Which Mammals Can Fly?

The only mammals capable of true and sustained flight are bats (Order Chiroptera).

Bats achieve powered flight using wings formed by a thin membrane of skin (called the patagium) stretched between their elongated finger bones, limbs, and body. There are over 1,400 known species of bats worldwide, making them the second-largest order of mammals (after rodents).

Gliding Mammals (Not True Flight)

While bats are the only true flyers, there are several other fascinating mammals that are often referred to as “flying” but actually glide or parachute between trees. Gliding is a controlled descent, while true flight involves active flapping of wings to gain lift and propulsion.

Notable gliding mammals include:

• Colugos (often called “Flying Lemurs,” though they are not true lemurs)
  • They are perhaps the most skilled gliding mammals, with a large skin membrane that allows them to glide significant distances with minimal loss of height.
• Flying Squirrels (a tribe within the squirrel family)
  • They use a furry membrane (patagium) that stretches from their wrist to their ankle to glide between trees.
• Wrist-winged Gliders (such as Sugar Gliders)
  • These are marsupials (possums) native to Australia and New Guinea.
• Anomalures (also known as “Scaly-tailed Flying Squirrels”)
  • These are African rodents that have evolved gliding abilities similar to flying squirrels.

Which Type of Mammal are Bats?

That’s a great follow-up question!

A bat is classified as a Placental Mammal belonging to the scientific Order Chiroptera.

Here is a breakdown of its classification:

Key Bat Facts:

• Only Flying Mammal: They are the only mammals capable of true, sustained, powered flight.
• Highly Diverse: Bats form the second-largest order of mammals (after rodents), comprising about 20% of all classified mammal species worldwide, with over 1,400 known species.
• Two Main Groups (Historically):
  • Megabats (Flying Foxes): Generally larger and primarily eat fruit, nectar, and pollen. Most do not use echolocation.
  • Microbats: Generally smaller, highly diverse in diet (mostly insects, but also fish, frogs, or even blood in the case of vampire bats), and use highly sophisticated echolocation to navigate and hunt.

How did they Evolve to Fly? How Can Almost Being Able to Fly Be a Survival Advantage?

That’s an excellent two-part question delving into the remarkable evolution of mammals!

Evolution of Bat Flight

The exact sequence of events that led to the first powered-flying bat is still one of evolution’s greatest puzzles, largely due to a gap in the fossil record. The oldest known bat fossils, such as Onychonycteris finneyi (dating back about 52.5 million years), already had the structures for powered flight, showing remarkable similarity to modern bats.

Scientists generally favor the “Trees-Down” Hypothesis, suggesting the evolution went through these stages:

1. Arboreal Ancestor: The common ancestor of bats was likely a small, quadrupedal, arboreal (tree-dwelling) insectivore, perhaps resembling modern shrews.
2. Gliding Stage (Hypothesized): It is widely hypothesized, though the fossil evidence is missing, that these ancestors developed a skin membrane (patagium) to help them glide or parachute between trees. This would have provided a selective advantage (see below).
3. Powered Flight: Selection pressures then drove a rapid transition, involving numerous molecular and anatomical changes:
   • Forelimb Elongation: The finger bones (digits 2-5) became greatly elongated and lightened, forming the main spars of the wing.
   • Membrane Expansion: The patagium expanded to stretch between the limbs, digits, and body, creating the large, flexible wing surface.
   • Muscular Adaptations: The pectoral (chest) muscles and back muscles increased in size and strength to provide the powerful upstrokes and downstrokes necessary for sustained flapping.
   • Metabolic Changes: Bats evolved a high metabolic rate and increased lung capacity to meet the extremely high energy demands of powered flight.

The earliest bats, like Onychonycteris, could fly but did not possess echolocation (a trait that evolved later), suggesting that flight evolved before the sophisticated biosonar system.

Survival Advantage of Gliding (“Almost Flying”)

The ability to glide or achieve “near-flight” provides several significant survival advantages for arboreal (tree-dwelling) mammals, which is why this adaptation has evolved independently at least six times in different mammalian lineages (flying squirrels, colugos, sugar gliders, etc.).

For the ancestors of bats, these initial gliding advantages-especially increased mobility and escape from predators-likely created a strong selective pressure that favored further modifications, eventually leading to the powered flight that defines modern bats.

You can see a documentary about the evolution of flying mammals, including bats, here: BATS, the Evolution of flying mammals.

Do flying Reptiles have a ‘gap in the fossil record’ Too?

Yes, flying reptiles (pterosaurs) also have a significant “gap in the fossil record,” particularly regarding the earliest stages of their evolution and the transition to powered flight.

Like the first flying mammals (bats), the oldest known pterosaur fossils, which date back to the Late Triassic Period (around 220 million years ago), are already fully adapted for powered flight. This means there is a gap in the record before that time, where the transitional forms-the gliding or “almost flying” ancestors-are largely missing.

The Pterosaur Fossil Gap Explained

The gap in the pterosaur record is focused on two main areas:

1. The Origin of Flight (The Big Gap)

• The Problem: The earliest known pterosaurs, such as Eudimorphodon, already have the anatomical features required for full, powered flight, including hollow, thin-walled bones, large wings, and specialized muscle attachments.
• The Missing Link: Paleontologists have long searched for the **”pre-pterosaurs”-**the ancestors that would show a step-by-step evolution from a ground-dwelling or tree-dwelling reptile to a flyer.
• A Recent Breakthrough: Recent studies have identified a group of small, flightless, dinosaur-like reptiles called lagerpetids as the closest known relatives to pterosaurs. Lagerpetids appeared about 20 million years before the first pterosaurs. Studies using modern scanning technology (CT scanning) have shown that lagerpetids already possessed certain skeletal and neurological features (like an inner ear structure related to balance) that were previously thought to be unique to flying pterosaurs. This discovery helps bridge the anatomical and evolutionary gap between the groups, but the physical, transitional fossils with developing wing structures are still missing.

2. The Transition Between Major Groups (A Shrinking Gap)

• The Problem: For years, there was also a notable gap between the two major groups of pterosaurs: the long-tailed non-pterodactyloids (early forms) and the short-tailed, often gigantic pterodactyloids (later forms).
• Gap-Filling Fossils: Discoveries like the intermediate darwinopterans and, more recently, Skiphosoura bavarica are helping to fill this later gap. These fossils show how certain features-like the head and neck-changed before others, creating a more continuous evolutionary sequence for the diversification of the group.

The fact that pterosaurs had light, delicate, and hollow bones-perfect for flight-made them extremely prone to destruction before fossilization, which is why complete, well-preserved specimens are rare and often only found in specific, exceptional fossil sites.

The video provides some visual context on how scientists piece together the evolutionary history of the ancestors of flying reptiles like the pterosaurs. Mysterious Triassic fossil reveals the origin of flying reptiles.