Big Free-Tailed Bat (Nyctinomops macrotis)

Big Free-Tailed Bat (Nyctinomops macrotis)

Order Chiroptera : Family
Molossidae : Nyctinomops macrotis (Gray)

Description
Similar to the Brazilian free-tailed bat, Tadarida brasiliensis, but much larger; ratio of foot to tibia about 53; second joint of fourth finger 2.5 mm in length; ears large, and joined at their bases for a short distance over forehead; upperparts ranging from light reddish brown to rich dark brown; underparts similarly colored, but paler. Dental formula as in Tadarida brasiliensis. External measurements average: total length, 134 mm; tail, 51 mm; foot, 9 mm; ear, 25 mm; forearm, 61 mm. Weight (non-pregnant females in June), 22 g; of fat, October-taken, non-gravid females, 24-30 g.

Distribution
Widely, but seemingly sparingly distributed from Iowa and southwestern British Columbia, in the north, southward through Mexico and the West Indies as far as Uruguay in South America. Known in Texas from scattered localities in the Trans-Pecos, Panhandle, and southeastern portion of the state.

Habits
This bat is rare in collections and little is known of its habits. In Texas, these bats have been recorded primarily from the Trans-Pecos where they seem to be seasonal inhabitants of rugged, rocky country in both lowland and highland habitats. With the exception of a single specimen from San Patricio County, which was found hanging on a screen door at the Welder Wildlife Refuge in December of 1959, no winter records of this species have been recorded for Texas. In summer, a segregation of sexes apparently occurs, as evidenced by the fact that few males have been taken in the Trans-Pecos.

Preferred roosting sites are crevices and cracks in high canyon walls, but these bats have also been captured in buildings. A specimen from Brazos County was obtained when it flew down a chimney and into the owner’s house. The only known nursery colony of these bats in the United States was discovered in the Chisos Mountains of Brewster County in Big Bend National Park by A.E. Borell May 7, 1937. His attention was attracted to a horizontal crevice in a cliff near the head of Pine Canyon in the Chisos Mountains by the squeaking of bats. He estimated the number of adults using the site to be about 150, and all those he collected were adult females, most of which were pregnant. He revisited the colony on October 19, 1938 and collected four more specimens, all females. On October 27, 1958, some 20 years later, one of us (Davis) visited the colony with Richard D. Porter. Our notes, written the next day, follow: "We hiked up Pine Canyon as far as the falls (a trickle of water over a cliff about 100 feet [30 m] high). The canyon is narrow and steep-sided and has a few large yellow pines, but most of them are dead. To the right of the falls the cliff is overhanging, and it has several more-or-less horizontal crevices paralleling the top. One of them, about 50 feet [15 m] above the talus and some 100 feet [30 m] north of the falls, contained the colony. We could clearly hear the bats chattering, much like the muted coo of doves." A considerable quantity of guano on the talus at the base of the cliff marked the place below which the bats were roosting. None of the bats voluntarily left the roost while we were there.

Borell found that the bats left the roost on May 7 at 8:20 p.m., when it was almost dark, and nearly an hour after the first western canyon bat was observed. The bats left in small groups during a period of 15 minutes. The swish of their wings was plainly audible, and their flight was rapid. It was so dark when they emerged that he could not determine whether they flew up or down the canyon. Possibly their habit of leaving their daytime roosts so late is the reason they seldom are seen and rarely collected.

Another maternity colony is thought to be in McKittrick Canyon in the Guadalupe Mountains National Park. In June, 1968 and August, 1970, Richard LaVal netted 14 N. macrotis at a pool 8 km inside the canyon, where steep walls rise nearly 540 m above the narrow canyon floor. In this section of the canyon the bats were heard vocalizing from far above the floor. All individuals captured were females. Eight of the 12 taken in June contained a single large embryo each. One of the two females captured in August was lactating.

The winter habits of this bat are unknown, although they may possibly hibernate in the Trans-Pecos. Richard LaVal found that individuals kept in a refrigerator at 5°C for 24 hours entered a deep torpor, from which they emerged within 15 minutes after their removal. Another bit of evidence suggesting hibernation is that adult, October-taken females were very fat and weighed about 20% more than non-pregnant, June-taken females. Because they are strong fliers and prone to wander somewhat in fall, these bats often turn up far from their normal range during this season. Records from the Panhandle and southeastern Texas may represent juveniles dispersing from breeding populations in the Trans-Pecos.

David Easterla and John Whitaker, Jr. examined the stomach contents of 49 N. macrotis and reported that by far the most important food items found were the bodies of large moths. The only other items regularly found were the remains of crickets and longhorn grasshoppers. Other items the bats had consumed were flying ants, stink bugs, beetles, and leafhoppers. In the stomachs that contained crickets and longhorn grasshoppers, these items usually made up less than 25% of the contents, but in a few they comprised as much as 50%. One stomach contained only small flying ants and one contained only large ants. These workers speculated that while in flight the bats captured the ground-dwelling insects (crickets, longhorn grasshoppers, and large ants) by picking them from the walls of the cliffs.

Little is known about reproduction and development of the young in this bat. Seemingly, each gravid female gives birth to a single offspring in late June to early July. Development is rather rapid because by October the young-of-the-year are nearly full-grown and difficult to distinguish from adults. The females gather in nursery colonies, from which adult males are excluded, to rear their young.

Remarks
This bat was formerly included in the genus Tadarida. See the account of N. femorosacca for an explanation of the use of the generic name Nyctinomops.

Reference: Bat Conservation International.