Nycticeius humeralis (Rafinesque, 1818)

Scientific Naming and Basic Size

The evening bat, with the scientific name Nycticeius humeralis (Rafinesque, 1818), is a small bat that weighs 7–15 g (0.25–0.53 oz). It is found across most of the midwestern and eastern United States.

Forearm Length

Its forearms measure 34–38 mm (1.3–1.5 in) in length.

Dorsal Fur Banding

Each hair on its dorsal side has a light gray tip, while the basal one to two-thirds of the hair is dark brown.

Overall Coloration

While a small number of individuals have white fur, the majority of the species is mostly brown in color.

Facial and Reproductive Anatomy

Evening bats have wide, dog-like muzzles, pronounced facial glands, and disproportionately large bacula. They are often mistaken for juvenile big brown bats, because they resemble that species but are smaller in size.

Jaw Structure

Compared to other insectivorous bats, evening bats have relatively robust jaws.

Calcar and Tragus Features

They have an unkeeled calcar and a short, round tragus; the curvature of the tragus distinguishes the species from otherwise similar bats in the genus Myotis.

Dental Traits

Each side of its upper jaw has one incisor, and the species has 4 molariform teeth in total.

Historical Distribution Assumption

Initially, scientists thought the evening bat was only a southeastern bat species.

Expanded Recorded Range

However, breeding evening bats have now been recorded as far north as Michigan and as far west as the 100th meridian.

Roosting Sites

Evening bats roost in a range of different structures, including Spanish moss, under bark, in tree cavities, and inside buildings.

Foraging Habitat Preferences

For foraging, evening bats in Georgia prefer pine forests, riparian zones, and open fields. Their home ranges cover approximately 300 hectares (1.15 mi²).

Migratory Behavior

The species is not present in the northernmost part of its range during winter, so it is likely that at least some evening bats are migratory.

Mating and Fertilization

Evening bats mate during fall and winter; female bats store the sperm until spring, when fertilization takes place.

Maternity Colonies

In May, females form maternity colonies that range from 15 to 300 individuals in size.

Litter Size

Among females that give birth, 90% have twins, though single pups and triplets also occur.

Nursing Behavior

While it is most common for evening bats to nurse their own offspring, a small share of pups are nursed by unrelated females.

Pup Development

Pups gain the ability to fly within one month of birth, and are weaned within 42 days of birth.

Natal Philopatry

Female evening bats display natal philopatry: as adults, they return to the roost where they were born to give birth.