Most mosses leave reproduction to luck. They shed spores by the million and trust the wind to drop a few somewhere damp enough to grow. One small family of bog mosses works to a different plan. Its members grow on dung and on the remains of dead animals, and instead of gambling on the breeze they recruit flies to ferry their spores from one dropping to the next. These are the dung mosses, and they are among the strangest plants you can find on a British moor.
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A capsule built like a flower
The family is the Splachnaceae, and what marks them out sits just below the spore capsule. In an ordinary moss the capsule tapers smoothly into its stalk. In a dung moss the base of the capsule swells into a broad coloured collar, the hypophysis, which in the showiest species is bigger and brighter than the spore-bearing part above it. The yellow parasol (Splachnum luteum) spreads a disc of clear lemon yellow on a tall red stalk; in S. rubrum the same skirt is a deep rose pink. Both are creatures of the far north, boreal and arctic bog mosses that reach Britain barely if at all, so a Scottish record of either counts as a real event. The collar does the same job wherever the moss grows, though. It works the way a flower works, as a signal.
The smell, and the flies that answer it
Mosses do not flower, as do mosses flower sets out, yet the dung mosses have arrived at something close to a flower's trick by a different road. The coloured collar is held up to be seen against dark peat, and it gives off a scent that insects read as fresh dung or carrion. Flies come to it expecting a meal and a place to lay their eggs. As they crawl over the capsule they pick up the spores, which are produced in sticky masses rather than as dry dust and cling to a fly's legs and body. The insect then heads off to the next cowpat or carcass, carrying the spores with it and depositing them more or less exactly where the moss can use them.
A living made from droppings
That targeting is the whole point. Dung and dead animals are rich, short-lived islands of nutrients dotted across ground that is otherwise poor and acid, and wind-blown spores would almost never land on one. A courier that actively seeks dung out is a far surer bet. Different members of the family take different substrates. Splachnum ampullaceum, the commonest in Britain, favours the droppings of sheep, cattle and deer in wet bogs, while the genus Tetraplodon turns up on bones, old carcasses, owl pellets and the scats of foxes and other meat-eaters. The nitrogen locked in those remains is what feeds a colony, and once the dung has rotted away the moss goes with it.
Where to look in Britain
Temper your hopes. The gaudy parasol mosses belong to Scandinavia and to the remotest corners of the Scottish Highlands, and several have declined badly as bogs were drained and grazing patterns changed. The one a hill-walker might realistically meet is Splachnum ampullaceum, which raises small flask-shaped capsules with a reddish-brown collar on old herbivore dung in acid mires and flushes, mostly in the north and west. I have gone looking for it more than once on upland bog and come back without it, which says something about how local it has become. Any dung moss in fruit is worth stopping and kneeling for.
Finding and naming them
Should you come across a moss fruiting straight out of a dropping on a moor, look first at the swollen collar below the capsule and note its colour and shape, because that carries most of the identification. Photograph it where it sits and leave it alone; these are scarce, slow plants, and lifting one does nobody any good. A record passed to your county recorder is worth far more than a specimen in a bag, and recording moss explains how that is done. For the broader tangle of creatures that depend on moss, from the flies here to the birds that nest in it, moss and wildlife picks the thread up.