Little Brown Bat vs. Big Brown Bat

Today we are going to explain how to tell the difference between the Little Brown bat (Myotis lucifugus) also known as the Little Brown Myotis and the Big Brown bat (Eptesicus fuscus). Both of these bats are known for being brown in color, being house bats, and hibernating in building during the winter months. But what is one way to really distinguish between these two bats? The easiest way comes down to is size.



The Little Brown bat is a small to medium sized myotis that has glossy fur that is dark yellow-brown to olive brown. Its face, ears, and membranes are dark, with the membranes (wings) sparsely or not furred. Little Brown bats range in size anywhere from 3"-4" and 8"-10" with their wings spread. An average weight for these bats are anywhere from 5-14 grams with the females being slightly larger than the males.


The Big Brown bat is a large, dark brown bat with long, glossy fur, a broad nose and wings, and short, black ears. Big Brown bats come in around 4"-5", have a 12"-14" wing span, and weigh up to 13-18 grams. With size being the easiest way to tell these two bats apart, and one bat only being roughly 1 inch bigger than the other, some would say size isn't the easiest way to tell these two bats apart without actually capturing and measuring them.


Little Brown Myotis

Little Brown bats mate primarily in the fall and they have a delay in fertilization until spring ovulation, after waking up from hibernation. Females will gather with the same nursery colony every year to have their pups. Gestation is 50-60 days with the young being born from June to July. Females will only have one pup each year. These nursery colonies like to roost in dark, warm places and leave their young in the roost during foraging flights. Pups can fly about 3 weeks after birth and can reach adult weight a month after their first flight.


Big Brown bats have a gestation period of about 60 days and generally only give brith to one pup. However, it has been found that occasionally twins can be born. Pups are able to fly at about three to five weeks and reach adult size after two and a half months. Big brown bats are found in virtually every American habitat but are most abundant in deciduous forest areas and suburban areas of mixed agricultural use. Most commonly found in small groups or all by themselves, big brown bats try to stay out of the heat durning the day, looking for a more dark, cool place to roost.


Little Brown bats make moths a major part of their diet as well as midges, mayflies, and aquatic insects. Little Brown bats are very effective at feeding in patches of insects and also sweeping low over water for drinks. Big Brown bats are efficient feeders and can fill their stomachs in about an hour, feeding mainly on beetles, moths, flies, flying ants, wasps, dragonflies, and other flying insects. These bats will not feed in heavy rain or when the temperatures dip below 50 degrees Fahrenheit.


Big Brown Bat

Even though these two brown bats look very similar and you would not be able to tell the difference between the two unless you did extensive research, taking measurements of their size, they are two completely different bats. Both of them serve a great purpose in the ecosystem, eating insects and other flying bugs. We love both the Big Brown Bat and the Little Brown Bat!