Bats may seem like birds at first glance, but they are actually mammals. Bats share a number of characteristics with birds, such as the ability to fly and being vertebrates. However, bats have a number of defining features that clearly distinguish them as mammals rather than birds.
Difference #1: Body Covering
One of the most obvious differences between bats and birds is that bats have fur or hair covering their bodies, while birds have feathers. Fur and hair are exclusive to mammals – no birds have either. Feathers are unique to birds. So just by looking at the body covering, it’s clear that bats are mammals.
Bats have thin fur or hair over most of their bodies, with especially dense fur on the chest and back. The fur helps insulate them and maintain their high metabolisms needed for flight. Birds rely on their feathers for insulation, so there would be no reason for a bird to have fur.
Difference #2: Reproduction
Bats give birth to live young and feed them milk from mammary glands. This is a exclusively mammalian mode of reproduction. Birds lay eggs from which their young hatch.
After fertilization, bats carry their developing embryos internally through pregnancy, just like humans and other mammals. The pups are born live and blind. Bats produce milk from their mammary glands to nurse their young, just like all other mammals. Birds lay eggs externally, incubate them outside their bodies, and don’t produce milk.
Difference #3: Wings
Bats and birds both have wings they use for flight. But bat wings and bird wings have very different structures.
Bird Wings
Bird wings are specialized forelimbs covered with long flight feathers. The wings are attached to a keeled breastbone (sternum) that anchors large flight muscles. Other bird bones like the humerus are pneumatic, meaning they are hollow and filled with air sacs. This makes the skeleton light but strong.
The flight feathers on bird wings provide lift and thrust via their asymmetrical shape and the bird’s flapping motions. The flight feathers are regularly preened and maintained. The wings are shaped differently across bird species depending on flight style and habitat.
Bat Wings
Bat wings are stretched membranes of skin called patagia that extend between their arm bones, fingers, legs, and tail. Under the thin patagia are the arm and hand bones of a normal mammal. Bats don’t have hollow, air-filled bones or pneumaticity. The patagia are hairless and resemble skin more than feathers.
Bird Wings | Bat Wings |
---|---|
Covered with feathers | Made of skin membranes (patagia) |
Attached to keeled sternum | Attached between limbs and digits |
Bones are hollow and air-filled | Bones are normal and dense |
So while the wings provide flight capability for both groups, the anatomical details highlight the birds vs mammals distinction.
Difference #4: Eggs vs Live Birth
As mentioned before, birds lay eggs while bats give live birth. But why is this difference so important?
Eggs contain and incubate developing birds outside of the mother’s body with only the egg itself providing protection and nutrients. The eggshell and membranes enclosure means no ongoing maternal support occurs. Once laid, the entire chick develops in the egg before hatching.
For bats, their embryos develop fully inside the mother’s uterus, attached to a placenta that provides nourishment. The growing pups are surrounded and sheltered within the mother’s body for a proper gestation period, just like other mammals. When born, bat pups are relatively immobile and require care, unlike the mobile and self-sufficient hatchling birds.
So bats support their young internally via pregnancy like mammals, while bird embryos develop externally in eggs.
Difference #5: Lactation and Nipples
Female bats nurse their pups with milk produced in mammary glands and delivered through nipples, a quintessentially mammalian trait. Birds do not lactate and lack nipples entirely.
Newborn bats suckle milk from their mothers to fuel growth and development. The milk provides needed proteins, fats, minerals, immune factors and water to the pups. Weaning occurs gradually over weeks as the pups grow more independent.
Birds utilize a completely different feeding strategy. Hatchlings are fed with regurgitated or disgorged food by the parents. The chicks are never fed milk or nursed as there are no mammary glands or nipples to deliver it. Parent birds can provide food but cannot lactate.
So the nursing strategy of bats is a clear sign they are mammals, while birds have an incompatible feeding system.
Difference #6: Jaws and Teeth
Bats have jaws and teeth adapted for diverse diets including insects, small vertebrates, fruit, nectar and even blood. Birds have strictly keratinous beaks without teeth.
The ancestral bat skull shows molars, premolars, incisors and canines adapted for omnivorous feeding on insects and plants. While insectivorous bats have smaller weaker teeth, vampires and fruit-eaters have larger fangs and grinding molars respectively. Jaws allow complex chewing motions.
Birds uniformly lack teeth and chewing abilities with their hardened beaks. They swallow food whole, relying on their muscular gizzards internally to grind up and digest food items. This feeding is suited to seeds, fishing and nestling provisioning.
The toothed jaws of bats thus align them with mammals like humans, while birds have a unique toothless mouth design.
Difference #7: Metabolism and Endothermy
Both bats and birds are endothermic, meaning they generate their own internal heat to maintain a high metabolism. But bats show the higher energy use typical of placental mammals.
The thermogenic capacity of bats reflects their small body size, rapid digestion, and energy demands for nighttime flight. Their basal metabolic rate is 2-3 times that of other mammals. Torpor and hibernation help conserve energy in colder months.
Birds have metabolic rates around 2 times that of similar-sized mammals. Their high rates support flight and fever responses to infection. But bats exceed even birds in relative metabolism, consistent with being placental mammals.
So bats aren’t just endothermic, they have the extremely high metabolism expected of a small nocturnal flying mammal.
Difference #8: Brain Structure
Bat brains share defining features with the mammalian brain plan, including a large and layered neocortex. The bird brain is smaller relative to body size and lacks a neocortex entirely.
Mammals including bats have six layered neocortex tissues that are important for sensory perception, motor commands, spatial reasoning, conscious thought and language. The neocortex makes up over 80% of the human brain.
Bird brains are small and lack layered neocortex tissues. Without a large neocortex, bird cognition shows more hardwired behaviors and less flexibility. Bat intelligence is enhanced by their complex layered neocortex.
Difference #9: Heart Structure
Bat hearts have a fully divided four chamber design typical of mammals and critical for complete separation of oxygenated and deoxygenated blood. Bird hearts have an incomplete division that still allows some mixing.
The four chambers in mammal hearts consist of two atria and two ventricles. This allows full oxygenation of blood without mixing of oxygen-rich blood from the lungs and oxygen-poor blood from the body.
Birds conversely have a three chambered heart with two atria but only one ventricle. Some mixing of oxygenated and deoxygenated blood occurs due to the incompletely divided ventricle. This provides adequate but not maximal oxygen circulation.
The four chamber heart of bats supports their high energy aerobic lifestyle. Birds manage with a three chamber heart but can’t fully isolate pure oxygenated blood.
Difference #10: Liver Structure
Bat livers have a lobed, bifurcated structure typical of mammals. Bird livers are not truly separated into lobes and pour their bile into the cloaca.
Mammalian livers like those in bats have distinct right and left lobes fed by separate hepatic arteries and ducts. The lobes are divided by connective tissues and blood vessels. Bile produced in the liver is routed to the gallbladder.
Avian livers lack clear separation into lobes and are more triangular and compact. Bile is not stored in a gallbladder but rather pours directly into the cloaca or gut. The cloaca collects both waste and eggs.
So bats have properly divided and lobed livers with traditional bile collection typical of mammals. Birds show major structural differences.
Conclusion
In summary, bats clearly belong to the mammalian class despite some superficial similarities to birds such as flight capabilities and high metabolisms. Defining mammalian features of bats include fur, live birth, mammary glands, toothed jaws, four chambered hearts, lobed livers, layered neocortexes, and extremely high energy demands. Bats utilize true powered flight like birds but their anatomy and physiology is consistently mammalian. While bats seemingly straddle both groups, their core structural attributes place them firmly as mammals. So next time you see a bat swooping through the night sky, remember it shares far more with you than just its ability to fly!