My wife and I stayed once in a bed-and-breakfast in rural Tasmania, the rugged island off the southern coast of Australia. A sheep rancher had built a guest cottage in the middle of his fields, and for an extra fee, lodgers could take a meal in the ranch house. Knowing we would probably never eat fresher lamb, we signed on.

Over dinner I asked innocently about the odd coloring—orange, red, blue, and green swatches—we had seen on the rumps of his sheep. "Ah, that's how we tell when the ewes mated," he explained with a laugh. "I hang a container of colored chalk in a rather strategic place on my rams. He leaves his mark when he does his duty, and that way I know that all the ewes with orange rumps were serviced on the 21st. When the due date rolls around—sheep deliver right on time and they're 100 percent fertile—I can herd the orange ewes into the barn and give them special care."

In the next few minutes I learned much more about the reproductive habits of sheep. Each ewe has only a six-hour window of receptivity to mating. This poses no problem to the ram, who has an infallible sense of which ewe might welcome him at any given moment. The rancher relied on 10 rams to service 4,000 female sheep, which meant that the rams worked themselves to exhaustion over several weeks, losing much of their body weight in the process. All work, no romance.

When I saw a scrawny, bedraggled ram—his chores done, his strength dissipated, good for nothing but the slaughterhouse, and even then unfit for human consumption—I breathed a prayer of thanks to God for adjusting the terms of sexuality in the human species. (Zoologists note that very few species—dolphins, some primates, and the large cats—engage in sex as a form of pleasure.)

The next morning ...