9 September 2006

Tyrannosaurus sex

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They dominated the Earth for 150 million years, but we know little about how they reproduced. Now, a band of brave scientists is trying to find out.
Tyrannosaurus sex

A Tyranosaurus rex female plays with her infants Credit: Kevin Stead/COSMOS

They were massive, lumbering creatures who were masters of all they surveyed. And while they may have been some of the most successful ceatures to have ever lived, few palaeontologists can tell you how they moved heaven and earth. How did the ‘terrible lizards’ have sex without crushing one another or becoming hopelessly tangled?

It may sound a like a question to provoke a giggle among schoolchildren, but it is actually a serious scientific concern. Success as a species involves being successful in reproduction, so obviously they were doing something right. But what, exactly?

How did spiny stegosaurs mate without stabbing each other to death? And where did Tyranosaurus rex stow his crown jewels – or did he let it all hang out?

The reason so little is known is that the soft body organs that would have been involved in reproduction do not preserve well, so the fossil record offers scant clues. This leaves palaeontologists in relatively unknown territory. Even the BBC television series, Walking with Dinosaurs, contains only one moment of passion between two Jurassic giants, a male mounting his mate.

Lovemaking must have required some delicacy and careful positioning since palaeontologists believe most dinosaurs did not have a true penis. Their genitalia, like those of modern birds and reptiles, are thought to have been tucked up beneath their tail in a small vent called the cloaca, Latin for sewer.

Cloacal organs in birds, reptiles and amphibians, are used for copulation, urination and defecation, and do not show on the outside.

If dinosaurs also had cloaca, according to theory penetration would have occurred when the male cloaca filled up with blood and bulged out into the cloaca of the female – much like a couple of plumber’s plungers pushing against each other. But lining up two cloacas was not as easy as you may think, particularly for dinosaurs the size of a house trying to line up cloacal openings that might have been only 20 cm in diametre and were tucked away under their thick tails, which could not be twisted easily.

The male would have had to move the opening of his cloaca close to that of the female so that his sperm could enter the female cloaca in what is referred to as a ‘cloacal kiss’. Today in animals with cloacas, this exchange can happen very fast, sometimes in just a few seconds.

Cloacal kissers may have experienced less difficulty in water, a lake or muddy flat, using buoyancy to overcome their crushing weight during the careful docking manoeuvre.

Sauroposeidon, the biggest dinosaur to walk the Earth, weighing in at 60 tonnes and growing up to 18 metres tall, certainly would have headed for the nearest water body when the heat was on.

Living 110 million years ago, the long-necked dinosaur inhabited the delta of a massive river system in what is now Oklahoma in the United States, where the giant fossils – each neck bone being about 120cm long – were unearthed in 1994.

Other dinosaurs, which feared the water, may have found ways around obstacles such as a big tail, spikes or club-shaped tail, in the same way some modern animals such as whales, echidna and giraffe do.

This article is inspired by a series of drawings of dinosaurs in different mating positions, including in water, by the late British palaeontologist L. Beverly Halstead, who believed all dinosaurs used pretty much the same mating position: “Mounting from the rear, [the male] put his forelimbs on her shoulders, lifting one hind limb across her back and twisting his tail under hers to align the cloaca.”


In an article in February 1988, the (now defunct) U.S. science magaaine Omni dedicated several pages to examining the plausibility of Halstead’s view of dinosaur sex. In it, he said the ancient animals were hampered by what palaeontologists called the ‘golden rule’: rear-mounting males always had to keep one foot on the ground to avoid crushing their mates.

“Their mating had to be done with great delicacy and great precision. It must have been utterly charming to watch, quite unlike our own species.”

Halstead’s article, which was illustrated by wildlife artists Sandy Fritz and Ron Embleton, entertained the idea that if dinosaurs “humped like birds, they’d have to have got past that thick, powerful tail; and for that they would need a corkscrew-shaped penis about three metres long.

“Yet there is no evidence they had such a grandiose organ,” Halstead explained. “Maybe they lay side to side, male to port and female to starboard, and sort of snuggled up together, bottom to bottom.”

That is the position Roger Seymour, professor in the department of environmental biology at the University of Adelaide in South Australia, is betting on. “I cannot direct you to any evidence of how dinosaurs did it,” he said. “It is only recently we have learnt how to determine the sex of dinosaur fossils.”

Seymour said without soft body parts to confirm dinosaurs even had cloacas, it was difficult to speculate accurately about how dinosaurs mated: “Nor do we have a set of six footprints to work from.”

But whether ankylosaur or armadillo, all cumbersome lovers had to follow basic physical laws.

In 1994, during a 15-minute presentation called, “Love’s Labours Lost: Mating in Large Dinosaurs”, Stuart O. Landry Jr, emeritus professor of biology at the State University of New York in Binghamton, told delegates attending an international conference of vertebrate morphologists at the University of Chicago that dinosaurs must have found non-standing positions to ensure a smooth exchange of gametes.

Having turned to the sex life of dinosaurs after the American Museum of Natural History in New York produced a poster depicting a rearing dinosaur defending its young, Landry believed big dinosaurs did it like crocodiles, in water or mud, to achieve the closest thing to weightless sex. He said the image of a giant dinosaur rearing up on two legs was highly unlikely. The heart would not be able to pump enough blood that far uphill to supply the brain.

Roger Seymour agreed. “If you calculate the vertical distance between where the heart is and where the head is – and some of these animals grew to up to 9 metres – you can measure accurately what the minimum pressure is at the bottom of a 9-metre column of blood. It works out to be seven times the normal mammalian blood pressure.

“Rear mounting is not a big problem if one keeps the neck horizontal,” he said, referring to recent studies of the lanky giraffe, whose blood pressure has been calculated to be about twice that of other mammals and whose hearts are proportionately 75 per cent larger.

Though water seems be the best medium in which big dinosaurs could mate without being crushed to death, what of terrestrial lovers? Stegosaurs and triceratops had more to worry about because they were covered in a lot of pointy bits, spikes and armour to ward off attackers.


Kenneth Carpenter in his 1999 book, Eggs, Nests and Baby Dinosaurs: A Look at Dinosaur Reproduction asks, “You want to know how a pair of three-ton stegosaurus did it? Probably like porcupines: very carefully.”

He suggests that ‘housecat-style’ could have been in the dinosaur armoury. “How stegosaurus, with the big plates on his back, managed to have sex is really not that difficult,” he wrote. “With the female squatting in the front while standing on her hind legs, the male could easily rest his forelimbs on one side of her broad pelvis.”

Roger Seymour is not so sure and after all, his team was the first to study mating echidna. “They do it very carefully. The female echidna rolls on to her side, as does the male who tucks its tail under the female’s and they go for it for at least four hours.”

They only mate once a year. So getting it right can mean the difference between life and a deadly splinter.

Modern megafauna such as the rhinoceros and elephant provide some insights, except they don’t have long, thick tails to contend with. A 1991 scientific paper, entitled “Fusion of Caudal Vertebrae in Late Jurassic Sauropods”, found that fusions of the tail vertebrae closest to the pelvic girdle were common among aptosaurs, diplodocus and camarasaurs. Why? Possibly, so their tails wouldn’t snap off during mating.

Authors Bruce M. Rothschild, of Northeastern Ohio Universities College of Medicine in Rootstow and his colleague David Berman suspect this fusion may have occurred only in female dinosaurs that could lift and arch their tails during mating.

“This probably made sex quicker and easier,” according to palaeontologist John Long, head of science at the Melbourne Museum ib Australia. A fossil fish expert, Long said the origins of sexual reproduction in vertebrates could be found in early fish such as the shark-like placoderms, in whom males had external clasping organs while females had wide pelvic plates.

Long said dinosaur eggs and nests were providing new insights into the social interaction of the giant beasts. Dinosaurs were prolific egg-layers and very protective of their young. They also built elaborate nests in grounds used seasonally by herds of different species.

But he added that because there is no fossil evidence for sexual organs, all the theories about dinosaur sex are just guesswork. He notes that some dinosaurs may well have had a penis that was tucked away when not in use – much like modern whales – but being soft tissue, we just don’t know. While very little is known about the mechanics of mating in dinosaurs, Long, in his 1995 The Rise of Fishes, describes reproduction as a driving force in evolution, “paramount to the continuation of the species”.

Unfortunately, that continuity was interrupted abruptly about 65 million years ago when a giant asteroid struck the Earth and wiped out anything weighing more than 25 kg. The age of the dinosaurs ended and the planet’s most successful lovers took their sexual secrets with them.

Carmelo Amalfi is a science writer in Perth, Western Australia.
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