, but paleontologists have found numerous eggs referred to sauropod dinosaurs. Some of these show a pathological condition in which eggs are coated with a second shell layer. According to Kenneth Carpenter, there are two possible ways for this to happen. One possibility is that the egg stalled while going through the shell gland and received a second covering because of the delay. But the other explanation is that some dinosaurs might have produced a larger number of eggs relatively rapidly, and sometimes so many eggs filled the reproductive tract of a mother dinosaur prior to laying that an egg might be pushed back up the oviduct where it would be coated in another shell coating. This pathology is often seen among crocodylians and other reptiles, but is rarer among birds, and the idea that sauropods laid eggs in large clutches seems to fit the nests attributed to these dinosaurs. Dinosaurs like Brachiosaurus and Mamenchisaurus laid nests of multiple eggs which were relatively small compared to their body size, so it is possible that they deposited entire clutches, while smaller dinosaurs such as oviraptorosaurs could lay a limited number of eggs at a time.

There is much we don’t know about dinosaur sex. From possible positions to anatomy, mysteries abound. But the subject has moved beyond silly speculation. A better understanding of dinosaur evolutionary relationships has given paleontologists a framework from which to hypothesize about different aspects of dinosaur reproduction, and those ideas have been tested by discoveries in the fossil record. Future finds and analyses will undoubtedly flesh out some of the remaining unknowns. We are only just beginning to discover some of the most intimate secrets of dinosaur lives.

This is the final installment of the dinosaur sex series. For more, please see my Smithsonian article “Everything you wanted to know about dinosaur sex” and the previous entries in the series:

How did the biggest dinosaurs get it on?

Sex and Dinosaur Necks

Intimate Secrets of Dinosaur Lives

References:

Brennan, P., Birkhead, T., Zyskowski, K., van der Waag, J., & Prum, R. (2008). Independent evolutionary reductions of the phallus in basal birds Journal of Avian Biology, 39 (5), 487-492 DOI: 10.1111/j.0908-8857.2008.04610.x

Brennan, P., Prum, R., McCracken, K., Sorenson, M., Wilson, R., & Birkhead, T. (2007). Coevolution of Male and Female Genital Morphology in Waterfowl PLoS ONE, 2 (5) DOI: 10.1371/journal.pone.0000418

Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs. Bloomington: Indiana University Press. pp. 78-81

McCracken, K. (2000). The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) The Auk, 117 (3) DOI: 10.1642/0004-8038(2000)1172.0.CO;2

Sato, T., Cheng, Y., Wu, X., Zelenitsky, D.K., Hsaiao, Y (2005). A Pair of Shelled Eggs Inside A Female Dinosaur Science, 308 (5720), 375-375 DOI: 10.1126/science.1110578

Wang, S., & Dodson, P. (2006). Estimating the diversity of dinosaurs Proceedings of the National Academy of Sciences, 103 (37), 13601-13605 DOI: 10.1073/pnas.0606028103

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About Brian Switek
Brian Switek

Brian Switek is a freelance science writer specializing in evolution, paleontology, and natural history. He writes regularly for National Geographic's Phenomena blog as Laelaps.

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