Fauna of Valle del TrejaBirdlife in the park includes moorhens, snipe, wood pigeon, kestrels and buzzards. Among the mammals represented here are the rare wild cat, porcupines, garden dormouse, edible dormouse, hedgehogs, badgers, weasels and foxes.
There is also a rare species of freshwater crab, Potamon fluviatile, apparently the same species discovered in 1997 to be living under Trajan's forum in the canals built by the Etruscans and connecting to the Cloaca Maxima. That population is thought to be the only freshwater crabs living in a major city. Based on a genetic analysis, which demonstrated that those crabs were similar to Potamon fluviatile in Greece, researchers believe that the crabs were brought by the Greeks, some 3000 years ago, before the founding of Rome. Potamon fluviatile sometimes ventures out of the water into the forest and, unlike Asian freshwater crabs, does not return to the sea to breed.