brooks island had no native land mammals prior to european contact. domestic mammals including cattle, sheep, goats, horses and dogs were introduced intentionally between the late 18th and mid-20th centuries. house mice ("mus musculus") and norway rats ("rattus norvegicus") were apparently introduced accidentally during this period. although mice were later displaced by voles, norway rats persist along the island's shores, where they have access to carrion and marine food sources, such as bay mussels ("mytilus trossulus"). during dry periods, rats may occasionally prey on voles. at one point, the island caretaker controlled the rat population through poisoning.