Other monogamous species include wolves, otters, a few hooved animals, some bats, certain species of fox, and the Eurasian beaver. This beaver is particularly interesting, as it is practicing monogamy in its reintroduction to certain parts of Europe; however, its American counterpart is not monogamous at all and often partakes in promiscuous behavior. The two species are quite similar in ecology, but American beavers tend to be less aggressive than European beavers. In this instance, the scarcity of the European beavers' population could drive its monogamous behavior; moreover, it lowers the risk of parasite transmission which is correlated with biological fitness. Monogamy is proving to be very efficient for this beaver, as their population is climbing.