Bats are often depicted as silent and stealthy hunters of the night, but they are also some of the loudest animals on the planet, capable of blasting out high-frequency calls with the same sound pressure as a jet engine. Many bat species use these high intensity shouts to echolocate their prey during foraging flights. Until recently, it was thought that the energetic cost of echolocation during flight was negligible for bats, since there was seemingly no difference in the flight costs for echolocating and non-echolocating bats. Yet, bats in the wild quite often hunt in groups and each bat must compete with a chorus of other calls to echolocate effectively, but at what cost? A new study by researchers from the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, shows that singing for your supper can be very costly indeed in noisy environments.
To test how these aerial shouting matches affect the energy consumption of bats during flight, the researchers flew Nathusius’ pipistrelle bats (Pipistrellus nathusii) in a wind tunnel and measured their metabolic rates while exposing them to different levels of background noise. To create a low-noise flight environment, the researchers simply used the ambient noise of the wind tunnel (69 dB); however, to achieve a high-noise flight environment, they placed loudspeakers under the wind tunnel playing ‘white noise’ at 109 dB and placed a microphone upstream of the bat in the tunnel to record the intensity of its echolocation calls. Finally, in order to measure the energetic expenditure of the bats during flight, the researchers started each experimental flight by injecting the bats with a carbon-13 isotope, which the animals would exhale as CO2. By taking breath samples from the bats before and after each flight, the team measured the difference in the 13C concentrations to estimate how much CO2 had been produced as a reliable proxy for the animal's metabolic rate.
Unsurprisingly, all of the bats’ calls were much louder (128 dB) when competing with the high background noise, in comparison with their calls when the ambient noise was low (113 dB). In fact, the high-noise wails were actually 30 times louder than the low-noise whispers, thanks to the exponential decibel scale. More importantly, while the team found that the costs of echolocating in low-noise environments are negligible, averaging just 0.8% of the flight costs, echolocating in loud background noise caused the bats to drastically boost their own calls, with the price of echolocation skyrocketing to almost 22% of the total flight costs. This completely overturns previous assumptions about echolocation energetics and the team calculated that the cost of calls exceeding 130 dB may be even greater than the cost of flight itself.
In the context of wild bats, the team concluded that during a single night of foraging in a high-noise environment, such as hunting in the vicinity of other bats, they would need to eat an additional 0.5 g of fresh insects to balance out the calling costs – which is a sizeable additional snack for a 7 g bat. It's not all bad news though as the team also found that the higher intensity calls allowed bats to find smaller insects at greater distances, albeit with diminishing returns for the most costly calls. On a final note, the team concluded that, interestingly, it's not just bats that cap their vocal intensity at ∼130 dB, as the peak intensity for many other mammals, birds and amphibians is also in the 130–140 dB range, suggesting that it may be energetic constraints, rather than body size, that limit the ability of animals to shout, scream and squawk.