In exchange for nesting sites and food resources, ants protect plants from herbivores. One of the best-known examples of ant-plant mutualism is in bullhorn acacias ("Acacia cornigera") and their tending "Pseudomyrmex" ants in Central America. This system was studied by Daniel Janzen in the late 1960s, who provided some of the first experimental evidence that ants significantly reduce herbivory rates of myrmecophytes. Since then many other studies have demonstrated similar results in other systems. In the bullhorn acacia system, in exchange for protection, the acacias provide domatia, Beltian bodies, and EFNs, and evidence indicates that the "Pseudomyrmex" ants can survive exclusively on these food resources without having to forage elsewhere. For many plants, including the bullhorn acacias, ants can significantly reduce herbivory from both phytophagous insects and larger organisms, such as large grazing mammals. Obligately associated ant species are some of most aggressive ants in the world, and can defend a plant against herbivory by large mammals by repeatedly biting their attacker and spraying formic acid into the wound.