Evening bat
Conservation status
Least Concern (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Chiroptera
Family:	Vespertilionidae
Genus:	Nycticeius
Species:	N. humeralis
Binomial name
Nycticeius humeralis (Rafinesque, 1818)

The evening bat (Nycticeius humeralis) is a species of bat in the vesper bat family that is native to North America.^[2] Hunting at night, they eat beetles, moths, and other flying insects.

Description

An evening bat in the hands of a researcher

The evening bat is a small bat weighing 7–15 g (0.25–0.53 oz)^[3] found throughout much of the midwestern and eastern United States. Their forearms are 34–38 mm (1.3–1.5 in) in length.^[4] The tip of each dorsal hair is a light gray, and one to two-thirds of the basal is dark brown. Though there have been some cases of white pelage, the majority of the population is mostly brown in color.^[4] They have wide, dog-like muzzles, pronounced facial glands, and disproportionately large bacula.^[4][5] Evening bats can be mistaken for juvenile big brown bats, due to their physical resemblance but smaller size.

Morphology

Evening bats have relatively robust jaws, compared to other insectivorous bats.^[6] They have an unkeeled calcar and a short, round tragus.^[7] The curvature of the tragus helps distinguish it from bats of the genus Myotis, which otherwise look very similar.^[7] Their skull has one upper incisor on each side with 4 molariform teeth.^[8]

Biology

The evening bat is a relatively short-lived, especially compared to other bats in its geographic range.^[9] It has a maximum age of 6 years, though few individuals live past 4 years.^[10][9] Its short lifespan for a bat could be explained by its considerably higher reproductive output. Bats that only have one pup per year would need to live much longer to have the same fitness as a shorter-lived species with two or three pups per year.^[9]

Reproduction

Evening bats mate in the fall and winter; the sperm is stored until the spring, when fertilization occurs.^[11] Female bats form maternity colonies in May,^[12] consisting of 15-300 individuals.^[3] Of females that give birth, 90% have twins, but singletons and triplets are also possible.^[11][12][13] Though it is more common for evening bats to nurse their own offspring,^[3] a small proportion of offspring are nursed by unrelated females. The pups are capable of flight within a month of birth.^[12] Pups are weaned within 42 days of birth.^[3] Female pups exhibit natal philopatry, meaning that as adults, they return to the roost where they were born to give birth.^[3]

Diet

These bats have varied diets. A majority of the bats' diet in Indiana and Illinois are beetles, including the spotted cucumber beetle, which is a serious agricultural pest.^[14] In southern Illinois, the spotted cucumber beetle is almost 25% of the evening bats' diet.^[15] Other beetles consumed include ground beetles and scarab beetles.^[14] Moths are also a significant dietary component.^[14] Bugs, winged ants, and flies are prey items of less significance.^[15][16] Evening bats partition resources with other insectivorous bats in their range, such as the eastern red bat and Seminole bat.^[16] Despite foraging in the same areas at the same time, these three bat species choose different prey items at different points throughout the summer.^[16]

Distribution

At first, the evening bat was thought of as a southeastern bat species.^[17] However, breeding evening bats have been found as far north as Michigan and as far west as the 100th meridian.^[13] Evening bats roost in a variety of structures, including Spanish moss, under bark, in tree cavities, and in buildings.^[18] For foraging habitat, evening bats in Georgia prefer pine forest, riparian zones, and open fields.^[19] Evening bats have home ranges of approximately 300 hectare (1.15 mi²).^[19] Because the evening bat is not found in the northernmost extent of its range in the winter, it is likely that at least some evening bats are migratory.^[4]

Conservation

While the evening bat is considered endangered in the state of Indiana,^[20] it has a cosmopolitan distribution throughout the southeast and midwest.^[18] Because evening bats do not enter or hibernate in caves, the species is not at-risk from white-nose syndrome, which has killed over six million bats in the United States since 2006.^[21] The evening bat's avoidance of this disease, along with die-offs of many other species, is possibly responsible for the evening bat recently expanding its range into Wisconsin in 2015 and Minnesota in 2016.^[21][22]