Do Bats Lay Eggs? [No, They Are Mammals!]

Photo: MPRshots / Shutterstock

If something flaps overhead during the night, casting a black shadow across the moonlight sky, it likely is a bat. These creatures are active in the spring through the fall as they feed and raise their young. 

You may have noticed that bats do not build nests, but they fly like birds. This has caused many to incorrectly assume that bats lay eggs.

Bats do not lay eggs. Bats are mammals because they are warm-blooded, breathe oxygen with lungs, have fur, and give birth to live young. They also share other mammalian characteristics such as a reproductive system involving an umbilical cord and placenta, a forearm bone structure that is like other mammals’, and mammary glands that produce milk.

This article will offer detailed facts and information about the mammalian characteristics of bats, along with unique features such as mating practices and the use of echolocation.

Bat Facts

There are over 14,000 species of bats globally dispersed over 6 continents. They are the second largest order of mammals (as compared to the largest: rodents: Rodentia).

They belong to the Chiroptera order, divided into two suborders: Megachiroptera (large Old World fruit bats) and Microchiroptera (global small bats). 

Bats vary in size with wing spans from 6-inches to 5 feet, weighing 0.07 ounces to 2.2 pounds. 

They may be insectivorous (insect), carnivorous (meat/blood), frugivores (fruit), or nectarivores (nectar).

Characteristics Of Mammals

Mammals have certain characteristics in order to be classified as one. 

These include the following:

  • Hair or fur 
  • Breathe oxygen
  • Warm-blooded
  • Mammary glands for producing milk
  • Three middle ear bones transmit sound waves 
  • A neocortex region in the brain for seeing and hearing
  • Specialized teeth (4 types: incisors, canines, premolars, and molars)
  • Four-chambered heart
  • Lower jaw is hinged to the skull
  • Diaphragm separates the heart and lungs from the abdominal cavity
  • Left aortic arch
  • Mature red blood cells lack a nucleus
  • Give birth to live young (except for echidnas and platypus)
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While there are some exceptions to these characteristics in some mammals, the most distinctive characteristics are breathing oxygen through the lungs, live births, production of milk, and being warm-blooded.

All species of bats meet the criteria for being a mammal, including the naked, hairless, bat (Cheiromeles torquatus).  

7 Mammal Characteristics In Bats: A Closer Look

Contrary to popular misconceptions, bats are not birds. They are mammals, and therefore do not lay eggs like birds.

Unlike the flying squirrel, which can glide in the air for short distances, bats are the only true flying mammal.

The following characteristics provide evidence that bats are in fact, mammals.

1. Fur

Bats do not have feathers. 

Most species have fur on their bodies and sometimes heads, in brown, red, tan, or gray colors. Some bats are born with fur or fine, downy hair, and others are not growing it as they mature.

Bats will groom the fur on themselves, pups, and each other to control parasitic infections and stay clean.

Their wings are hairless with supple skin, which stretches from the forelimb to an elongated, hooked finger.  

2. Warm-Blooded Energy Conservation

Bats cannot sweat, so they may lick or fan their wings, or pant. Many species of bats can also create physiological responses to help manage undesirable temperature conditions or to conserve energy. 

Bats are warm-blooded endothermic homeotherms, meaning that they maintain a constant body temperature, creating heat through the metabolism of food. 

They can also conserve energy or tolerate hot temperatures by dropping their body temperature as low as 38.5°F.

In addition to temperature control, researchers have found that some species can control their heart rate during rest, torpor (slowed physiological changes), or hibernation (dormancy).

Not all bats go into torpor or hibernate, but will migrate to be in ideal climates. 

Some species of bats have a resting heartbeat of approximately 300 beats per minute (bpm) but can slow their heart rate down to 200 bpm. This can help to save up to 10% of their caloric needs.

3. Mating Practices

The location and when bats mate can vary depending on the species, but undergo a unique process. Often mating practices take place in late summer through autumn.   

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Practices can vary with aerobatic swarming, meeting in a secluded spot, or at roosts. Some species mate upside down, while others do it in rock crevices or on the walls of caves. 

Males may release pheromones and vocalizations to draw females in. The male mounts the female from behind in brief or long encounters. Some bats are monogamous and others breed with multiple partners.

The sperm is stored in the female’s reproductive tract, preventing immediate fertilization, until ovulation occurs.  

Bats that live in temperate zones (with cold seasons) will store the sperm until next spring in their reproductive tracts for up to 198 days. Tropical bats store sperm for much shorter periods. 

Regardless of the mating and fertilization practices, bats are still mammals. They are internally inseminated and then produce live young.

4. Live Birth And Milk

Typically, most bat species give birth to only one pup at a time annually. 

Once fertilization has occurred, the gestation period of bats ranges from 6 to 9 weeks, or up to 8 months depending upon the species (such as the vampire bat). 

Weather, climate, and food availability can affect pregnancy duration as well.  

The baby is connected with an umbilical cord and placenta, sharing blood, nutrients, and waste processes.

Giving Birth

Bats will gather in maternity roosts during the birthing seasons, generally during the mid-spring to mid-summer months. On average, a maternity roost colony can have up to 50 females.

Bats give birth to their young, called a pup, while hanging upside down, from their vaginal cavity.

Since birthed bats don’t know how to fly, the agile mother catches it or pulls it up by the umbilical cord once delivered, to prevent a fall. 

Once the umbilical cord is broken off, the baby will have a belly button.

Feeding And Care

Female bats have teats with mammary glands on their upper bodies, from which they will nurse their young until about 3 to 4 weeks old.

Some species of mother bats also regurgitate nectar and the pup will lick it from their mother’s mouth. 

Females will carry their pups, before they can fly, in different ways depending upon their anatomy.

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Some carry pups latched onto a teat, cradled into a folded skin under its wings, or in a “pocket” of skin near the mammary glands.

When the young bat is no longer relying on the mother for sustenance, it learns how to fly and get its food. 

5. Breathing Oxygen With Lungs

Bats have a respiratory system, which involves lungs that take in oxygen and release carbon dioxide.

Bats have one or two nostrils that take in air. They do not use their mouths to breathe, since a trachea is not attached to them.

Bat lungs have an efficient respiratory system with a large lung volume, about 72% bigger than similarly-sized, non-flying mammals. This means that they can utilize significantly more air with each breath.  

As bats fly, they produce a protein, myoglobin. Myoglobin assists in facilitating the transfer of oxygen from the blood to the muscle.

An efficient lung capacity with myoglobin can allow bats to fly fast. Some bats can fly at nearly 100 miles per hour.

6. Bone Structure

Bats have forearm bone structures like other mammals.  

Bat bones are denser to handle the flight, but their wings have the same types of bones as humans do in their forearms.   

Their forearms, connected by muscle and a skin-like webbing consist of the following bones

  • Humerus
  • Ulna
  • Radius
  • Carpals
  • Metacarpals
  • Phalanges

7. Echolocation

Echolocation is not a common mammalian trait but is seen in two other mammals: whales and dolphins.

Bats are not blind, but they are active during the night. They use different tones of echolocation to help them navigate an area, find food, and make social calls using ultrasonic waves.

In Conclusion

Bats are, in fact, mammals and do not lay eggs. 

They are warm-blooded, have fur, birth live pups, and breathe oxygen. A female bat’s reproductive system has a placenta and umbilical cord that care for the pup internally. It delivers the pup through a vaginal cavity, while hanging upside down.

Bats also have mammary glands to feed their young. They have forearm bone structures like other mammals.

As bats fly, they find places to mate and roost and use echolocation for social and food sources.

James Ball

James has had a lifelong passion for animals and nature, tracing back to his childhood where he first began fostering intimate knowledge and connection with pet frogs and snakes. He has since honed this interest into a career as a trained Wildlife Biologist, specializing in Biogeography, sustainability and conservation. In addition to his professional pursuits, James maintains an active lifestyle, regularly indulging in outdoor activities such as hiking, and musical pursuits like playing piano and swimming.

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