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  The Mammals of Texas - Online Edition

Pocketed Free-tailed Bat
Order Chiroptera : Family Molossidae : Nyctinomops femorosacca (Merriam)

Description. Similar to the Brazilian free-tailed bat, Tadarida brasiliensis, but the bases of the ears are joined at the midline; second phalanx of the 4th digit less (not more) than 5 mm; anterior part of hard palate narrowly, rather than broadly, excised; upper incisors placed close together, their longitudinal axes nearly parallel, not convergent, distally; the presence of a fold of skin stretching from the inner (medial) side of the femur to the middle of the tibia. This fold produces a shallow pocket on the underside of the interfemoral membrane in the vicinity of the knee, a structure to which the common name alludes. Dental formula: I 1/2, C 1/1, Pm 2/2, M 3/3 X 2 = 30. External measurements average: total length, 112 mm; tail, 46 mm; foot, 10 mm; ear, 23 mm; forearm, 46 mm. Weight, 10-14 g.

Distribution in Texas. Southwestern United States and northwestern Mexico; records from southern California, southern Arizona, southeastern New Mexico, western Texas; southward in Mexico to the state of Michoacan. Known in Texas only from Big Bend National Park, Brewster County.

Habits. This species is an inhabitant of semiarid desertlands. It has been found using day-roosts in caves, crevices in cliffs, and under the roof tiles of buildings. Nothing is known about the winter habits of these bats; apparently, they are absent from Texas during this time.

These bats leave their day roost late in the evening to forage, exhibiting swift, powerful flight. Philip Krutzsch recorded the following observations made at a colony of 50-60 of these bats he found at a daytime roost in a crevice in the face of a cliff in San Diego County, California, on March 17. The first bats left the colony at 6:15 p.m.; others followed in twos and threes for another half-hour. The bats dropped from 1 to 1.5 m before taking wing. Their flight appeared to be a rapid, complete wing beat. When first taking flight, they uttered a shrill, sharp, high-pitched, chattering call, which was repeated while the bats were in full flight. They also squeaked a great deal while in the roost. He described the odor of this bat as similar to that of the house bat, but not quite so strong. Little data on their food habits are available. The stomachs of thirteen bats captured in Big Bend National Park were found to contain moths, crickets, flying ants, stinkbugs, froghoppers and leafhoppers, lacewings, and unidentified insects.

Data on reproduction in this species are also scarce. Fifteen females captured at Big Bend National Park between June 10 and July 12 each contained a single embryo. Lactating females have been caught in this area from July 7 to August 8, suggesting that a single young is born to the female in late June to early July.

Remarks. There is some confusion about the generic name of this bat and its relative, N. macrotis. Patricia Freeman, in a comprehensive study of the family Molossidae world-wide, separated the New World and Old World species of Tadarida (exclusive of Tadarida brasiliensis) into two distinct genera — applying the name Nyctinomops to the New World species. Other mammalogists, however, have not followed this arrangement and place all of the Texas species in the genus Tadarida.