There are species which have adopted monogamy with great success. For instance, the male prairie vole will mate exclusively with the first female he ever mates with. The vole is extremely loyal and will go as far as to even attack other females that may approach him. This type of behavior has been linked to the hormone vasopressin. This hormone is released when a male mates and cares for young. Due to this hormone's rewarding effects, the male experiences a positive feeling when they maintain a monogamous relationship. To further test this theory, the receptors that control vasopressin were placed into another species of vole that is promiscuous. After this addition, the originally unfaithful voles became monogamous with their selected partner. These very same receptors can be found in human brain, and have been found to vary at the individual level—which could explain why some human males tend to be more loyal than others.