Bat Evolution: Bats are incredible creatures with unique skills and adaptations, including the power of flight, and many bats can even do this in complete darkness using echolocation. Bats are mammals in the order Chiroptera, the second-largest order of mammals on the planet, consisting of over 1,400 species. Bats comprise about 20% of all classified mammal species worldwide.
Historically, people believed that bats were bad omens, and they appear in various myths and legends. Partly this was due to their nocturnal nature and the association with the supernatural. In reality, bats, as an order, play a vital ecological role in the pollination of plants, the distribution of seeds, and the control of insects.
Bats are the only mammals capable of powered flight. This gives a vital clue as to why they diversified so successfully into many species. It is analogous to the diversification which occurred in other flying animals, namely birds and insects. The evolutionary theories for the development of flight in bats are hindered by the lack of fossil records. The issue is that bats suddenly appeared in the fossil record around 50 million years ago.
Whilst modern-day bat evolutionary development can be linked to the Cenozoic era, scientists have been unable to trace any mammalian ancestral connections as to why bats developed the capability to fly, or why they were the only mammals to evolve this capability.
By comparison, the fossil records of other animals can be linked due to genetic similarities. For example, the ancestors of Tyrannosaurus rex (or more commonly known as the T-rex) can be traced to Proceratosaurus fossil records from the middle Jurassic period.
In turn, we can trace T-rex’s distant relatives to modern times! So lacking fossil records, what are the hypotheses supporting the evolution of flight in bats? To answer this question, we need to expand our thinking on other flying animals.
Powered flight has evolved in four animal groups independently:
Insects
Potentially, there was more than one origin of flight in each of these groups, but the first group to develop the ability to fly was the insects, and this occurred during the mid-Devonian period. The ability to fly likely originated from the use of controlled aerial descent by the wingless archaeognathans.
Due to a gap in the fossil record, intermediate stages between this controlled aerial descent and winged flight are unknown. However, around 400 million years ago, we see the first known winged insect, Rhyniognatha hirsti.
It is thought that the small body mass of insects may have allowed them to take advantage of gusts of wind to become airborne, similar to seed dispersal in some plants.
Pterosaurs
As vertebrates, pterosaurs had inherently heavier bodies, so they could not evolve in the same way as insects. The first winged vertebrate occurred in the fossil record 228 million years ago; the basal pterosaur, Preondactylus.
Unlike insects, whose wings are formed by cuticle membranes pterosaur wings were formed by skin membranes known as patagia, extending from highly derived forelimbs, along with patagia that extended towards the neck, and between the hind-limbs. Much of the characteristic anatomy of pterosaurs is present in their sister group, the Protosauria.
Proto-pterosaurs likely possessed a wing precursor in the form of a membrane extending from one of their digits onto their bodies. This membrane is believed to have been used for display long before evolving as a flight apparatus.
Birds
Pterosaurs were not the only animals believed to have had wing precursors used for display rather than aerial movement. The early pre-bird avian dinosaurs are likely to have possessed feathered forelimbs that may have had display functions, given evidence of high melanosome diversity in the preserved feathers of these animals.
From there, animals such as Archaeopteryx emerged around 150 million years ago, with full-feathered wings capable of primitive flapping flight.
The insects, pterosaurs, and birds ruled the skies until the Cretaceous–Paleogene extinction event around 66 million years ago. Mammals then entered their age of dominance on the planet.
BATS
Powered flight is a highly complex evolutionary capability, but it gave bats access to a valuable aerial niche and enabled their populations to expand into every continent on the planet except Antarctica. Besides flight, the other most notable characteristic of bats is their often highly developed echolocation capability.
Echolocation is the process by which bats emit sound and can interpret the returning signals that bounce off the environment to produce a three-dimensional internal map of their surroundings. Thus enabling the bats to orient themselves in their immediate physical environment in the dark, and to hunt and catch prey.
Echolocation can take several forms in bats, but the group can be broadly split into those that are capable of echolocation via vocalization from the larynx and those that are incapable of it.
Understanding the evolution of powered flight in the group is inherently linked to understanding the evolution of laryngeal echolocation. This forms the basis for fundamental questions, such as: which came first, echolocation or flight?
The reason they are intrinsically linked is that echolocation requires significant amounts of energy to transmit signals over distances. It has been calculated that bats need over 10 times more energy to echolocate compared to their resting metabolism energy consumption. In addition, the energy cost of flying is around 15 to 20 times their resting metabolic rate.
Bats evolved an efficient method to echolocate whilst minimizing the extra energy for each capability. They synchronize their breathing with their wing flaps: exhaling on the upward stroke and inhaling on the downward stroke. Since the muscles that control their wing movements also manage their breathing, this dual function allows them to echolocate with virtually no additional energy expenditure, except for the periods when they are feeding whilst flying.
Bat Evolution Hypotheses
Due to the lack of ancestral fossil records in bats before the Cenozoic era, six different hypotheses exist amongst the scientific community on the evolution of flight in these animals. Or to put that more simply, we don’t know for sure.
Gliding Model Hypothesis
Bats likely evolved true flight from an ancestral gliding state; the aerodynamics of this have been subject to detailed theoretical modelling.
A full evolutionary pathway from gliding to powered flight in bats begins with an ancestral gliding stage via a fore-limb to hind-limb skin membrane, which is followed by the development of flapping and a skin membrane between the digits. This is followed by the abandonment of gliding entirely.
However, the question of exactly how and why a flapping motion originated from gliding remains unexplained. Possible explanations are that the musculature strain of controlled gliding resulted in the development of pectoral muscles capable of generating a powerful flapping motion. Or that other ecological factors resulted in a morphological change that favored a structure that was more aerodynamic during flapping than during gliding. The development of sufficient metabolism for flight remains unexplained in this model and is assumed to have occurred during the transition.
Despite this fundamental question remaining unanswered, the gliding model is generally assumed to describe the sequence of events. Given that echolocation is critical to the ecology of most bats and is physiologically coupled with powered flight, the next aspect to address is the order in which flight and echolocation developed.
Echolocation First Hypothesis
The echolocation-first hypothesis proposes that nocturnal pre-bats used a reach-hunting technique to capture flying insect prey. This technique involves reaching out with the forelimbs from a stationary perch and is likely to have involved a complex sensory system to calculate and predict prey movements.
Forelimbs would already be elongated to adapt to an arboreal lifestyle, and it is possible that ultrasound (used for communication) could have become modified into primitive echolocation to aid in prey capture. Over time, the echolocation would have become more sophisticated, and the fore-limbs would extend further and include an interdigital membrane to improve prey capture while the pre-bat remained perched, by effectively creating a large net.
Gliding and then flight would have developed later, as the pre-bats leapt from their perch to reach insects further away, and echolocation would have been secondarily lost in Pteropodids. A point in favour of the echolocation first hypothesis is the fact that some other mammals (notably some shrews and hedgehogs) echolocate without any flight ability. Coupled with this, since these mammals are sister taxonomic groups, it is possible that their common ancestor had echolocation without flight capability.
A point against the echolocation-first hypothesis is that it relies heavily on reach-hunting in the ancestral bat, a capability that has never been observed in wild bats. Further, modelling indicates that perch-hunting is an energetically inefficient strategy.
Flight First Hypothesis
The flight-first hypothesis proposes that the ancestors of bats developed gliding flight while jumping between trees. Subsequently, gliding was replaced by flapping flight since it allowed greater control and maneuverability.
It is postulated that this is where the divergence between pteropodids (i.e., bats that predominantly feed on fruit and nectar) and microchiropterans (i.e., bats that predominantly feed on insects) occurred. The Microchiropterans subsequently developed echolocation to capture prey more effectively.
The argument against this hypothesis is the comparison with the evolution of flight in birds. The evolution of flight in birds, though not entirely understood, is unlikely to have involved a gliding stage since the form and, to an extent, the aerodynamics required to allow gliding versus maneuverability while flying are different. Also, the fact that birds never developed an echolocation capability is argued to make the flight-first hypothesis unlikely.
Further, consideration that gliding mammals such as flying squirrels (scientific name Pteromyini) aren’t capable of powered flight, and that they have evolved to glide without ever developing the capability to echolocate while also exploiting nocturnal niches.
The main piece of evidence in favour of the flight-first hypothesis is the energetic coupling of flight and echolocation in bats. i.e., the synchronization of breathing with wing-flaps. This suggests that, for echolocation to be energetically advantageous, flight capability must have been present before the evolution of echolocation.
However, this is argued to be too much of a literal leap in the dark, as it would have required pre-bats as nocturnal animals to firstly, develop acute night vision to enable them to leap from branch to branch in the dark.
Bats that echolocate don’t possess highly acute night vision. This suggests that whilst they were evolving echolocation, their visual capabilities would have effectively had to have devolved. The implication being that during the transition from visual to echolocation capability, an intermediate form possessed reduced visual capabilities and a not fully developed echolocation capability. A state with no selective advantage would surely not have arisen via natural selection.
Tandem Development Hypothesis
This hypothesis proposes that an echolocation system developed with flapping flight, such that the length of leaps between branches would increase as echolocation became more sophisticated. This hypothesis overcomes the issue with the flight-first hypothesis, i.e., that an animal wouldn’t leap into darkness in the hope of encountering a suitable landing site. It also makes a lot of sense in terms of energy efficiency since the metabolic efficiency used by bats is also coupled to the evolution of flying and echolocation.
The major drawback of this theory is that it suggests that modern-day bats, which don’t have echolocation, e.g., the fruit-eating bats, somehow developed echolocation and then subsequently lost the capability and swapped back to a highly developed night vision capability.
Diurnal Frugivore Hypothesis
Our penultimate hypothesis is the Diurnal Frugivore Hypothesis, namely that the ancestors of modern-day bats were fruit-eating mammals who were predominantly active during the day. The scenario proposes that there was a proliferation of fruiting plants during the Cretaceous period, and that bats glided from branch to branch during the day, eating fruit and occasionally insects to supplement their diet. Subsequently, as birds proliferated and started to out-compete the bats, they were forced to move into nocturnal niches.
The diurnal frugivore hypothesis is essentially another version of the flight-first hypothesis, and therefore has the same major pitfall. i.e., the fact that birds never developed an echolocation capability is argued to make the hypothesis unlikely.
Interdigital Webbing Hypothesis
Our final hypothesis was developed by students in the School of Biology studying at the University of St Andrews, and was published by the Mammal Society in the 2020 Mammal Review.
It is postulated that the hypothetical common ancestor would have been a nocturnal tree-dwelling mammal approximately 66 million years ago, and already had a low form of echolocation used purely for communication.
As an adaptation for nocturnal life, our pre-bat would also have already had a highly developed hearing capability, which is still present today in bats. Further, the pre-bat would already have elongated fingers for climbing, and webbing between the fingers similar to that seen on modern-day flying frogs and marbled cats.
According to scientific analysis, the two major bat groups, Yangochiroptera and Yinpterochiroptera diverged, and this occurred before the subsequent divergence of the Pteropodidae and Rhinolophoidea.
It is proposed that ancestral Rhinolophoidea were the first group to develop laryngeal echolocation and that this was used to aid perch-hunting (given the ability of rhinolophoids to echolocate while stationary with minimal energy expenditure).
It would be more likely the case that the common pre-bat ancestor already had a fore-limb to hind-limb skin membrane. If not, this must have developed independently on all three bat families. Powered flight would follow in all three bat families, with laryngial echolocation evolving in the Yangochiropterans and improved vision developing in the Pteropodidae. The result being the 3 major bat groups of the modern day.
The hypothesis makes sense since the morphological and behavioral differences between the two groups of echolocating bats, i.e., that tandem development would naturally lead to subtly different solutions. It is also conceivable that a tree-dwelling pre-bat could have already had inter-digital webbing since other tree-dwelling animals have this feature.
In conclusion, the Interdigital Webbing Hypothesis is essentially a refinement of the gliding model hypothesis, which is generally favored amongst the scientific community.
REFERENCES
The following reference material was used in the production of this documentary.
- Anderson, S.C. and Ruxton, G.D. (2020) ‘The evolution of flight in bats: a novel hypothesis’, Mammal review, 50(4), pp. 426–439. [Accessed 23/05/2025] ]
- Speakman, J.R. (2001) ‘The evolution of flight and echolocation in bats: another leap in the dark’, Mammal review, 31(2), pp. 111–130. [Accessed 23/05/2025]
- Gardner, N.M. and Dececchi, T.A. (2022) ‘Flight and echolocation evolved once in Chiroptera: comments on “The evolution of flight in bats: a novel hypothesis”’, Mammal review, 52(2), pp. 284–290. [Accessed 23/05/2025]
Thank you to the the publishers and authors of The evolution of flight in bats: a novel hypothesis for the the use of source material which has been reproduced in this video under the following license: “Open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited”
CREATIVE COMMONS IMAGE CITATIONS:
The following fossil images were used in this video documentary & article under Creative Commons Licensing:
- Onychonycteris fossil image by Matthew Dillon [23/05/2025] ↩︎
- Proceratosaurus fossil image by The Trustees of the Natural History Museum, London – https://data.nhm.ac.uk/dataset/collection-specimens/resource/05ff2255-c38a-40c9-b657-4ccb55ab2feb/record/2140243, CC BY 4.0, [Accessed 23/05/2025] ↩︎
- Rhyniognatha hirsti fossil image: A General List of the Fauna Together With Illustrations [Accessed 23/05/2025] ↩︎
- Preondacvlus buffarinii fossil image By Ghedo – Own work, CC BY-SA 4.0, [Accessed 23/05/2025] ↩︎
- Sharovipteryx fossil image. By Ghedoghedo – Own work, CC BY-SA 4.0, [Accessed 23/05/2025] ↩︎
- Microraptor fossil image by User: Captmondo – Own work (photo), Copyrighted free use, [Accessed 23/05/2025] ↩︎
- Archaeopteryx lithographica fossil image By H. Raab (User: Vesta) – Own work, CC BY-SA 3.0, [23/05/2025] ↩︎
