Bat prevent them from spending too much time in unfamiliar places, and decrease predation. In some species, pregnant individuals may not use torpor. The smallest bat is Kitti's hog-nosed bat ("Craseonycteris thonglongyai"), which is long with a wingspan and weighs . It is also arguably the smallest extant species of mammal, next to the Etruscan shrew. The largest bats are a few species of "Pteropus" megabats and the giant golden-crowned flying fox, ("Acerodon jubatus"), which can weigh with a wingspan of . Larger bats tend to use lower frequencies and smaller bats higher for echolocation; high-frequency echolocation is better at detecting
Aerodramus It has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by DNA sequence comparison. This suggests that as in bats, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment. A study suggested that the echolocation subunits were mainly located in the central nervous system, while the subunits in the vocal apparatus were already present and capable of use before echolocation even evolved. This study supports the hypothesis of independent evolution of echolocation in "Aerodramus" and "Collocalia", with the subsequent evolution of complex behaviour needed to