. >> nancy simmons is the curator in charge of mammology, at the museum of natural history. >> why ares nocturnal? >> well, the thought is that actually, the ancestors of all mammals are all nocturnal. small animals scurrying around in the age of dinosaurs. it gave them access to resources that other animals like dinosaurs couldn't use. so bats basically never gave up that lifestyle. whereas the ancestors of us, primates, gave up the nocturnal lifestyle to be diurnal. >> but working at night it helps the bats how? >> if you think about what the animals are that are active in the air, during the day, it's birds. and by being active at night, bats are not competing directly with birds. so the bats basically fulfill all the same ecological roles that birds do, only they do it at night. >> bats are unique mammals. though structurally in some ways they're similar to humans. so bats have four limbs, two of them are attached to the wings. >> two of them are the wings. >> okay. >> yes. they have all the same bones that humans do. the upper arm, the forearms, the wrist bones, and then the bones