A Tyranosaurus rex female plays with her infants

Credit: Kevin Stead/COSMOS

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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."