A hoary bat

The hoary bat makes much quieter echolocation calls, or none at all, when looking to be incognito ©Oregon State University/Flickr CC BY-SA 2.0

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Bats go silent to avoid competition when looking for mates

The phrase 'blind as a bat' is looking increasingly out of date as new research suggests bats use their sight and memory after going silent when looking for mates.

Hoary bats make much quieter echolocation calls or stop calling altogether when other bats were nearby. A new study suggests that bats go quiet to prevent competitors from overhearing their location.

While the bats still use echolocation to navigate in many situations such as when hunting, the findings show that bats are much better at navigating with their other senses than previously thought.

The study, led by researchers at the University of Colorado, was published in Nature Scientific Reports.

Dr Aaron Corcoran, the lead author of the paper, says, 'We were shocked by these findings. Bats are virtually synonymous with echolocation and researchers have assumed for decades that bats have to use echolocation to orient in the dark.

'This opens up a whole series of questions about what bats are capable of and how they perceive the environment around them.'

A hoary bat with the recording equipment attached

Recording equipment was used to measure the calls and acceleration in the air  ©Nicole Jean Hill

Manoeuvres in the dark

While echolocation is perhaps best known in bats it has evolved in several groups, including shrews, toothed whales and some birds.

As bats fly, they give out ultrasonic sound pulses with very short wavelengths. They then use the returning echo to assess the direction and distance of obstacles, prey and other bats in the surrounding area.

For some time, it was thought that bats relied on echolocation for almost all of their navigation. However, more recently studies have shown that other senses, including sight and hearing, are also important to allow them to find their way around.

Earlier studies looking at hoary bats found that while the species does use echolocation when hunting out in the open, there were also short, quieter calls known as 'micro calls' and also periods of flight in complete silence. But their ground-based recording equipment meant they didn't capture many of these moments.

To counter this, the new study used recording devices strapped to the backs of 10 male Hoary bats. These devices were programmed to record 10 seconds of sound and acceleration at varied time intervals, allowing researchers to listen in during flights.

They found that as the bats passed other individuals, they were mostly silent or making micro calls, which suggests that they are trying to be more inconspicuous. These quiet calls were also much more likely to occur near to bat chasing than by chance alone.

But crucially, during these quieter periods the bats were also continuing to manoeuvre and accelerate a similar amount to when they were making more intense calls. The amount of echolocation was also not affected by light levels, suggesting that echolocation was not being used less if bats could see their surroundings more clearly.

A hoary bat on a tree stump.

The micro calls are used to avoid obstacles such as trees ©Forest and Kim Starr/Flickr CC BY 2.0

Stealth flights 

The findings point to bats making silent flights in an effort to be undetected by others, in a similar strategy to submarines which run silent when trying to evade detection. 

When trying to hide from others, the micro calls of the bats allow for essential echolocation so that they avoid larger objects like trees. Vision and their memory of the area may fill in the gaps to help them avoid smaller objects. 

The researchers suggest that the silence and quiet calls may be related to competition between males in the hunt for mates, although the fact that they only studied male bats during mating season may have influenced things. 

Once the bats are detected by a competitor, they begin to make their high intensity calls again as they chase them away from the area. 

The researchers now hope to conduct more research into the mating behaviour of the hoary bats to find out more about if the quiet flights offer an advantage to males during the mating season.