A male bat sings while performing a wing flapping display in front of his territory
Male free-tailed bats sing intricate and complex "love songs" to woo prospective female mates, new research has found.
A detailed analysis of the structure of the bats' song has found they use a defined syntax and order of syllables.
That makes the bats' songs more similar to those produced by birds than by other mammals such as mice.
Among mammals, only whales may produce more complex songs, scientists report in the journal PLoS One.
In 2008, a team of researchers including Dr Kirsten Bohn of Texas A&M University in College Station, Texas, US discovered that Brazilian free-tailed bats (Tadarida brasiliensis) sang songs that appeared to contain defined phrases and syllables.
Within the broad rules, the bats are quite versatile
Dr Kirsten Bohn
"But we only had songs from a few individuals in a captive colony," says Dr Bohn.
Now Bohn and her colleagues have examined over 400 songs produced by 33 individual free-tailed bats living in different colonies in two different regions.
The researchers found that all the bats produce songs with a common hierarchical structure.
"All are constructed from the same four types of syllables and all syllables are combined in the same way to form three types of phrases," says Dr Bohn.
These phrases can be either chirps, trills or buzzes.
"This type of hierarchically structured vocalisation is extremely rare in mammals," she adds.
Songs of attraction
The Brazilian free-tailed bat is common from California to Mexico.
During the mating season, the males establish territories, which they vigorously and aggressively defend against other encroaching males.
But males do let females enter, and the males' love songs are thought to both attract these females and deter rival males.
Free-tailed bats listen out for each others' tunes
As well as finding that all the males structured their songs similarly, the scientists discovered that individual males also add their own particular flourishes.
"Even though these songs are highly structured and follow specific rules, they are also quite variable both for any particular bat and across bats," Dr Bohn explains.
At various times, individual males changed the number of syllables they combined into each of the three phrases, and they also varied the number of times a phrase was repeated within any one song.
"Within the broad rules, the bats are quite versatile," says Dr Bohn.
She suspects that males may vary their songs to generate different meanings, or make themselves more attractive to females, ideas that she now hopes to test.
Other mammals such as mice have in the past few years been shown to produce songs, whilst one other bat species, the Greater sac-winged bat (Saccopteryx bilineata) is also known to sing, though it sings quite differently and evolved this ability independently.
A captive free-tailed bat takes flight
But the hierarchy and complexity of the free-tailed bats love songs make them stand out among mammals.
Only birds and among mammals, the whales, can compare, says Dr Bohn.
"Bird and whale songs have units, syllables or notes, that are combined into specific phrases, or themes, that are combined to form songs," she says.
"In most other mammals, including mouse songs, sequences are produced non-randomly. But there is no evidence as yet for specific syntactical rules and hierarchical structure."