Swap pecorino for parmesan without adjusting anything else, and the dish turns out saltier — often by a lot. Both are hard, dry, aged grating cheeses, but pecorino is sheep's milk salted heavily as its main route to being shelf-stable in a matter of months, while parmesan is cow's milk given a year or more to reach the same stability instead. Pecorino Romano carries something like double the sodium of parmesan, and that one fact explains most of what people notice when they reach for the wrong one.
Why pecorino tastes saltier: it's built into the cheese, not added to the dish
Both cheeses have to survive months on a shelf, and salt and time are the two tools that make that possible. Parmesan gets there almost entirely through time: a wheel ages at least a year, often two or three, while enzymes slowly break its proteins down into free amino acids and the water available for anything to grow in keeps dropping. Pecorino Romano gets there mostly through salt. A wheel can be sold after as little as five months, and it's dry-salted hard, repeatedly, during that short window to make up for the aging it skips. That salt doesn't wash out later. It's why pecorino carries roughly double parmesan's sodium gram for gram, and why a dish built around it, like cacio e pepe, usually has no added salt in the recipe at all — the cheese already supplied it.
The tang isn't just salt — it's sheep fat
Cut the salt back and pecorino still wouldn't taste like parmesan, because the base fat is different too. Sheep's milk runs close to double the fat of cow's milk, and more of that fat is made up of short and branched-chain fatty acids that read as sharp and tangy on their own, the same family of compounds behind the assertive smell of sheep and goat cheeses generally. Parmesan's cow's-milk fat carries fewer of them, so its flavor leans on what the long aging built instead — free glutamic acid and amino acid crystals that taste savory and nutty rather than sharp. That's also where parmesan's occasional white crunch comes from: tyrosine crystallizing out after enough months, something a young pecorino rarely gets time to develop.
What to do with the one you have
Swapping parmesan into a recipe written for pecorino gives a milder, rounder result — safe, but add back a pinch of salt to cover what pecorino would have supplied. Swapping pecorino into a recipe written for parmesan does the opposite: taste before adding more salt, and expect the dish to land sharper and saltier than intended, especially in anything reduced or simmered where flavors concentrate as the liquid cooks off. Neither melts smoothly on its own. Both are dry and low enough in moisture that direct heat turns them oily and stringy rather than pooling them into a sauce, so both belong grated into something that already has liquid in it, not melted solo the way mozzarella is.