Holy Echolocation! How Bats Got Their Amazing Superpowers
Phylogenically speaking, we live on a bat-planet. With more than 1,240 species in the order Chiroptera, these winged creatures account for one-fifth of all mammal species and occupy every continent in the world but Antarctica. Contrary to popular belief, bats are not closely related to rodents, birds, or insects, and occupy a distinct branch of the mammalian tree.
Reconstructing the evolutionary history of bats is rather difficult, as their delicate bones do not fossilize well and thus there are few early bat and proto-bat specimens in the fossil record. This also makes it rather difficult to determine how and when bats acquired the abilities of flight and echolocation, leading some in the anti-science community to cite this as proof of special creation.
The fact-based version of events, while more complicated, is far more interesting.
A Complicated Bat-Taxonomy
The order Chiroptera is split into two suborders - Megachiroptera or megabats and Microchiroptera or microbats - in traditional Linnaean taxonomy. While all bat species are capable of flight, only the microbats (and one genus of megabats) are capable of echolocation. Megabats are also generally eat fruit, nectar, and pollen, while microbats are insectivorous or blood-sucking in addition to eating fruit. As the name implies, microbats are generally smaller (though not always) and have a few other physical distinctions from megabats, such as a missing claw on the second toe of their forelimb and a lack of insulating underfur.
Research using comparative anatomy and molecular genetic evidence has indicated that both suborders evolved from a common ancestor that already had capability of flight. How these proto-bats diverged into the branches we have today is a subject of much debate in chiroptologic circles. A proposal based on cladistic analysis - and supported by genetic evidence - would split the bat order into the suborders Yinpterochiroptera and Yangochiroptera. The Yinpterochiropteran suborder would include most of the current megabat families and a handful of current microbat families. The Yangochiropteran suborder would include all bats who use laryngeal echolocation - generating ultrasonic pulses in the voicebox rather than using a tongue click.
Same Bat-Timing, But Different Bat-Channels
Microbats generate sounds in the 14,000Hz to 100,000Hz range, and at intensities up to 140 decibels - as loud as a pistol firing or a jet engine. Different bat species use different sound frequencies depending on their environment, hunting technique, and type of prey. Some bats use constant frequencies during a hunt, others sweep through a range of sound frequencies, and still others produce multiple harmonics in their echolocation calls.
Constant frequencies are useful when hunting in open environments, allowing the bat to determine the range and velocity of more distant targets by subtle changes in Doppler shift. On the other hand, a sweep of frequencies (frequency modulation) is best for close and cluttered environments such as caves, as the multiple frequencies prevent call and echo from overlapping and allow the bat to easily distinguish moving targets from background noise.
In general, the timing of echolocation calls follows a standard pattern across microbat species. During the initial search phase, sounds are produced at a slow rate of ten to twenty per second. As a target is identified, the rate of sound pulses increases - often with an increase in sound intensity. On final approach, the rate of calls can be as high as 200 pulses per second - known as the terminal buzz.
Echolocating bats have precisely tuned inner ear organs, allowing them to hear the echoed ultrasonic pulses with extreme precision. Microbats also have a comparatively larger auditory cortex than other mammals, reflecting the importance of this sense to their survival. Hunting bats use the slight timing differences between sounds received in both ears to locate the direction of the target prey, and it is hypothesized that they use echoes from the tragus (the flap of skin in front of the ear canal) to determine the elevation of the target.
Some Days You Just Can't Get Rid of An Allele
There are several competing hypotheses for how and when bats evolved their abilities of flight and echolocation. Although some studies from the 1980s seemed to indicate that the megabat species evolved from a shared ancestor with primates, most current genetic evidence indicates that bats likely evolved from an earlier flying mammal 52 to 54 million years ago and later split into the Yinpterochiroptera and Yangochiroptera lines.
Until recently, there had been some debate over whether flight or echolocation had evolved first in the chiropteran lineage. However, discovery of the Onychonycteris finneyi fossil in the Green River Formation of the midwestern United States seems to have resolved this debate, as the fossil was capable of flight but did not appear to have the cochlear structure necessary for echolocation.
Where laryngeal echolocation fits into this lineage complicates the bat family tree. There are currently two competing hypotheses. One hypothesis is that echolocation evolved once in microbats, then was lost in the pteropid line and later regained in the Egyptian Rousettus bat, which uses tongue-clicks rather than laryngeal sounds to locate prey. A second hypothesis is that echolocation evolved separately in the Rhinolophoidea superfamily and the Yangochiropteran bats. Genetic analysis so far points to the single-origin hypothesis.
The bat lineage is further complicated by the geographic distribution of bat families. Genetic evidence hints that bats likely evolved in tropical regions of Laurasia or Africa and spread via land bridges to South and North America. However, fossil evidence of early bats is lacking in Africa and South America, and reconstruction of the geographic radiation of bats is difficult without further bat-discoveries.
The differing strategies of echolocation used by bat species in different environments offers one of the best examples of adaptive radiation in the natural world. Coevolution also plays a part, as moths and other prey species have adopted "jamming" techniques to counteract bat echolocation calls in some ecosystems.
Although there are many unanswered questions in the lineage of bat evolution and many holes in the fossil record, genetic evidence from modern bats has produced a strong working model of the lineage of these fascinating creatures.
To The Bat-Quiz!
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- PLoS ONE: Accelerated FoxP2 Evolution in Echolocating Bats
Gang Li et al
- Molecular evidence regarding the origin of echolocation and flight in bats
Emma C. Teeling, et al
- The evolution of echolocation in bats
Gareth Jones, Emma C. Teeling
- Integrated fossil and molecular data reconstruct bat echolocation
Mark S. Springer, et al.
- The evolution of ﬂight and echolocation in bats: another leap in the dark
John R. Speakman
- A Nuclear DNA Phylogenetic Perspective on the Evolution of Echolocation and Historical Biogeography
Geeta N. Eick, David S. Jacobs and Conrad A. Matthee
- Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation
Nancy B. Simmons, Kevin L. Seymour, Jörg Habersetzer & Gregg F. Gunnell