Keen's Long-eared Myotis is a medium-sized Myotis species with dark, glossy fur and darker indistinct spots on the back of the shoulder; the underside is paler. Its long ears extend beyond the nose when pressed forward; the tragus is long, narrow and pointed. The ears and wing membranes are dark brown but not black. The outside edge of the tail membrane has a fringe of tiny hairs that can be seen with a hand lens. The calcar has an indistinct keel. The skull has a relatively steep forehead.
IdentificationThe Fringed Myotis (Myotis thysanodes), Northern Long-eared Myotis (Myotis septentrionalis) and Western Long-eared Myotis (Myotis evotis) are other Myotis species with a long ear and tragus. However, the distribution of Keen's Long-eared Myotis overlaps only with that of the Western Long-eared Myotis. No external features that we know of will positively distinguish the two species. Keen's Long-eared Myotis has relatively shorter ears, extending less than 5 mm beyond the nose when pushed forward, and its ears are also paler; but these traits are variable and somewhat unreliable. Positive identification can only be made from cranial characters; the distance from the last upper pemolar to the last upper molar is less than 4.2 mm in Keen's Long-eared Myotis and greater than 4.2 mm in the Western Long-eared Myotis.
Dental Formulaincisors: 2/3
Measurementstotal length: 84 (63-94) n = 33
tail vertebrae: 40 (32-44) n = 33
hind foot: 9 (8-10) n = 32
ear: 18 (16-20) n = 38
tragus: 11 (9-12) n = 41
forearm: 36.0 (34.2-38.5) n = 43
wingspan: 241 (209-262) n = 33
weight: 5.1 (4.0-5.9) n = 35