Jack Cohen, who has died aged 85, was a reproductive biologist known for his theory of why men produce so many redundant sperm; he was also well known in the sci-fi world as an “alien creature designer” for science fiction writers and as the co-author, with Sir Terry Pratchett and Ian Stewart, of four Science of Discworld books which used the setting of Pratchett’s cult fantasy terrain to explain the real science of our universe.

Cohen first met Pratchett in the 1980s when he accidentally spilt a pint of beer over Pratchett’s lap at a science fiction convention. The friendship was subsequently sealed at a convention in The Hague, at which members of an audience of sci-fi fans objected when Pratchett, seeking to defend a fellow author who had earned their hostility, pointed out that he and his co-guests were so rich they did not need to attend such conventions, but were doing so out of the goodness of their hearts.

“Someone threw a tomato and it got him,” Cohen recalled. “Terry lost it. ‘What the f--- do you think you’re doing,’ he said, and really went over the top. I stood up and said ‘shut up’. I was at the back right of the audience and all eyes turned towards me. I said, extemporising wildly: ‘Money is like air and love. If you’ve got it, it doesn’t matter. If you haven’t got it, that’s desperate.’

“Everyone stood up and clapped and Terry said: ‘Is that Jack Cohen? Then I’ll buy you a drink.’”

As their friendship developed Cohen introduced Pratchett to his friend Ian Stewart, a professor of mathematics and colleague at Warwick University, with whom he had written several books including Figments of Reality: The Evolution of the Curious Mind (1997). Together they developed the idea of publishing works of popular science based on Pratchett’s Discworld, where everything runs on magic.