The words of the Hedgehog song notwithstanding, naturalists realised as late as the 1960's that nobody had actually officially witnessed the mating of hedgehogs. Everybody had just made presumptions about how it was done, the biggest of which was that at some point somebody else had verified the presumptions by actual observation. (They do it very carefully, don't they? It's one of those Well-Known Facts, innit?) After all, there is no shortage of baby hedgehogs, so the species must have at some point worked out the mechanics of the business for themselves...
So somewhat voyeuristic observations were made of hedgehogs, porcupines and spiny echidnae in moments of private intimacy, and the Well-Known Fact was scientifically verified and validated: creatures blessed with spiny protection on all upper surfaces, but which have a soft vulnerable underbelly, do indeed Do It Very Carefully.
By analogy, what must have evaded Discworld philosophers and the astronomers of Krull is that star turtles must have a similar vested interest in Doing It Very Carefully, in this case lest things spill. (such as fifth elephants?) After all, at least one world turtle still exists, and we know of eight others which are still in the infant stage. What must help the astrochelonians is that there is of course no gravity in space: Great A'tuin can bank, swerve, and in extreme cases loop-the-loop, either to break the monotony or to avoid cosmic hazards, and the relative positions of elephants and discworld remains unchanged and people on the disc see or feel nothing unusual, except for the fact it must confuse the hell out of astronomers... and of course, there may be mating positions available in Space which are not normally an option on the Disc, where "male mounts-female-from-rear" would appear to be the standard option. (And obviously the philosophers have extrapolated from observations of turtles and other wildlife at ground level, to presume that this is the only way Star Turtles would mate. A moment's thought about possibilities inherent in gravity-free deep space might suggest less drastic methods with less potential damage to cargo carried on the shell. And in all probability, a lot more fun for the turtles, who wouldn't normally get much...) --AgProv 11:17, 28 June 2007 (CEST)
But the outer space of the dicsworld gravity-free? In "The Colour of Magic", it is mentioned that Tethys fell of the edge of his world and fell for some time, until he landed on the Discworld. This would indicate a universal direction to gravity, which would make the belly-to-belly impossible, or at least make the bottom world turtle become a world-less turtle.
Universal gravity doesn't necessarily make Great A'Tuin's swerves and loop-de-loops a problem, as long as the elephants and disc is on the inward curve, Great A'Tuin could balance out the centrifugal force with the "altered" gravity. (unsigned comment by Furbo 12 January 2012)
Based on the assertion Great A'Tuin had five elephants and lost one, and this makes it unusual, the fact the baby world turtles had four elephants indicates Lamarckian evolution takes place in world turtles. The infants inherit the characteristics parents acquire during their life. Or possibly having five elephants was unusual and the infants bred true with only four. Or, least likely of all, there was a slight error the description of the infant turtles. (unsigned comment by IJuggler 21 January 2019)