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A sculpture of Pentaceratops outside the New Mexico Museum of Natural History and Science. Could sexual selection account for the prominent ornaments of this dinosaur? Photo by the author.

Non-avian dinosaurs were weird. That’s one of the reasons we love them so much. There’s nothing quite like a slender-necked Barosaurus, a beautifully-crested Dilophosaurus or lavishly-ornamented Pentaceratops alive today. If such dinosaurs were anything, they were bizarre, but why were they so strange? Each case demands its own explanation, and paleontologists have continuously tussled over whether particular ornaments were weapons, sexual displays or something else.

According to an in-press paper at Trends in Ecology & Evolution, at least some weird dinosaur features may best be understood in the context of mate competition, mate choice and sexual signalling. The paper, by entomologist Robert Knell and colleagues, is the latest in a long-running debate over whether sexual selection had any influence on dinosaur lives and how to detect the hallmark of such pressures.

The debate has been going on for years but only recently increased in intensity. In a 2010 study, paleontologists Kevin Padian and Jack Horner rightly noted that sexual dimorphism–or a significant anatomical difference between the sexes–has never been conclusively demonstrated among non-avian dinosaurs. The idea had been proposed for a variety of dinosaurs using a number of skeletal landmarks, but none of the hypotheses have stuck. Even if sexual dimorphism existed among dinosaurs, we lack the sample size to identify the phenomenon. More than that, Padian and Horner cited the lack of sexual dimorphism as a sign that sexual selection probably wasn’t an important facet in the origin and modification of bizarre dinosaur features. Instead, the researchers hypothesized, the various horns, crests, plates and other ornaments evolved because of species recognition–the ability for dinosaurs to quickly and easily identify members of their own species.

Other researchers disagreed. Knell and Scott Sampson had a brief exchange in the journal pages with Padian and Horner. This was followed by a paper by Dave Hone and co-authors that suggested that mutual sexual selection might explain the mystery of why dinosaurs had bizarre ornaments but don’t seem to exhibit sexual dimorphism. Under this hypothesis, both males and females may prefer mates with elaborate visual signals, and therefore the same prominent structures would be expressed in both sexes. This kind of sexual selection has been documented in modern avian dinosaurs, but, until now, hasn’t been considered as an explanation for the ornamentation of non-avian dinosaurs. Even though mutual sexual selection has not been proven as an evolutionary driver among extinct dinosaurs, it’s a possibility worth considering.

The new paper by Knell and co-authors also draws on modern examples to investigate how we might identify examples of sexual selection among prehistoric species. The paper covers a wide variety of creatures, from ammonites to birds, but, since this is the “Dinosaur Tracking” blog, I’ll focus on how the argument applies to the ever-controversial adornments of non-avian dinosaurs.

As the researchers state, there’s no simple, tell-tale way to identify sexual selection. This is partly because many strange structures are multifunctional, and structures may be co-opted for different functions during the course of their evolution. Think of sauropods. The elongated necks of these dinosaurs allowed them to feed over a wide swath of greenery, but they could have also been used as visual displays. A big fleshy neck is prime advertising space. In this case, a feeding advantage appears to have preceded any signalling function, but the mosaic nature of evolution hinders our efforts to tease apart the influence of different, interacting pressures.

All the same, there are a few clues that can help paleontologists identify possible cases where sexual selection was at play in the deep past. One possible line of investigation is sexual dimorphism, although, as I said above, this has yet to be conclusively demonstrated in dinosaurs. (And, as Knell and co-authors argue, sometimes the sexes might differ for reasons other than sexual selection.) The way prominent displays grew is another phenomenon worth looking into. We would expect that features that make a difference in mating would only appear as the dinosaur approached sexual maturity. Juvenile, and presumably sexually-immature, Lambeosaurus don’t have the full-blown crests of older individuals. Perhaps this is because the crests are sexual signals that only grow as the dinosaurs approach mating age, although it’s possible that the development of crests are related to the overall growth of the dinosaur’s skeleton.

The diversity–or disparity–of ornament shapes among closely-related species may also be important. Even closely-related species of ceratopsid dinosaurs, Knell and collaborators note, had very different horn shapes and arrangements. This could be a sign of sexual selection by way of competition and mate choice, but, as Padian and Horner pointed out, the same evolutionary pattern could be the result of selection for distinct-looking species. Finally, Knell and co-authors cite “costliness” as another potential indicator–if a trait is flashy, requires a good deal of energy to grow and comes at a cost to the organism’s survival potential, then it may be a sexually-selected trait.

Obviously, each line of evidence comes with caveats. Sexual selection can be difficult to identify even among living species, much less extinct ones. It would be strange if sexual selection played no role in dinosaur evolution, but we’re left with the question of how to detect and test the hypothesis of sexual selection. Paleontologists will have to very carefully test hypotheses about bizarre structures, paying careful attention to distinguish between competing alternatives. Ultimately, paleontologists may only be able to identify possible scenarios for the origin and evolution of bizarre features, but studies of modern species can at least provide guidelines for what researchers should look out for.

If we’re truly going to understand the visual signals of dinosaurs, though, we need better sample sizes. We need to know how individuals of the same species varied from one life stage to the next. Without this anatomical foundation, researchers will be left to argue from a typological standpoint that may misconstrue how certain features changed with age and evolved over time. Recall the “Toroceratops” debate–if Triceratops changed into a Torosaurus-form late in life, most likely beyond the onset of sexual maturity, that is certainly going to influence how paleontologists investigate and discuss dinosaur visual signals.

The influence of sexual selection, or lack thereof, will undoubtedly be debated for some time to come. But, as Knell and colleagues conclude, investigating the possible influence of sexual selection in prehistory “is neither a forlorn nor impossible task.” We may yet find out what’s sexy to a dinosaur.

For more on this study, see this post by Dave Hone, one of the paper’s authors.

[My thanks to Darren Naish, another of the paper's authors, for sending me the new study.]

Reference:

Knell, R., Naish, D., Tomkins, J., Hone, D. (2012) Sexual selection in prehistoric animals: detection and implications, Trends in Ecology & Evolution DOI: 10.1016/j.tree.2012.07.015.

Will we ever find mating dinosaurs? No one knows, but the discovery of fossils showing dinosaur behavior—such as this nesting oviraptorosaur—hint that we may someday find dinosaur sex set in stone. Photo by Steve Starer, image from Wikipedia.

Earlier this month, I wrote a short article for Nature News about 47-million-year-old turtles that died at a very inopportune moment. Several pairs of prehistoric turtle were fossilized in the act of mating—the tragic consequence of sinking to the toxic depths of a prehistoric lake. An unfortunate fate for the reptiles, but a boon for the paleontologists who found the sexy fossils.

The discovery got me thinking about dinosaur sex. I’ve written quite a bit about the topic before—I ran a four-part series on what we know about dinosaur nooky earlier this year—but much of what we know about dinosaur reproduction only outlines the mating habits of Apatosaurus and company. There’s still a lot we don’t know. In fact, some of the most basic questions are the most persistent. What, exactly, “dinosaur style” looked like has been a subject of frequent speculation but very little rigorous research, and no dinosaurs have ever been found fossilized in the act to show us how it was done. But does this mean that we’ll never find dinosaur sex preserved in stone?

Copulation is typically a brief moment in time. For such an intimate snapshot to become part of the fossil record, exceptional circumstances are required. In the case of 320-million-year-old sharks preserved in what may be part of a mating ritual, a quick death and rapid burial in fine-grained sediment locked the fishy forms in rock. We also know a little about how prehistoric insects reproduced thanks to mating pairs trapped in amber. And as for the turtles, the copulating reptiles drifted down to a layer of water that not only killed them, but kept their bodies safe from scavengers as sediment settled on their bodies. For sex to make it into the fossil record, a quick death, rapid burial and high-definition preservation are all required.

Given these conditions, I’m not very hopeful that paleontologists are going to find mating dinosaurs. Even the smallest dinosaurs were too big to be trapped in amber, and as fully-terrestrial animals, dinosaurs did not copulate in the sort of aquatic environment where fast death and burial would have been possible. Dinosaurs just didn’t mate in the kind of habitats where there was a high potential for the amorous pairs to perish and be entombed in sediment. Good news for them, but frustrating for paleontologists.

Still, I shouldn’t be too hasty in saying that we’ll never find mating dinosaurs. I never expected that paleontologists would discover turtles caught in the act, for one thing. And the fossil record is full of surprises, including fossils that detail some aspects of dinosaur behavior. Paleontologists have previously discovered dinosaurs preserved in nesting and sleeping positions, and there’s the fighting dinosaur pair. Maybe someday a fortunate paleontologist will help us solve the prehistoric mating mystery by finding dinosaurs that made love, not war.

A restoration of Lambeosaurus magnicristatus, a dinosaur once thought to represent the male form of Lambeosaurus lambei, but now known to be a distinct species. Art by Богданов, via Wikipedia.

Figuring out how dinosaurs mated is a frustrating task. There is relatively little that can be gleaned from the fossil record, and much of what paleontologists suspect about behavior and soft tissue anatomy comes from comparisons to birds (specialized, living dinosaurs) and crocodylians (the closest living relatives to the dinosauria). Even worse, exactly how to tell male and female dinosaurs apart from one another has puzzled scientists have decades. If we even can’t sort the females and the males, how can we accurately envision dinosaurian sex?

For a time, it seemed like the skeletal construction of dinosaurs might hold the answer. The clue paleontologists were looking for was sexual dimorphism. This is a difference between males and females of the same species as expressed in secondary characteristics—not the fiddly bits actually used during mating, but traits like size, bizarre ornamentation, coloration and similar features. Detecting such differences in dinosaurs requires a large sample of individuals of the same species which are about the same age and come from the same time and place (the more closely a paleontologist can approximate a population in a sample, the better). If such a group can be separated out into two distinct types—say, with one being larger than the other and with a larger crest—then there is a possibility that those two forms represent females and males.

Paleontologists have hypothesized sexual dimorphism for multiple dinosaur species, from Protoceratops to Tyrannosaurus. None of the proposed cases is especially well supported. What might seem to be a split between robust and gracile forms of a species—often taken as males and females, respectively—might actually represent different growth stages of the same dinosaur, different species of dinosaur, or individual variation in a small sample size.

The case of Lambeosaurus is a good example of the difficulties involved in distinguishing the dinosaur sexes. In 1975, paleontologist Peter Dodson undertook a review of the many, many species of hadrosaur described from the roughly 77-million-year-old strata of Alberta, Canada’s Oldman Formation. Paleontologists had named three genera and twelve species of crested hadrosaurs from this area, but after comparing the skulls of these dinosaurs, Dodson concluded that only the dinosaurs Corythosaurus casuarius, Lambeosaurus lambei and Lambeosaurus magnicristatus were present. More than that, Dodson proposed that he had discovered sexual dimorphism in each of these species, with the anatomy of these dinosaur’s crests being the primary way to tell females from males.

But paleontologists David Evans and Robert Reisz found a different pattern when they re-examined the sample of Lambeosaurus from Alberta. In the case of Lambeosaurus magnicristatus, in particular, a combination of a small sample size and an incomplete fossil had caused the confusion. Dodson included only two individuals of the hadrosaur species in the study, and since the crest of one individual was larger than that of the other, concluded that the two skulls represented the two sexes. As Evans and Reisz pointed out, the crest of the specimen Dodson regarded as female had been broken and so seemed superficially smaller. If the missing part had been in place, the difference between the two individuals would have disappeared.

Other paleontologists suggested different dimorphic schemes. James Hopson proposed that individuals grouped under the species Lambeosaurus lambei were females, and the species Lambeosaurus magnicristatus were males, and Kenneth Carpenter advocated a similar lumping. The problem with such a scenario is that the two species are not found at the same stratigraphic level. Evans and Reisz pointed out that Lambeosaurus lambei is found in greater numbers at a lower geologic level than the much rarer Lambeosaurus magnicristatus. The species did not overlap and so cannot represent different sexes of the same species.

Other attempts to set sex differences for dinosaurs have met similar frustrations. For a time, it was thought that male and female Tyrannosaurus could be distinguished on the basis of a tiny bone at the base of the tail. A row of small, spike-like bones called chevrons runs beneath much of the tail in dinosaurs, and it was thought that the first chevron in female Tyrannosaurus—the one closest to the hips—was reduced in size so that eggs could more easily pass out of the body. A similar observation had been reported before in crocodylians, and the fact that the trait seemed to be associated with larger specimens of Tyrannosaurus appeared to indicate that female tyrants were more robust than males of the same age. But this turned out to be a mistake. Crocodylian expert Gregory Erickson and colleagues discovered that the report on the reduced chevron in crocodylians was in error, and the fact that a complete chevron was found in the huge Tyrannosaurus “Sue” further eliminated the connection between the bone and sex identification.

As paleontologists Kevin Padian and Jack Horner pointed out in a Journal of Zoology paper published last year, sexual dimorphism “has never been conclusively established in dinosaurs.” Yet there is a way to identify at least one of the dinosaur sexes. The clues can’t be seen in the gross anatomy of skeletons or flashy ornaments, but in the structure of dinosaur bones.

In 2005, researchers Mary Schweitzer, Jennifer Wittmeyer and Jack Horner reported that they had found “gender-specific reproductive tissue” in a Tyrannosaurus specimen given the name “B-rex.” The specific type of tissue, called medullary bone, indicated that the particular dinosaur was female. Comparison to modern birds provided the key to this puzzle. Medullary tissue forms inside the long bones as a source of calcium when female birds are laying eggs. The same tissue is not naturally found in males. While there is no methodology to identify male dinosaurs in a similar way, the presence of medullary tissue inside dinosaur limb bones can be used to identify egg-laying females.

Paleontologists Andrew Lee and Sarah Werning ran with this finding to investigate how dinosaurs reached sexual maturity. Not only did Lee and Werning find medullary bone in two other dinosaurs—the ornithischian herbivore Tenontosaurus and the theropod Allosaurus—but, by combining these findings with evidence of dinosaur growth, they found that dinosaurs began reproducing when they were still actively growing. Tenontosaurus, Allosaurus and Tyrannosaurus had the dinosaurian equivalents of teen pregnancies, and this finding fit with the idea that dinosaurs lived fast and died young. Dinosaurs started having sex before they were skeletally mature, which corresponds to a lifestyle of rapid growth and a high likelihood of death before reaching maximum body size.

With any luck, future discoveries and studies of medullary bone will help us better understand when and how dinosaurs reproduced. Perhaps, paired with analyses of dinosaur skeletal anatomy, this peculiar type of bone may even help test ideas about sexual dimorphism in dinosaurs. If you can identify at least some female dinosaurs in a sample, you can then look to see if that subgroup contains any particular skeletal features that set them apart. The trouble is that medullary bone only works for identifying egg-laying females—males or females that are not reproducing cannot be distinguished this way. Still, the fact that paleontologists are able to pick out even a few female dinosaurs is a wonderful discovery that has the potential to show us previously unknown aspects of dinosaur biology. We are only just beginning to learn the more intimate secrets of dinosaur lives.

This post is the third in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

Previous Installments:

How did the biggest dinosaurs get it on?

Sex and Dinosaur Necks

References:

Dodson, P. 1975. Taxonomic implications of relative growth in lambeosaurine hadrosaurs. Systematic Zoology, 24 (1), 37-54

Erickson, G., Kristopher Lappin, A., & Larson, P. (2005). Androgynous rex – The utility of chevrons for determining the sex of crocodilians and non-avian dinosaurs Zoology, 108 (4), 277-286 DOI: 10.1016/j.zool.2005.08.001

Evans, D., & Reisz, R. (2007). Anatomy and Relationships of Lambeosaurus magnicristatus, a crested hadrosaurid dinosaur (Ornithischia) from the Dinosaur Park Formation, Alberta Journal of Vertebrate Paleontology, 27 (2), 373-393 DOI: 10.1671/0272-4634(2007)27[373:AAROLM]2.0.CO;2

Lee, A., & Werning, S. (2008). From the Cover: Sexual maturity in growing dinosaurs does not fit reptilian growth models Proceedings of the National Academy of Sciences, 105 (2), 582-587 DOI: 10.1073/pnas.0708903105

Padian, K., & Horner, J. (2011). The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? Journal of Zoology, 283 (1), 3-17 DOI: 10.1111/j.1469-7998.2010.00719.x

Schweitzer, M., Wittemeyer, J., Horner, J. (2005). Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex Science, 308 (5727), 1456-1460 DOI: 10.1126/science.1112158

Did sexual selection cause sauropods, such as this Barosaurus at the Natural History Museum of Utah, to evolve ludicrously long necks? Photo by the author.

Yesterday I wrote about the possible mating mechanics of immense sauropod dinosaurs such as Brachiosaurus and Argentinosaurus. But there’s more to mating than the act itself. It is not as if two Diplodocus nonchalantly walked up to each other, had a quickie, and plodded off to feed on a nearby patch of ferns. There was probably some kind of behavioral lead-up to copulation—a way for one sex to strut its stuff and the other to be choosy about a mating partner. With this in mind, one paleontologist proposed that sex might hold the secret of why sauropods evolved such long, gorgeous necks.

The idea that mating behavior might have something to do with sauropod anatomy was inspired by giraffes. Scientists have been puzzling over why giraffes have such spectacular necks for over a century and a half. The most popular notion is that the long necks of the mammals are an adaptation for feeding high up in the trees where competing herbivores can’t reach, but in 1996 zoologists Robert Simmons and Lue Scheepers proposed something different.

Male giraffes fight each other in a peculiar form of combat called “necking.” It’s not as nice as it sounds. Male giraffes swing their long necks to batter each other with the stout ossicones on the tops of their heads. These bouts determine hierarchies among males, and dominant males mate more often than subordinate ones do. Since males with bigger, stronger necks would seem most likely to win the contests, Simmons and Scheepers argued, those males are more likely to pass on their traits to the next generation, and therefore necking might have been the reason why giraffes evolved longer necks. Female giraffes just happened to get evolutionarily carried along even though they don’t engage in the same behavior.

The “necks for sex” hypothesis has been controversial from the start. At present, the weight of the data supports the idea that giraffe necks primarily evolved as a way to sample a wide range of food, not as a weapon involved in battles for mating rights. Studies since 1996 have indicated that long necks really do help giraffes avoid competition with other species for the most nutritious food by going higher up, especially when food may be scarce, and studies of fossil giraffes hint that long necks may have began to evolve in response to changes involved with the spread of grasslands around 14 million years ago. Still, the idea proposed by Simmons and Scheepers has remained a sexy hypothesis, and in 2006 paleontologist Phil Senter applied the idea to Apatosaurus and kin in a paper called “Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation.”

Without living sauropods to study, Senter proposed six predictions for what a sexually-selected sauropod feature would look like. For example, on the basis of previous theoretical work, Senter suggested that a feature that was primarily used for display or mate competition would provide no benefit to the survival of the animal and might, in fact, be a risk. In the case of sauropods, Senter argued that the long necks of sauropods would not have provided the dinosaurs with any major advantage over other herbivores in terms of accessing food. Just as Simmons and Scheepers proposed that the long necks of giraffes did not provide a feeding benefit, Senter suggested the same for dinosaurs like Camarasaurus. As a corollary to that, Senter also pointed out that predatory dinosaurs must have targeted the long necks of sauropods to quickly bring down the giants. “The evolution of more neck, and hence more vulnerability to a fatal bite, therefore incurred a survival cost for all but the longest-limbed sauropods [which would have carried their necks higher off the ground],” Senter wrote.

Senter only briefly entertained how fancy, flashy sauropod necks might have been involved in dinosaur mating behavior. Maybe males smacked necks when fighting for territory, or perhaps competitors simply eyed each other to see whose neck was bigger. There was no way to tell. Overall, though, Senter believed that the necks of sauropods were more consistent with what would be expected for a sexually-selected feature than an adaptation for feeding.

However, in a paper published last year, paleontologists Mike Taylor, Dave Hone, Matt Wedel and Darren Naish refuted Senter’s arguments. The long necks of sauropod dinosaurs certainly could have provided survival benefits, particularly in terms of accessing high-quality foods that were beyond the reach of smaller herbivores. Senter had assumed that sauropods held their heads low to the ground and therefore would not have been capable of much vertical reach, but there is osteological evidence to the contrary. Sauropods were physically capable of holding their heads high, and may have favored an elevated neck posture most of the time. More than that, the exceptionally long necks of many sauropods would have provided an energetic benefit by allowing the animals to stand in one place while sampling food over a wide range.

Senter also overstated the vulnerability of dinosaurs like Barosaurus to attack. As Taylor and co-authors pointed out:

The [sauropod] neck was not simply a mass of external blood vessels and nerves, but was constructed from tough elements including the often robust cervical ribs, bony laminae, ligaments and tendons. A theropod could hardly dispatch a moving apatosaur with one swift bite, and a raised neck would further reduce vulnerability.

That is assuming that predatory dinosaurs hunted adult animals at all. Like many modern predators, Mesozoic hunters like Allosaurus and Torvosaurus probably targeted young, relatively small sauropods more frequently.

Underlying all of this, though, was a conceptual flaw in taking a hypothesis proposed for one species—the evolution of necks for sex in giraffes—and applying it to a disparate, wide-ranging, and long-lived clade of vertebrates. If the long necks of sauropods were so costly to evolve and provided no significant survival benefit, then why did so many dinosaurs retain this feature for so long? Taylor and co-authors summarized the rhetorical flaw this way:

If the long necks of sauropods had negative survival value, their retention across the whole clade is analogous to a hypothetical situation where the maladaptively long tails of birds-of-paradise are found throughout Passeriformes [perching birds, about half of known bird species], or where the enormous antlers of the Irish Elk Megaloceros are ubiquitous in Artiodactyla [even-toed, hoofed mammals and descendant forms, including whales].

The proportionally long necks of sauropods must have had some adaptive advantage for the trait to be so widespread and persistent. This doesn’t mean that sauropod necks were only used for feeding, though. As Taylor and co-authors pointed out, traits used in mate competition may also provide survival benefits. As the researchers stated, “It remains possible that the sauropod neck originally arose either as a sexually selected feature or to help gather food, but it cannot be demonstrated that the necks remained monofunctional throughout their evolution, or that they could not be co-opted for a secondary function.” The neck of the giraffe is a perfect example. Male giraffes swing their necks in competition, but their long necks have also been shown to provide them with a competitive edge when it comes to reaching food resources other herbivores just can’t exploit. The question is which impetus was more important in the trait’s evolution.

For sauropod dinosaurs, feeding ecology was more important than sexual selection in the evolution of long necks. But once long necks had evolved, who knows how they might have been used for communication and display? Such prominent necks would have been elongated, fleshy billboards which could very well have been used to establish dominance, attract mates, or otherwise advertise an individual’s prominence. If adult sauropods were too big to be harried by predators, and therefore did not require camouflage, would sauropods have developed bright, striking color patterns along their necks to gain the attention of potential mates and show off that they were the healthiest, sexiest dinosaurs around? Those are the kinds of questions that can keep a paleontologist up at night.

For more on sauropods and the “necks for sex” debate, see these posts by Darren Naish, Dave Hone and Mike Taylor.

This post is the second in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

Previous Installments:

How did the biggest dinosaurs get it on?

References:

Senter, P. (2006). Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation Journal of Zoology, 271 (1), 45-53 DOI: 10.1111/j.1469-7998.2006.00197.x

Taylor, M., Hone, D., Wedel, M., & Naish, D. (2011). The long necks of sauropods did not evolve primarily through sexual selection Journal of Zoology, 285 (2), 150-161 DOI: 10.1111/j.1469-7998.2011.00824.x

The backside of Diplodocus, photographed at the Utah Field House of Natural History. Photo by the author.

In 1991, the American Museum of Natural History unveiled one of the most fantastic fossil displays ever created. Placed at the center of the renovated Theodore Roosevelt Rotunda, an adult Barosaurus rears back to protect its offspring from an oncoming Allosaurus. The defending sauropod’s head is 50 feet up in the air, although whether or not such an immense, long-necked dinosaur could have pulled off such a feat has been a continuing point of contention. Even in a typical posture, Barosaurus must have had a powerful heart to pump blood along its 25-foot neck, and who knows how hard the dinosaur’s heart would have to work to continue bloodflow to the animal’s head if it reared up? Some paleontologists consider such a feat physically impossible, but as paleontologist William Gallagher pointed out while teaching my Paleontology 101 class at Rutgers University, male Barosaurus had a good reason to rear up. How else would the huge dinosaurs have positioned themselves to mate?

Exactly how dinosaurs got it on has inspired no small amount of speculation. The largest dinosaurs of all, the sauropods, have been especially perplexing. We often say that these dinosaurs “shook the earth” with their footsteps, but did they also make the bed rock with their lovemaking? (I apologize for that joke, and will keep the geology puns to a minimum. Promise.) Paleontologist Beverly Halstead famously wondered about dinosaur sex in public lectures and articles, and he suggested that standard “dinosaur style” was for a male to come alongside the female and throw its leg over the female’s back as she lifted her rump into the air to move her tail out of the way. In the case of sauropods such as Diplodocus, Halstead even imagined that the amorous dinosaurs might intertwine their tails. While other paleontologists have considered the tail-twisting aspect unlikely—sauropod tails were balancing organs and were too stiff to intimately coil around each other—the basic dinosaur position Halstead promoted has remained a prominent possibility for the dinosaur kama sutra.

But not everyone agreed that giants such as Apatosaurus and Brachiosaurus were capable of such nupital acrobatics. These animals were so immense—Apatosaurus, not even the largest sauropod, is estimated to have weighed more than 23 tons—that some researchers thought the kind of positions Halstead was promoting would give the dinosaurs fractured legs and broken spines. At a symposium of vertebrate morphologists held at the University of Chicago in 1994, biologist Stuart Landry, Jr. gave a short presentation entitled “Love’s Labors Lost: Mating in Large Dinosaurs.” He did not see how sauropods could have mated on land. A large, rearing sauropod, he told his audience, “would have to support 10 to 20 tons in a precarious position two or three meters off the ground.” A male Apatosaurus would be liable to tip over and possible take the female with him. Instead, Landry suggested that the largest dinosaurs looked for muholes or bodies of water to buoy themselves up. When a conference attendee asked if he was proposing that all dinosaurs mated in water, Landry responded, “I would say the very large ones must have.” Of course, this hypothesis required a large number of Jurassic and Cretaceous hot tubs of just the right depth for sauropods to reproduce, and scientific models of sauropods have suggested that these dinosaurs were actually quite buoyant and unstable in water. Sauropods were diverse, disparate and widespread animals that roamed in terrestrial habitats all over the world—there’s no reason to presume that the largest dinosaurs had to seek out the nearest deep lake when they got the itch.

Biomechanics expert R. McNeill Alexander also considered the weight problem in his 1989 book Dynamics of Dinosaurs & Other Extinct Giants, but came to a different conclusion. Even though a male sauropod would have rested a great deal of weight on the back of the female during mating, Alexander pointed out that the stresses and strains would not have been any more severe than those caused while the female dinosaur was walking. (After all, walking requires shifts in weight as the dinosaur balances and goes through each step cycle, and so a dinosaur’s skeleton had to be strong enough to cope with these shifts.) “If dinosaurs were strong enough to walk they were strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”

Without living specimens to observe, we will never know all the intimate details of sauropod sex. Still, there are only a limited number of positions that could have worked for the dinosaurs. For reasons that I’ll write about later this week, the consensus among paleontologists is that male dinosaurs probably had relatively small penises. (Shhh! Don’t tell Tyrannosaurus. He’s already upset about all those “useless forelimbs” jokes.)  An amorous male would have to position his cloaca—the orifice used for both expelling waste and mating in crocodylians, birds and probably dinosaurs—right up to the cloaca of a female, and the female’s tail would have undoubtedly presented an obstacle. Rather than simply leaning straight against the top of a female like an elephant or rhinoceros does, a male sauropod would probably have to rear up at a relatively oblique angle, and the female would have to assist by moving her tail (which is also a way in which female dinosaurs could have exerted mate choice and confounded any hot-under-the-collar males they would rather not mate with). Perhaps some museum will look into the problem and try to mount a pair of coupling sauropod skeletons—much like the mating Tyrannosaurus at Spain’s Jurassic Museum of Asturias—but even then we are limited by what we can imagine. Whether we want to imagine a pair of Brachiosaurus in flagrante delicto is another matter altogether.

This post is the first in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

References:

Alexander, R. M. 1989. Dynamics of Dinosaurs & Other Extinct Giants. New York: Columbia University Press. pp. 57-58

Anderson, J. The Perplexing Puzzle of Maladroit Mating. Chicago Tribune. August 30, 1994.